Publications of the University of Eastern FinlandDissertations in Forestry and Natural Sciences No 188

Publications of the University of Eastern Finland

Dissertations in Forestry and Natural Sciences

isbn: 978-952-61-1893-2 (printed)

issn: 1798-5668

isbn: 978-952-61-1894-9 (pdf)

issn: 1798-5676

Sari Oksanen

Spatial ecology of the grey seal and ringed seal in the Baltic Sea– seeking solutions to the coexistence of seals

and fisheries

The Baltic seal populations have

recovered since the 1990s and the

conflicts between seals and fishery

interests have aggravated at the

same time. This thesis provides

new information on the spatial

ecology of the grey seal and ringed

seal in relation to coastal fisheries

in the Baltic Sea and discusses the

implications for conflict mitigation

and population management

measures.

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Sari OksanenSpatial ecology of the grey seal and ringed seal in the

Baltic Sea– seeking solutions to

the coexistence of seals

and fisheries

SARI OKSANEN

Spatial ecology of the grey seal and ringed seal in the

Baltic Sea – seeking solutions to the coexistence of seals and

fisheries

Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences

No 188

Academic Dissertation To be presented by permission of the Faculty of Science and Forestry for public examination in Auditorium N100 of the Natura Building, University of Eastern

Finland, Joensuu, on November 20, 2015, at 12 noon.

Department of Biology

Grano Oy Jyväskylä, 2015

Editors: Research Dir. Pertti Pasanen, Profs. Pekka Kilpeläinen, Kai Peiponen, and Matti Vornanen

Distribution: University of Eastern Finland Library / Sales of publications

P.O.Box 107, FI-80101 Joensuu, Finland tel. +358-50-3058396 www.uef.fi/kirjasto

ISBN: 978-952-61-1893-2 (printed) ISSNL: 1798-5668 ISSN: 1798-5668

ISBN: 978-952-61-1894-9 (PDF)

ISSNL: 1798-5668 ISSN: 1798-5676

Author’s address: University of Eastern Finland Department of Biology P.O.Box 111 80101 JOENSUU FINLAND email: sari.oksanen@uef.fi

Supervisors: Senior researcher Mervi Kunnasranta, Ph.D.

University of Eastern Finland Department of Biology P.O.Box 111 80101 JOENSUU FINLAND email: mervi.kunnasranta@uef.fi

Professor Raine Kortet, Ph.D. University of Eastern Finland Department of Biology P.O.Box 111 80101 JOENSUU FINLAND email: raine.kortet@uef.fi

Reviewers: Senior researcher Bernie McConnell, Ph.D Scottish Oceans Institute East Sands University of St Andrews St Andrews Fife KY16 8LB UNITED KINGDOM email: bm8@st-andrews.ac.uk

Wildlife biologist Jamie Womble, Ph.D National Park Service Glacier Bay Field Station 3100 National Park Road Juneau, Alaska 99801 USA email: Jamie_Womble@nps.gov

Opponent: Senior researcher Jonas Teilmann, Ph.D

Department of Bioscience Frederiksborgvej 399 4000 ROSKILDE DENMARK email: jte@bios.au.dk

ABSTRACT

A thorough understanding of the habitat use of seals can enhance modern management of their populations and of the seal-fishery conflicts. The Baltic grey seal (Halichoerus grypus) and ringed seal (Phoca hispida botnica) populations have been subject to various anthropogenic impacts such as extensive hunting and environmental pollution, which led them to the brink of extinction during the 20th century. Populations have recovered since the 1990s and seal-fishery interactions have increased at the same time. The spatial ecology and movement patterns of free ranging Baltic grey seals and ringed seals were quantified in relation to fisheries using satellite telemetry. Also, by-catch and its mitigation in fyke nets were studied.

The results of this work point to considerable mobility among the grey seals and ringed seals in the Baltic Sea. During the post-moulting season, however, most grey seals (i.e. residents) occupied home ranges of 890 km2, inside which they made repeated foraging trips from a couple of haul out-sites to foraging areas near their capture sites. By comparison, the ringed seals occupied larger home ranges (6 690 km2) and often had several spatially remote foraging areas and an average of 26 haul-out sites. In general, the grey seals showed much stronger foraging and haul-out site fidelity than the ringed seals during the post-moulting season. The present observations support existing cumulative evidence that some adult male grey seals specialise in feeding in fishing gear, whereas no comparable indications of specialisation were observed among the ringed seals.

This thesis provides evidence that incidental by-catch in fyke nets increases juvenile mortality of the ringed and grey seals. The sex ratio was equal among the by-caught ringed seals, but male-biased among the grey seals. The seal sock, a cylindrical net attached to the fish chamber of the fyke net, proved to be effective in reducing by-catch mortality among ringed seals, but did not perform as well with grey seals.

The divergence in movement patterns and habitat use between the two seal species suggests profound differences in the effectiveness of conflict mitigation and population management measures. The results imply that selective removal of individuals near the fishing gear could be a more effective method of mitigating depredation by grey seals than by ringed seals. The stronger haul-out and foraging site fidelity of grey seals suggests that the spatial management of key habitats is more straightforward for this species than for ringed seals. Nevertheless, despite the nomadic behaviour of ringed seals during the post-moulting season, their foraging areas did form clusters. Thus the results indicate that management of key ringed seal habitats could to some extent be implemented in the marine protected areas that overlap with these “foraging hot spots”.

Universal Decimal Classification: 574.3, 591.522, 591.525, 591.526, 599.745.3 CAB Thesaurus: seals; Phoca hispida; Halichoerus grypus; fisheries; bycatch; mortality; animal behaviour; population ecology; populations; habitats; spatial distribution; movement; foraging; tracking; telemetry; Baltic Sea Yleinen suomalainen asiasanasto: hylkeet; halli; norppa; kalastus; kuolleisuus; käyttäytyminen; populaatioekologia; populaatiot; habitaatti; liikkuminen; seuranta; Itämeri

Preface

I am grateful to all the people who have contributed to this thesis and supported me during this project. First of all, I owe my warmest thanks to my main supervisor Mervi Kunnasranta, who has provided me with both the freedom to find my own way of doing things but also help and support when I have got stuck. I wish to thank also my other supervisor Raine Kortet, who gave me valuable comments to my thesis and advice in many practical matters.

During the past four years I have been privileged to work in and combine data from several larger projects that have been carried out in the former Finnish Game and Fisheries Research Institute. I wish to thank Markus Ahola, Esa Lehtonen and Nina Peuhkuri for the fruitful collaboration and the opportunity to combine the data from different projects into my thesis. I thank also my other co-authors Jyrki Oikarinen for his help in putting the ringed seal project into action and Marja Niemi especially for the co-operation in finding ways to analyse the data. I sincerely wish to thank the reviewers Bernie McConnell and Jamie Womble for their thorough reviews and constructing feedback.

The sponsors are acknowledged for making this research possible. The European Fisheries Fund, the ESKO Fishery Action Group and Sandmans stiftelse granted financial support for the projects. I am grateful for the Maj and Tor Nessling Foundation for funding my PhD studies and part of the field work in the Bothnian Bay. Many thanks also to Hilary Devlin, Rosemary Mackenzie and Malcolm Hicks for the linguistic help during these years. Thanks are also due to Lauri Mehtätalo and Juha Heikkinen for the statistical support, Pirkko Söderkultalahti for help regarding the fishery data and for Kirsti Kyyrönen for help with the graphics.

The collecting of the data is very dependent on a good field team. I express my deepest gratitude to the whole fieldwork team

of the grey seal projects, especially fishermen Onni Välisalo, Juha Välisalo, Heikki Salokangas, Mikael Lindholm, Magnus Lindholm, Yngve Thomasson and Bengt Blomqvist. Also Tapio Mäkelä and veterinarians Nina Aalto and Marja Isomursu were of invaluable help. I was fortunate to take part in the field work of the ringed seal project in the Bothnian Bay and my special thanks are extended to all those people working there. I wish to thank the fishermen Sauli Kehus, Esa Pirkola, Mauno Posti, Timo Matinlassi and Mikko Viitanen for their efforts in catching the seals. Thanks are also extended to Eero Helle, Paavo Hepola and Martti Lahti for sharing the old local know-how about capturing seals. Special thanks are due to Juha Vierimaa and Petri Timonen for their effort in the field but also for their company during the long days of waiting for action in a small cottage. Jouni Aspi, Miina Auttila, Pihla Kauppinen, Terho Laitinen, Juha Taskinen, Mika Vehmas, Kalevi Viipale and Jari Ylönen are thanked for their participation and valuable help. Peter Boveng and Josh London from NOAA are thanked for the flipper tags and all the help with them. I thank the technical support from the university, Kari Ratilainen, Tuomo Nilsen and Matti Savinainen, for their help in many practical matters over the years.

I wish to express my gratitude to the entire staff of the Department of Biology for the nice atmosphere and excellent conditions for my research. Particularly, I wish to thank the ‘research group of cute animals’, i.e Anni N, Anni R, Lauri, Marja, Meeri, Mervi, Mia, Miina and Riikka for all the merry and sometimes also somewhat crazy moments, but also for the support when things did not go as planned. Thanks also to the colleagues at the coffee room for the conversations. I have truly enjoyed working with you all.

All my friends, Kaisa, Jyrki, Nina, Arto, Olutkerhon tytöt, just to mention a few, thank you for your company and for giving me a refuge from work when needed. Finally, I wish to thank my parents, my brother Oskari and my sister Kati and her family for all the love and support I have received throughout my life. Jari, thanks for always being there for me and thanks for being you. I am so lucky to have you all in my life!

LIST OF ORIGINAL PUBLICATIONS This thesis is based on data presented in the following articles, referred to by the Roman numerals I–III.

I Oksanen S M, Ahola M P, Lehtonen E and Kunnasranta M. Using movement data of Baltic grey seals to examine foraging-site fidelity: implications for seal-fishery conflict mitigation. Marine Ecology Progress Series 507: 297-308, 2014.

II Oksanen S M, Niemi M, Ahola M P and Kunnasranta M.

Identifying foraging habitats of Baltic ringed seals using movement data. Movement Ecology 3: 33, 2015.

III Oksanen S M, Ahola M P, Oikarinen J and Kunnasranta M.

A Novel tool to mitigate by-catch mortality of Baltic seals in coastal fyke net fishery. PLoS ONE 10: e0127510, 2015.

The above publications are reproduced at the end of this thesis with their copyright holders’ permission.

AUTHOR’S CONTRIBUTION The original ideas for the papers were developed jointly by the authors. The present author took part in the data collection for papers II and III, but not for paper I. She carried out the analyses for all the papers and was the author responsible for the manuscript in all cases.

Contents

1 Introduction ................................................................................... 13 1.1 Seal-human interaction in the Baltic region ............................ 14 1.2 Mitigation of the conflict ............................................................ 16 1.3 Aims of the present research ..................................................... 20

2 Materials and methods ................................................................ 21 2.1 The Baltic Sea ............................................................................... 21 2.2 Live-capturing and tagging of the seals .................................. 23 2.3 Analyses of movement and habitat use ................................... 24 2.3.1 Temporal and behavioural division of the data ........................... 24 2.3.2 Defining home ranges and foraging areas .................................. 25 2.3.3 Habitat characteristics ................................................................ 25 2.4 Analysis of by-catch and possibilities for its mitigation ....... 26 2.5 Ethics of the research .................................................................. 27

3 Results and discussion ................................................................. 29 3.1 Male grey seals visit the pontoon traps ................................... 29 3.2 Species variation in movement patterns ................................. 30 3.3 Stronger foraging and haul-out site fidelity in grey seals ..... 34 3.4 By-catch mortality in fyke nets and its reduction .................. 38 3.5 Implications for conflict mitigation and the management of seal populations ................................................................................ 39 3.5.1 Selective removal as a tool for reducing depredation .................. 39 3.5.2 Spatial management of key habitats ........................................... 40 3.5.3 International seals require international management .............. 41 3.5.4 Development of fishing practises ................................................ 43

4 Conclusions .................................................................................... 45

5 References ...................................................................................... 47

6 Appendices .................................................................................... 61

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1 Introduction

Identification of the key habitats for life history priorities such as breeding and foraging, is often an initial step in understanding animals’ spatial habitat use and designing effective conservation and management strategies. These principles also hold true for mobile aquatic predators like pinnipeds (Harwood, 2001; Russell et al., 2013; Womble & Gende, 2013). The lives of many seal species are currently affected by anthropogenic influences such as interactions with the fishing industry and the effects of climate change (Kovacs et al., 2012; Laidre et al., 2015). The ice-associated pinnipeds are dependent on sea-ice habitats, especially for breeding, whereas a reduction in the extent of the sea ice might actually benefit land-breeding seals by creating more suitable habitats (Kovacs et al., 2012). The impacts of climate change on regional ecosystems and food webs are hard to predict; however, variable trends in seal populations will likely be seen in the future (Kovacs et al., 2011, 2012; Laidre et al., 2015).

Many seal species interact with fisheries while foraging (Graham et al., 2011; Harvey et al., 2012; Bowen & Lidgard, 2013), which can result in conflicts, usually of a complex nature. Seals may reduce fish catches and damage fishing gear (Read 2008) and may compete for the same resources with fisheries (Yodzis, 2001; Bowen & Lidgard, 2013). Seal populations can also face increased resource competition (DeMaster et al., 2001) and habitat loss (Harwood, 2001), as well as increased mortality due to culling and incidental by-catching (Hall et al., 2000; Read, 2008). Although these conflicting interactions have a long history, the recent expansion of human activities, together with the recovery of seal populations, has led to an increase in seal-fishery interactions in many parts of the world (Bowen & Lidgard, 2013; Roman et al., 2015). An improved understanding of the foraging habitats of seals may help us to assess actions to mitigate the impact of these conflicts (Matthiopoulos et al., 2008; Augé et al.,

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2014). For example, the detrimental effects that some seals have on fisheries could potentially be reduced with locally focused removals when seals show strong foraging site fidelity with respect to certain areas that are important for the commercial fishing (Graham et al., 2011). Similarly, the targeting of marine protected areas (MPAs) towards the conservation of important feeding grounds for mobile predators have successfully mitigated incidental by-catch and resource competition (Pichegru et al., 2010; Gormley et al., 2012).

1.1 SEAL-HUMAN INTERACTION IN THE BALTIC REGION

The grey seal (Halichoerus grypus) has a long history of conflict with fisheries in many parts of its coastal distribution area in the North Atlantic (Hall & Thompson, 2009; Bowen & Lidgard, 2013; Roman et al., 2015). The ringed seal (Phoca hispida hispida) generally inhabits remote locations throughout its circumpolar distribution, while its Baltic subspecies (P. h. botnica) inhabits areas where human activities are ubiquitous (Korpinen et al., 2012). Thus the grey seal and ringed seal may be said to have significant interactions with humans in the Baltic region.

The colonisation of the coastal areas of the Baltic Sea after the last Ice Age, some 9 000 years ago, was closely linked to hunting and fishing, and seal hunting remained a traditional source of income in the coastal settlements until the 19th century (Ylimaunu 2000). By the beginning of the 20th century, however, seals had lost much of their economic value, as seal oil and pelts had been replaced by cheaper alternatives. Instead, seals became more distinctly regarded as competing with the fishermen. The estimated population sizes at that time were from tens of thousands to over 100 000 grey seals and from 100 000 to over 200 000 ringed seals (Harding & Härkönen, 1999; Kokko et al., 1999). Several culling bounty systems were introduced (Ylimaunu, 2000). Although sealing in the Baltic region was not practiced on an industrial-scale, small-scale traditional hunting occurred and resulted in a serious decline in seal populations.

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Populations were further affected by reproductive problems caused by pollutants and the combination of extensive hunting and toxins caused both populations to decline to only about 5 000 seals by the 1970s (Harding & Härkönen, 1999). As a result of conservation measures and reductions in the concentrations of organochlorines in the water that they inhabit, the Baltic seal populations have been recovering since 1990s (Harding & Härkönen, 1999; Nyman, 2000; Routti, 2009). Reproduction in the grey seal has recovered to normal levels and the prevalence of uterine occlusions in the ringed seal has declined from ca. 60% in the late 1970s (Helle 1980a) to 8% (Bäcklin et al., 2013). The present census sizes are 32 000 grey seals (Ahola & Leskelä, 2014) and 13 000 ringed seals (Sundqvist et al., 2012), although the most recent estimate for the ringed seal indicates an even larger population (17 600 seals; T. Härkönen, personal comment). A third seal species, the harbour seal (Phoca vitulina), inhabits southern Baltic Sea and its current population estimate is 800 seals (Härkönen et al., 2013).

Attitudes towards seals vary throughout the Baltic region and depend on the local abundances of the various species. The recovery of the grey seal population has occurred mainly in the northern and central Baltic Sea (Härkönen et al., 2013) and that of the ringed seal at the northernmost tip of the Baltic Sea, in the Bothnian Bay (Fig. 1). The grey seal has recolonised its former distribution area in the south only very slowly, and the southern subpopulations of the ringed seal in the Gulf of Riga and Gulf of Finland are recovering extremely slowly, if at all. The grey seal causes most of the seal-induced damage to fishing and fish farming around the Baltic Sea (Lunneryd et al., 2003; Kauppinen et al., 2005), which means that it is largely considered a game species or even a pest in its main distribution area in Finland and Sweden. On the other hand, in parts of the southern Baltic Sea where its numbers are low, i.e. in Poland and Germany, it is considered a species that needs more effective conservation (Schwarz et al., 2003; HELCOM Seal Expert group 2010). A similar north-south dichotomy also exists in attitudes towards ringed seals, with their growing numbers in the Bothnian Bay allegedly

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causing fishermen substantial catch losses and changing public opinion against their conservation (Storm et al., 2007).

1.2 MITIGATION OF THE CONFLICT

The main policy instruments for resolving the issue of seal-induced fish losses and gear damage in the northern Baltic Sea are financial compensation and the development of fishing technology and practices (Bruckmeier & Höj Larsen, 2008; Varjopuro, 2011). In addition, lethal removal of the seals has become a key policy instrument for reducing seal-induced damages to fishing prospects (Finnish Ministry of Agriculture and Forestry, 2007; Bruckmeier & Höj Larsen, 2008; Varjopuro 2011). The combined annual hunting quota for grey seals in Finland and Sweden is approximately 1850 individuals (Kauhala et al., 2012; Naturvårdsverket 2015), and the actual catch has been ca. 400–600 seals (Moraeus et al., 2014; Finnish Wildlife Agency, 2015; Ålands Landskapsregering, 2015). A quota of 100 ringed seals has recently been established in Finland (Finnish Ministry of Agriculture and Forestry, 2015), while in Sweden several dozens separate hunting licenses are permitted annually. In spite of these measures, however, depredation and associated damage caused by seals still continues to be a notable problem for coastal fisheries (Varjopuro, 2011; Holma et al., 2014).

Detailed knowledge of the spatial ecology of seals is essential for developing strategies for management and mitigation of the seal-fishery conflict. Grey and ringed seals, like many other phocids, have three key elements in their annual life cycle, breeding, moulting and foraging. In the Baltic region they mate and give birth in February–March (Hook & Johnels, 1972; Hall & Thompson, 2009), which coincides with the annual maximum ice coverage. The Baltic grey seals preferably give birth on drifting ice floes, but they can also do so on land (Hook & Johnels, 1972; Jüssi et al., 2008) while the ringed seal pups are born in subnivean lairs, mainly in stable pack ice (Hook & Johnels 1972; Smith & Stirling, 1975; Jüssi, 2012). The lactation period is typically 2–3

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weeks for grey seals (Haller et al., 1996) and 5–7 weeks for ringed seals (Hammill et al., 1991; Reeves, 1998). Moulting reaches its peak in April–May in both species and is characterised by extended haul-out periods (Helle, 1980b; Born et al., 2002; Kunnasranta et al., 2002). Foraging is limited during breeding and moulting, when the seals lose weight (Beck et al., 2003b; Kelly et al., 2010; Young & Ferguson, 2013). The post-moulting season, on the other hand, is an important foraging period when seals restore their energy reserves for the next winter (Ryg et al., 1990; Beck et al., 2003b; Härkönen et al., 2008; Young & Ferguson, 2013). It is also the time period when the coastal fishing effort is the highest in the Baltic Sea, and it therefore coincides with the majority of interactions between seals and fishermen.

Globally marine mammal populations are increasingly being subjected to fragmentation and degeneration of their marine habitats (Harwood, 2001). This is also the case with the Baltic Sea, which is influenced by various human activities including eutrophication, fishing and inputs of hazardous substances (Korpinen et al., 2012). Marine protected areas have been found to be an effective way of limiting interactions between aquatic predators and fisheries (Pichegru et al., 2010; Gormley et al., 2012). For example, protection of the key foraging areas of the New Zealand sea lion (Phocarctos hookeri) has been proposed as a means of reducing interaction with fisheries in the form of resource competition, by-catch mortality and habitat loss due to bottom trawling (Augé et al., 2014).

Both the grey seal and the ringed seal are listed as annex II species in the EU Habitats Directive, implying that special areas of conservation should be set aside to maintain their important habitats (European Comission 1992). Also the Baltic Marine Environment Protection Commission (HELCOM) requires maintenance of the status and habitats of these species. For many pinnipeds it is relatively easy to define their critical habitats for breeding or resting, but defining their foraging habitats is more difficult (Harwood 2001). Therefore, despite the fact that many pinniped species spend the majority of their time in the water, the delimitation of protected areas to reduce the impacts of human

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activities has mainly been focused on terrestrial haul-out sites (Cordes et al., 2011; Augé et al., 2014). Sanctuaries have been established, for example, to protect certain grey seal haul-out sites in the Baltic Sea. By contrast, little attention has been paid until recently to foraging areas, which are harder to identify but where interactions with fisheries most often occur (Cordes et al., 2011; Augé et al., 2014).

Quantifying the spatial patterns of free-ranging seals should allow for identification of key foraging habitats and estimation of the degree of spatial overlap between the seals and coastal fisheries. The magnitude of such interactions depends partly on the degree of spatial and temporal resource overlap between the seals and the fishing activities (Matthiopoulos et al., 2008). In general, few studies carried out previously on the movements of seals in the Baltic have mainly concentrated on grey seal juveniles, demonstrating that although they prefer shallow areas near haul-out sites, the juveniles concentrate their movements less than do adults (Sjöberg et al., 1995; Sjöberg & Ball, 2000). More information is needed on the habitat use of adults and their overlap with fisheries, as diet studies indicate that older grey seals in particular feed on large, economically important fish species (Lundström et al., 2010; Kauhala et al., 2011). Seasonal and inter-annual haul-out site fidelity has been demonstrated for Baltic grey seals (Karlsson et al., 2005), but we have little knowledge of their foraging habitats, foraging site fidelity or potential overlap with coastal fisheries. In addition, recent results based on photo-identification (Königson et al., 2013) suggest that grey seals show a varying degree of individual specialisation in visiting pontoon traps, but again the detailed habitat use of the individuals visiting the pontoon traps has remained unknown. Information is also lacking on possible specialisation in ringed seals. In general, very little information exists on the interaction between ringed seals and fisheries in either the Baltic Sea or globally. The Baltic ringed seal is traditionally looked on as relatively sedentary (Härkönen et al., 2008), but its spatial ecology in terms of foraging habitats or range of movement has not been studied in detail.

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The further development of fishing methods and deterrents can provide a means of mitigating seal-induced damage and incidental by-catch mortality. Physical barriers such as wire grids mounted on the fish traps may prevent seals from entering the traps (Lunneryd et al., 2003; Lehtonen & Suuronen, 2004; Hemmingsson et al., 2008; Königson et al., 2015b). Exclusion devices that facilitate the seals’ exit from the fishing gear (Hamilton & Baker, 2015) or techniques for trapping the seals alive inside the fishing gear have also been developed (Lehtonen & Suuronen, 2010). Small-scale coastal fishermen using trap nets, gill nets and long lines in the Baltic Sea are highly susceptible to seal-induced damage (Köningson et al., 2009), and this is one reason why certain alternative types of seal-proof fishing gear have been developed. Cod pots equipped with seal exclusion devices, for example, provide an alternative to long lines and gillnets for coastal cod fishing, as they reduce both the grey seal depredation and by-catching (Königson et al., 2015a, 2015b). Also trap nets with modified fish chambers, i.e. pontoon traps, reduce grey seal-induced losses and by-catch (Suuronen et al., 2006; Hemmingsson et al., 2008). Relatively little attention, however, has been paid to estimating and reducing the by-catch mortality of seals in other stationary gear in the Baltic Sea. An information gap regarding pinniped by-catch mortality and methods for reducing it in coastal and small-scale fishing has also been recognized globally (Lewison et al., 2004). In addition to seal hunting, by-catch mortality may be another significant component in the anthropogenic mortality of seal populations. The estimated annual by-catch of the grey seals in the Baltic is of the order of 2 000 animals or more (Vanhatalo et al., 2014), but the magnitude of that for ringed seals is unknown. A broader knowledge of the causes and magnitude of by-catch of seals and methods for avoiding it would be of great importance for the sustainable management of seal populations and fisheries.

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1.3 AIMS OF THE PRESENT RESEARCH

The primary objectives of the research described in this dissertation were to provide scientifically based information on the spatial ecology and habitat use of the grey and ringed seals in the Baltic Sea in relation to the seal-fishery conflict and conservation of the seal populations. The specific objectives were:

1. to examine the movement patterns and foraging habitats of the Baltic grey and ringed seals in relation to coastal fisheries, protected areas and environmental attributes (I, II),

2. to quantify the demographic properties including sex, age and species of seals that are bycaught in fyke nets and to test the effectiveness of the seal sock, a pinniped by-catch reduction device for use with fyke nets (III), and

3. to provide information that can be used in management and mitigation of the seal-fishery conflict, in the development of practices for sustainable fishing and in the conservation of seal populations (I-III).

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2 Materials and methods

2.1 THE BALTIC SEA

The Baltic Sea (surface area 400 000 km2) is a semi-enclosed brackish water system (average salinity 7‰), with an ocean connection through narrow, shallow Danish straits, where saline oceanic water enters the Baltic basin and mixes with freshwater from its catchment area (Leppäranta & Myrberg, 2009). The brackish water limits species diversity in the basin in such a way that this diversity decreases along with the salinity gradient from the more saline south-western region (surface water salinity 18–26‰) to the almost freshwater conditions (0–6‰) of the Bothnian Bay in the north and the Gulf of Finland in the east. The mean depth of the Baltic Sea is 54 m and its maximum depth 459 m. The present research was mainly conducted in the Gulf of Finland (mean depth 37 m, maximum 123 m) and the Gulf of Bothnia (mean 55 m, maximum 293 m), where the latter is taken to comprise (from north to south) the Bothnian Bay, the Quark and the Bothnian Sea (Fig. 1).

Most Finnish commercial fishermen (ca. 2 100 fishing in the Baltic Sea) use trap nets and gill nets in the coastal areas (Finnish Game and Fisheries Research Institute, 2014). The annual catches with such fishing gear nevertheless make up only about 9% of the total catch of the Finnish marine fishing and the majority comes from trawling for Baltic herring (Clupea harengus membras) and sprat (Sprattus sprattus) (Finnish Game and Fisheries Research Institute, 2014).

A trap net is defined here as a bottom-anchored floating item of fishing gear with wings and a leader net to guide fish into a fish chamber (or bag). Different types of trap nets such as pontoon traps (Hemmingsson et al., 2008, I) and fyke nets (II, III) are commonly used in the Baltic area, mainly for targeting Baltic herring, Atlantic salmon (Salmo salar), European whitefish

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(Coregonus lavaretus), and in the Bothnian Bay also vendace (Coregonus albula).

Figure 1. The Baltic Sea region. Grey seals were captured with pontoon traps in the Gulf of Finland and Bothnian Sea (I) and ringed seals with fyke nets and floating seal nets in the Bothnian Bay (II). Seal socks were tested for use in fyke nets in the Bothnian Bay (III).

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2.2 LIVE-CAPTURING AND TAGGING OF THE SEALS

The seals were captured live for two purposes: firstly, for telemetry studies (I, II), and secondly for testing the performance of the seal-sock, a novel device consisting of a strong, seal-proof cylindrical net, as a means of reducing by-catching in fyke nets (III). The grey seals for the telemetry studies were captured in the Bothnian Sea and the Gulf of Finland from 2008 to 2011 with commercial pontoon traps equipped with “non-return” gates (see Lehtonen & Suuronen, 2010) (Fig. 1, I). The ringed seals for tagging were captured in the Bothnian Bay from 2011 to 2013 either with fyke nets equipped with seal socks or with floating seal nets (Fig. 1, II). Attempts were also made to catch ringed seals with fyke nets having a fish chamber that was partly above the surface in 2012 and with a flexi-fyke net and a fyke net of small mesh size, both equipped with seal socks, in 2013. However, only ringed seals that were too small to be tagged (< 40 kg; 7 alive and 2 dead) were captured with these nets, and they had to be excluded from the material.

Altogether 21 grey seals from the pontoon traps (I) and 26 ringed seals from fyke nets (N = 10) and seal nets (N = 16) (II) were equipped with GPS phone tags (Sea Mammal Research Unit, University of St. Andrews, UK)(Appendix 1). To complement the GPS data, additional Argos flipper tags (SPOT5, Wildlife Computers Inc.) were deployed on four ringed seals, and they extended the overall tracking period by a couple of months (II, Appendix 1). To ensure later identification, a uniquely numbered plastic ID tag (Jumbo tag, Dalton) was attached to a hind flipper of each seal captured live (N = 73) (I, II). Seals were classified as adults or juveniles according to body weight by reference to an age–weight database (Natural Resources Institute Finland), i.e. weighed ringed seals with a body weight > 50 kg were classified as adults (estimated age ≥ 4 years) (II, III) and male grey seals with a body weight > 92 kg and females > 65 kg were categorized as adults (estimated age ≥ 5 years) (I, III).

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2.3 ANALYSES OF MOVEMENT AND HABITAT USE

2.3.1 Temporal and behavioural division of the data Overall, the 16 GPS phone tags on grey seals provided data on an average of 136 ± 69 days (range 37–301 d, 167–6030 locations) and the 24 tags on ringed seals 125 ± 39 days (range 74–202 d, 336–6452 locations). The remaining 7 tags (5 deployed on grey seals and 2 on ringed seals) only provided data from 0 to 19 days or only provided a few GPS locations, and these were not included in the home range analyses (I, II). The location data were filtered by a method following that of McConnell et al. (1992). The GPS phone tags had wet-dry sensors whose information was used to classify the GPS locations to either at-sea or haul-out locations (I, II). For the grey seals the at-sea locations were further categorized as active ones (representing movements) if the seal was more than 1 km away from its haul-out site (Cronin et al., 2012) (I). The GPS phone tags utilise Fastloc GPS technology (Wildtrack Telemetry Systems) and the accuracy is ± 55 m when ≥ 5 satellites are used to locate the tag (Bryant 2007).

For the movement analyses the tracking data were divided into two seasons on the basis of the behaviour patterns of the seals (I, II). Different criteria were used for the grey seals (I) and ringed seals (II), as most of the grey seals made a clear transition away from their open-water home ranges at the time of ice formation, so that a temporal division could be made into an open-water season (22 June – 2 January) and an ice-covered season (14 December – 8 May) based on the timing of the transition in each seal (I). By contrast, ringed seals did not show any comparable clearly definable behaviour related to ice formation. Ringed seals forage intensively between August and January to store energy for breeding and moulting (Härkönen et al., 2008; Young & Ferguson, 2013), so that their data were divided according to the foraging (29 August – 31 January) and breeding seasons (1 February – 31 March) (II). In practise the open-water season for the grey seals and the foraging season for the ringed seals virtually coincide, and for clarity these periods are referred to as the post-moulting season in the present summary.

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2.3.2 Defining home ranges and foraging areas To examine the overall movement patterns, the home ranges of seals were assessed by means of minimum convex polygon (MCP; Worton, 1987) and adaptive local nearest neighbour convex hull (LoCoH; Getz et al., 2007) analyses (I, II). Figures for the individual total home ranges (for the whole tracking period) were obtained with the MCP and LoCoH estimators (95% of the locations in MCP and 95% of the isopleths of the utilisation distribution in LoCoH). Home ranges for the post-moulting season were obtained with the LoCoH estimator for both the grey seals (I) and the ringed seals (Table 1). Land areas were excluded from the MCP home range estimates.

The main foraging areas of the grey seals were determined in terms of active core areas, i.e. the 50% isopleths of the utilisation distribution constructed from each individual’s active locations by means of the LoCoH estimator (I). The foraging habitats of the ringed seals were detected with first passage time (FPT) analyses (II), where FPT is defined as the time required for a tracked individual to cross a circle of a given radius (Johnson et al., 1992; Fauchald & Tveraa 2003) and can be used to detect any movement patterns leading to increased residence (Lemieux Lefebvre et al., 2012). High FPT values (high-residence locations) were separated from low FPT values by obtaining a threshold value from a histogram of FPT values for each track (Pinaud & Weimerskirch 2007). The high-residence locations were then used to detect one or more foraging areas within each track, following the method published by Lemieux Lefebvre et al. (2012). For comparative purposes, core areas for the ringed seals were also estimated from the LoCoH analyses (Table 1).

2.3.3 Habitat characteristics The habitat preferences of the grey seals with respect to water depth were investigated during the post-moulting season, when the greatest overlap with coastal fisheries is expected (I). Manly’s selection ratios (Manly et al., 2002) were calculated for each seal by relating areas used (i.e. the 95% LoCoH home range) to the area available in seven water depth classes (employing

26

bathymetric data from HELCOM, 2015a). Two fishery datasets were used to examine the spatial and temporal overlaps between fishing activities and the tracked grey seals (I). First, catch report data (in 50 x 50 km ICES statistical rectangles) based on EU fishing logbooks and fishery forms applying to the Finnish coastal waters in 2008–2011 were used to examine the overlap between grey seal tracking data, the trap net fishing effort and seal-induced damage. Secondly, a spatially limited, but more fine-scaled dataset of trap nets (location, type and fishing season for each trap net) was used to examine fine-scaled spatial overlap of grey seals and trap nets in the Gulf of Finland on a weekly basis during 2010 and 2011. The percentage of tracking days when each seal was within close proximity (< 250 m) of the trap nets was calculated.

To investigate the foraging habitat characteristics of ringed seals, the water depth of high-residence locations and their distance from the coastline were calculated using HELCOM’s GIS data (HELCOM, 2015a)(II). Also, the percentage of high-residence locations within the MPAs and within those ICES statistical rectangles which had annual coastal fishery catches (in 2007) over the median value for the Baltic Sea (HELCOM 2015a) were calculated.

2.4 ANALYSIS OF BY-CATCH AND POSSIBILITIES FOR ITS MITIGATION

The effectiveness of seal socks (Fig. 2) in reducing by-catching has not previously been tested (III). To do this, the numbers of seals caught alive in a set of fyke nets (4–6 nets/year) were compared between years when the socks were not in use (2008–2010) and when they were in use (2011–2013). The fishing took place from May to October–November within the same area in the Bothnian Bay in each year (Fig. 1). The fishermen recorded the number, species and capture date of all seals caught in this way in a logbook, and also the sex and weight (kg) or age class (juveniles and adults) of the seals concerned during the seal sock test period

27

in 2011–2013. The weighed seals were further divided into age classes by reference to an age-weight database (Natural Resources Institute Finland).

Figure 2. The seal sock attached to the fish chamber of the fyke net. When the fyke net is set for fishing the sock points upwards and has a float to keep it on the surface. A diagram of the sock and fyke net can be found in the paper III.

2.5 ETHICS OF THE RESEARCH

The capturing and tagging protocol was approved by the Finnish Wildlife Agency (licences nos. 2011/00087, 2011/00082 and 2013/00197) and the Animal Experiment Board in Finland (licences nos. ESLH-2008-04828/YM-23, ESAVI-2010-05432/Ym-23 and ESAVI/1114/04.10.03/2011). Our goal was always to reduce handling times as far as possible and thereby to minimise the stress caused to the animals.

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29

3 Results and discussion

3.1 MALE GREY SEALS VISIT THE PONTOON TRAPS

The sex and age distributions of the seals varied between the capturing methods and species. The grey seals captured with pontoon traps (N = 28) were all males, most of them were adults (82%) and three of them were recaptured in the same pontoon traps (I). In contrast, the ringed seals captured with various types of fyke net (N = 49) were mostly juveniles (94%) (III), while those captured with seal nets (N = 16) were mainly adults (81%) (II). None of the ringed seals were recaptured.

The results (I) support the observations of male dominance among grey seals feeding in or near pontoon traps in the Baltic (Lehtonen & Suuronen, 2010; Königson et al., 2013; Kauhala et al., 2015). The reason why males forage close to trap nets more than females remains unclear. Male dominance has also been observed among terrestrial carnivores predating on livestock, where the biased sex ratio seems to be related to gender differences in diet and foraging strategy and may ultimately be linked to sexual dimorphism and gender-related differences in mating strategies (Linnell et al., 1999). The grey seal is a polygynous, size-dimorphic species in which the males are larger than the females (Hall & Thompson 2009) and larger size often means higher physiological energy intake demands (Lavigne et al., 1986). The genders of grey seals in the Atlantic Ocean reportedly differ in their seasonal patterns of energy storage, diet, diving and foraging strategy (Beck et al., 2003a, 2003b, 2007; Breed et al., 2006, 2009). Also in the Baltic Sea, adult males in particular feed on larger fish (such as Salmo sp.) in addition to Baltic herring, which is the predominant prey for both sexes and all age groups (Lundström et al., 2010; Kauhala et al., 2011; Suuronen & Lehtonen, 2012). The correlation between body mass and mating success is weak in the case of male grey seals, however (Godsell,

30

1991; Lidgard et al., 2001), and therefore the consumption of abundant but risky food may not be directly beneficial in terms of breeding success. Male grey seals shot near or in trap nets are usually in normal bodily condition (Königson et al., 2013; Kauhala et al., 2015), but the males that become entangled in trap nets are generally in a poorer condition than other males (Bäcklin et al., 2011; Kauhala et al., 2015), possibly indicating increased risk-taking on the part of hungry individuals.

3.2 SPECIES VARIATION IN MOVEMENT PATTERNS

Extensive movements were observed among both the grey and ringed seals (I, II). Both species ranged on average 400 km from their tagging site (I, II, Table 1). Similar figures have been reported for the Atlantic grey seal and Arctic ringed seal, which generally range over distances of several hundreds of kilometres during post-moulting season (e.g. Heide-Jørgensen et al., 1992; Sjöberg et al., 1995; McConnell et al., 1999; Austin et al., 2004; Born et al., 2004; Freitas et al., 2008; Kelly et al., 2010, I-II). In fact, even distances of a couple of thousand kilometres from tagging sites have been reported for both species (McConnell et al., 1999; Teilmann et al., 1999; Kelly et al., 2010; Harwood et al., 2012).

Surprisingly, the total home ranges of the grey seals and ringed seals in the Baltic were similar in size (I, II, Table 1). On average, an individual’s MCP home range for the total tracking period (grey seals 33 675 km2 and ringed seals 31 565 km2, Table 1, I-II) covered ca. 8% of the surface area of the Baltic Sea. Varying patterns of habitat use have been observed within and between grey seal and ringed seal populations (Sjöberg & Ball, 2000; Austin et al., 2004; Niemi et al., 2012; Brown et al., 2014; Harwood et al., 2015). The LoCoH home ranges observed here (6 858 km2 for grey seals and 8 030 km2 for ringed seals, Table 1, I-II) were fairly similar to those previously reported for grey seals in the Baltic Sea (6 294 km2, Sjöberg & Ball 2000) and for ringed seals in eastern Canada (“locals” 2 281 km2 and “long rangers” 11 854 km2, Brown et al., 2014). Also, the resident grey seals in eastern

31

Canada have similar-sized home ranges (3 965 km2, Austin et al., 2004). The home ranges of long-ranging grey seals in eastern Canada (70 680 km2, Austin et al., 2004) and those of ringed seals in the Beaufort Sea (64 141 km2, Harwood et al., 2015) are clearly larger, however, whereas ringed seals in Lake Saimaa understandably occupy smaller home ranges (92 km2, Niemi et al., 2012). Baltic ringed seals have been considered quite sedentary due to the limited movements observed in a previous study (Härkönen et al., 2008), but the contrasting observations made in the present work indicate that the movements of ringed seals in the Baltic Sea are of a similar order of magnitude to those in the Arctic Sea. In addition, genetic studies (Palo et al., 2001; Martinez-Bakker et al., 2013) also indicate that Baltic ringed seals may be more mobile than had earlier been suggested.

Despite the similar range of overall movements between grey and ringed seals of the Baltic, seasonal movement patterns varied between the species. Arctic ringed seals can move long distances during the ice-free post-moulting season (Teilmann et al., 1999; Freitas et al., 2008; Kelly et al., 2010; Harwood et al., 2015), whereas grey seals are central-place foragers and often concentrate their activities near certain haul-out sites (McConnell et al., 1999; Sjöberg & Ball 2000; Karlsson, 2003; Austin et al., 2004; Harvey et al., 2008, I). Most of the grey seals studied here (14/16) were “residents”, occupying relatively small home ranges, and the other two being “transients”, that left the area of capture during the post-moulting season (Fig. 3, I). By comparison, the Baltic ringed seals were more nomadic (II), having larger home ranges and core areas than the grey seals (Fig. 3, Table 1). The difference is even more profound between the ringed seals and the resident grey seals, the post-moulting home ranges of the resident grey seals averaging 886 km2 (I) and those of the ringed seals 6 691 km2 (Table 1). The core areas of the resident grey seals averaged 58 km2 and those of the ringed seals 908 km2.

32

Figure 3. Post-moulting habitat use (95 % LoCoH home ranges) of grey and ringed seals.

33

Tabl

e 1.

Dist

ance

s fro

m ca

ptur

e site

and

hom

e ran

ges (

MCP

95%

and

LoC

oH 9

5%) o

f tra

cked

gre

y se

als (

N =

16)

an

d ri

nged

seal

s (N

= 2

4). C

ore a

reas

(LoC

oH 9

5%) a

re co

nstr

ucte

d fro

m a

ll lo

catio

ns fo

r tot

al tr

acki

ng p

erio

d an

d fr

om a

ctiv

e loc

atio

ns fo

r pos

t-mou

lting

seas

on (J

une-

Janu

ary)

. Pos

t-mou

lting

core

are

as co

uld

not b

e est

imat

ed fo

r 4

grey

seal

s and

1 ri

nged

seal

.

LoC

oH c

ore

area

(k

m2)

rin

ged

980

737

758

908*

*

667

829

gre

y

1 01

7

1 67

0

98

492*

**

1 50

5

64

Diff

eren

ces

betw

een

spec

ies

was

tes

ted

with

Man

n W

itney

U t

est

***

stat

istic

ally

sig

nific

ant

differ

ence

(p<

0.00

1)

LoC

oH h

ome

ran

ge

(km

2)

rin

ged

8 03

0

4 79

6

7 98

1

6 69

1***

3 84

0

5 68

6

gre

y

6 85

8

7 93

6

3 15

2

1 84

6***

3 80

2

662

MC

P h

ome

ran

ge

(km

2)

rin

ged

31 5

65

16 6

40

30 4

75

- - -

gre

y

33 6

75

38 9

36

22 7

79

- - -

Dis

tan

ce f

rom

ca

ptu

re (

km)

rin

ged

392

195

400

399*

**

186

406

gre

y

370

267

337

183*

**

192

154

mea

n

SD

med

ian

mea

n

SD

med

ian

Tota

l tra

ckin

g p

erio

d

Pos

t-m

oult

ing

sea

son

34

Grey seals generally prefer open-water areas, as they do not make breathing holes in the ice (Hook & Johnels, 1972), whereas adult ringed seals in particular prefer fast or pack ice as their habitat (Crawford et al., 2012). This difference is probably one key factor affecting their habitat preferences during the ice-covered period, thus giving rise to differences in seasonal movement patterns. Like the grey seals of the northwest Atlantic (Harvey et al., 2008), most of the present Baltic grey seals left their summer home ranges for the winter period, as these were in sea areas that are typically covered by ice in winter. They tended to favour parts of the Baltic Proper or the Gulf of Riga where there would be some areas of drift ice suitable for breeding. In contrast, the adult ringed seals were mostly located in the ice-covered regions during the breeding time in February-March, while the juveniles were associated more with the ice edges or open water areas (II). The results obtained here thus suggest that the Baltic ringed seals may exhibit similar habitat partitioning between adults and juveniles during the breeding season to that reported in the Arctic (Crawford et al., 2012). Given that ringed seals are known for their breeding site fidelity (Kelly et al., 2010; Valtonen et al., 2012), it is likely that the two adult females that migrated from the Bothnian Bay to the Gulf of Riga in November-December had been feeding in the Bothnian Bay and returned to the Gulf of Riga to breed.

3.3 STRONGER FORAGING AND HAUL-OUT SITE FIDELITY IN GREY SEALS

The grey seals showed stronger foraging and haul-out site fidelity during the post-moulting season than did the ringed seals. Resident grey seals made repeated trips from a small number of off-shore haul-out sites (4 ± 3 sites) to certain in-shore foraging areas and thereby showed both haul-out and foraging site fidelity (I, Fig. 4). Their foraging areas were mostly situated close to their capture sites (Fig. 4). In contrast, the more nomadic ringed seals had an average of 26 ± 16 haul-out sites, and most of them (17/26) had several remote foraging areas (mean distance 328 km) or did

35

not concentrate foraging in any specific area (II, Fig. 4). The remaining nine, however, were more local foragers, having either one foraging area or several areas close (distance 67 km) together.

Figure 4. Schematic illustration of the movements, home ranges and habitat use of the Baltic grey seals (blue) and ringed seals (red) during the post-moulting season. Triangle: capture site; black rectangle: haul-out site. Foraging areas of grey seal (light blue polygon) and ringed seal (red circles).

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Both the grey and ringed seals foraged in relatively shallow waters, which suggests a potential overlap with coastal fisheries (Fig. 5, I-II). The resident grey seals showed a preference for shallow coastal areas (< 30 m deep) and avoided deeper areas (> 50 m) (I), which is similar to earlier findings (Sjöberg & Ball, 2000). The foraging areas of the ringed seals were also situated in shallow waters (median depth 13 ± 49 m) (II). The resident grey seals overlapped both temporally and spatially with commercial trap net fishing (I), so that their foraging areas were characterised by a high fishing effort with trap nets and the seals were observed in the vicinity of these nets (≤ 250 m away) on 30% of the tracking days. Both the core areas (Fig. 5) and the foraging areas (II) of the ringed seals overlapped with coastal fishing to some extent, indicating potential interaction, especially in certain identifiable clusters of foraging “hot spots” in the northern Bothnian Bay and northern Bothnian Sea and Quark (II).

Indirect methods were used to detect foraging (I, II). The active core areas of the grey seals were assumed to reflect the important foraging grounds, although they probably also featured forms of behaviour other than feeding (i.e., transit, socialising and resting, I). Foraging is nevertheless known to account for about 50% of the total time budget of seals (McClintock et al., 2013) and it is likely to dominate their behaviour at sea. In the case of the ringed seals, on the other hand, the foraging areas were estimated using the FPT approach and the haul-out locations were included in the analysis (II). The low proportion (8%) of time that the ringed seals were observed to spend hauling out indicates, however, that this activity contributes relatively little to their use of high-residence areas (referred to here as foraging areas) (II). As the open-water season is the most important foraging and weight gain time for ringed seals (Ryg et al., 1990; Härkönen et al., 2008; Young & Ferguson, 2013), the high-residence areas very likely refer to areas of increased foraging effort. The analytical approaches adopted here (I, II) were heuristic rather than statistical (McConnell et al., 2010), but our results and conclusions should be quite robust with respect to the weaknesses of these approaches, given the accuracy

37

of the GPS positioning, the large number of daily fixes and the study questions, which were related to broad-scale habitat use (I, II). It is clear, however, that more fine-scaled analyses of foraging behaviour and habitat preferences in the Baltic grey and ringed seal populations, based on state-space methods, for example, should be encouraged in the future.

Figure 5. Overlap of post-moulting core areas of the ringed seals (N = 23) and resident grey seals (N = 11) with commercial coastal fishing. Core areas could not be estimated for 3 grey seals and 1 ringed seal. Fishing data from 2007, HELCOM, includes all species reported by fishermen.

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3.4 BY-CATCH MORTALITY IN FYKE NETS AND ITS REDUCTION

The present results indicate that the majority of the seals by-caught in fyke nets are juveniles of both sexes (III). This was especially evident in the case of the ringed seals, 95% of which were juveniles and both sexes were equally represented. Most of the by-caught grey seals were also juveniles (67%), although the sex ratio was biased towards males (71%). The sex and age distributions of the grey seals by-caught in fyke nets (III) were similar to the trends reported for various forms of fishing gear, with juveniles showing an even sex ratio caught in summer and adult males in autumn (Bäcklin et al., 2011; Kauhala et al., 2015; Königson et al., 2015b). The predominance of juveniles in the by-catch could to some extent be explained by their naivety, as juveniles of many seal species are more vulnerable to perishing in fishing gear (Bjørge et al., 2002; Sipilä, 2003; Niemi et al., 2013). The by-caught pups have also been observed to be in poorer condition than other pups, suggesting that individuals with lower energy stores may be more prone to take risks when foraging than those that are in good condition (Kauhala et al., 2015).

The seal socks increased the survival rate for the ringed seals, as 83% of all those caught found their way into the sock and 70% remained alive (III). Survival was not increased among the grey seals, however, as only 17% found their way into the sock, and only 11% remained alive. Only 3 out of the 8 grey seals that reached the fish chamber found their way into the sock, which may point to behavioural differences between the species. Ringed seals keep breathing holes open in the sea ice, which may be several metres thick in the Arctic (Smith & Stirling, 1975), and swimming upwards in a narrow funnel might therefore be a more typical form of behaviour for them. In conclusion, the basic structure of a well-functioning seal sock was a tube of diameter 0.7 m having hoops to keep the sock open and a float to keep it on the surface. The length of the sock should allow for changes in the water level. It is recommended that these characteristics

39

should be taken into account in the commercial manufacture of seal socks.

3.5 IMPLICATIONS FOR CONFLICT MITIGATION AND THE MANAGEMENT OF SEAL POPULATIONS

3.5.1 Selective removal as a tool for reducing depredation The results suggest that selective removal of individuals near the fishing gear could be a more effective conflict mitigation method in the case of grey seals than in that of ringed seals (I, II). It is based on the underlying assumption that only a small proportion of the individuals in the predator population, i.e. problem individuals, are responsible for a disproportional impact, by specializing in feeding either in certain areas or on certain prey species (Linnell et al., 1999). No indication of problem individuals could be detected among ringed seals, however, as most of them ranged over larger areas and did not exhibit as clear within-season foraging site fidelity as did the grey seals (II). In addition, no ringed seals were recaptured during the study (II), whereas some grey seals were recaptured from one to three times in the same pontoon traps (I). In fact evidence has accumulated that some male grey seals specialise foraging in or near pontoon traps (Lehtonen & Suuronen, 2010; Königson et al., 2013), which combined with the observed strong foraging site fidelity of this species and its spatio-temporal overlap with coastal fishing (I), may indicate that some males are likely to be problem individuals. As most grey seal hunting takes place on the spring ice (Bäcklin et al., 2011; Kauhala et al., 2012) and is not targeted at individuals that interfere with fishing, selective removal of individuals appearing around the fishing gear could provide a more locally focused means of reducing the conflict. Also, the conflict appears to be largely independent of the total size of the grey seal population, apparently due to their coastal occurrence (Bowen & Lidgard, 2013). This suggests that selective removal could be a more effective measure than traditional hunting, which is aimed at limiting or reducing the population size.

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However, lethal removal of individuals was reported to reduce depredation for one month (Königson et al., 2013), and it may thus provide only a short-term help as other individuals may replace the one that has been removed.

3.5.2 Spatial management of key habitats The differences in haul-out and foraging habitat use between grey and ringed seals affect the spatial management of their key habitats. Both species are listed as annex II species in the EU Habitats Directive, implying that special areas of conservation should be designated to maintain their important habitats (European Comission, 1992). Seal sanctuaries have been established to protect some grey seal haul-out sites, and 79% of the resident grey seals studied here were found to make use of seal sanctuaries or the frontier zone between Finland and Russia (I). This illustrates the importance for grey seals of haul-out sites with limited anthropogenic activities. In contrast, Baltic ringed seals do not typically haul out in groups at terrestrial sites, and their movements have not been studied in detail previously. Thus planning for spatial conservation measures such as seal sanctuaries for this elusive species is challenging. The present ringed seals ranged over larger areas and showed much lower levels of haul out and foraging site fidelity during the post-moulting season than did the grey seals (II). Roughly half of their haul-out sites, however, were inside the foraging areas, with the foraging areas of different individuals forming two clusters of “foraging hot spots” (Fig. 6, II). It was especially in these hot spots that the foraging areas of the seals partly overlapped with MPAs and Natura 2000 sites, but even so, the presence of ringed seals was listed as a criterion for protection in 7 out of 15 MPAs and in only 5 out of 30 Natura 2000 sites that overlapped with high-residence areas for the species (European Enviroment Agency, 2015; HELCOM, 2015b). The results therefore indicate that management of the important resting and feeding habitats could to some extent be implemented in and adjacent to the existing networks of protected areas. Identified foraging areas of ringed

41

seals should therefore be taken into account when updating management plans for overlapping protected areas.

Figure 6. Overlap of high residency locations of Baltic ringed seals with marine protected areas (MPA) and Natura 2000 sites. Count of high residency (HR) locations in 5 x 5 km grids for tracked ringed seals (N =26) from August to January, 2011-2014.

3.5.3 International seals require international management Detailed information on seasonal movement patterns is important for the planning of management and conservation measures. Grey seals and ringed seals exhibit fidelity to breeding sites (Pomeroy et al., 2000; Kelly et al., 2010; Valtonen et al., 2012),

42

and therefore understanding their spatial distribution throughout the annual cycle is essential, as threats encountered during the post-breeding season may influence population dynamics (Womble & Gende, 2013). For example, the extensive movements of both grey and ringed seals observed in this work (I, II) suggest that local mitigation measures carried out in the northern Baltic Sea, such as hunting and selective removal, may actually target some individuals that breed in the south, where the subpopulations have not recovered well and local abundances are low. Hunting in the north may therefore compromise the attainments of conservation goals set out for the southern part of the Baltic Sea. Interactions may increase further in the northern Baltic Sea due to warming of the climate. The degree of overlap between grey seals and coastal fishing activities during the winter may increase (Meier et al., 2004), as the grey seals can be expected to stay closer to the coasts if the fast ice diminishes in extent and duration, as they generally avoid areas with fast ice (Hook & Johnels, 1972). The decreasing extent of the sea ice may also bring the ringed seals closer to the shores, as they haul out, moult and breed on the ice.

Just as climate change will probably have globally and regionally variable effects on seal populations (Kovacs et al., 2011, 2012; Laidre et al., 2015), the Baltic populations may respond differently. Grey seals breed on drifting ice floes or else on land (Hook & Johnels, 1972; Jüssi, 2012). Although their breeding success is lower on land (Jüssi et al., 2008), it is likely that it may not be affected as much as the breeding of ringed seals. It may be possible for the grey seals to expand their winter distribution northwards as the extent of the ice cover decreases, whereas the overall distribution of ringed seals is threatened by the warming climate (Kovacs et al., 2011, 2012; Sundqvist et al., 2012). The importance of the Bothnian Bay as the main distribution and breeding area for the Baltic ringed seal may be emphasized in the future, as the warming climate reduces the ice cover, thereby detracting from the breeding success of the more southerly subpopulations (Meier et al., 2004; Sundqvist et al., 2012).

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3.5.4 Development of fishing practises The present results indicate that by-catch in fyke nets specifically increases juvenile mortality (III). Therefore, it may not reduce population growth and viability as much as a similar increase in adult – especially female – mortality would be likely to do (Harding et al., 2007). Nevertheless, in addition to hunting mortality, by-catch mortality constitutes another significant anthropogenic cause of mortality for the seal populations. The hunting pressure in the northern Baltic Sea has been some 400–600 seals a year in recent times (Moraeus et al., 2014; Finnish Wildlife Agency ,2015; Ålands Landskapsregering, 2015) while annual by-catch mortality in grey seals is approximately 2 000 seals (Vanhatalo et al., 2014), indicating that drowning in fishing gear is the most common human-induced mortality factor for this species. Mortality due to by-catch is unknown for the Baltic ringed seals; however, quite a considerable number was bycaught during this study (III). Mortality due to by-catch is likely to vary substantially between regions and types of fishing gear. Information on by-catch will be essential for sustainable management of these seal populations, and therefore further research into the volume and composition of by-catches should be encouraged.

Seal socks can be used for reducing by-catch mortality, especially that of ringed seals in coastal fyke nets. Reducing by-catch mortality may become an increasingly important issue for population management, as population growth of this ice-dependent species is predicted to decrease due to climate change (Meier et al., 2004; Sundqvist et al., 2012). Reducing incidental mortality is also becoming an increasingly important consideration in fisheries management in order to respond to growing demands for sustainable seafood. For example, LIFE (Low impact and Fuel-Efficient) fishing is aimed at reducing the impacts of fishing on the ecosystem and the Marine Stewardship Council (MSC) is granting ecolabels to fishing companies that follow their sustainability standards (Suuronen et al., 2012; Kirby & Ward, 2014).

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In addition to reducing by-catches, the seal sock tested in here has potential applications for capturing seals alive. Like the gate that restrains seals inside pontoon traps (Lehtonen & Suuronen 2010), the seal sock also allows quick, ethically acceptable removal of individuals that repeatedly visit fishing gear. In general, the development of items such as pontoon traps and cod pots has been helpful in reducing grey seal-induced losses and incidental by-catching (Suuronen et al., 2006; Hemmingsson et al., 2008; Königson et al., 2015a, 2015b), and it has also been reported that the modified fyke nets used in eel fishery have reduced damage from harbour seals (Königson et al., 2007). Little attention has been paid to the reduction of depredation by ringed seals, however, and this should be studied in detail in the future, especially as selective removal may have only a limited impact on depredation by ringed seals. Gill net fishing has already been abandoned in some parts of the Bothnian Bay due to severe depredation by ringed seals, and research into alternative forms of fishing gear and fishing practices, such as selection of the fishing grounds, can be helpful (Königson et al., 2015a, 2015b). Acoustic deterrent devices (ADD) can in some cases reduce losses caused by grey and harbour seals (Fjälling et al., 2006; Harris et al., 2014) and their effectiveness on the ringed seals should be systematically tested. The development of fishing practises will remain one key measure in reducing seal-induced losses to coastal fisheries.

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4 Conclusions

This thesis not only provides basic ecological information on the habitat use and movement patterns of the grey and ringed seals, but also points to profound differences in the effectiveness of conflict mitigation and population management measures (I, II). Although the extent of overall movement was similar in both species, their seasonal patterns of movement differed. The Baltic grey seals generally behaved like typical central-place foragers during the post-moulting season and showed much more pronounced fidelity to given foraging and haul-out sites than did the nomadic ringed seals. Individuals of both species were observed to make long migrations, often from their foraging grounds to their breeding areas further south. This observation points of a necessity for international co-operation in the management of Baltic seal populations.

The present findings suggest that the planning of management measures for grey seals may be more straightforward than for ringed seals. The strong site fidelity of grey seals with regard to coastal fishing areas indicates that selective hunting could be an effective method of reducing grey seal depredation (II). Their foraging site fidelity also suggests that, in addition to information on their haul-out sites, foraging distribution data could be used to delineate marine protected areas in order to protect their key habitats. The development of fishing gear and practises to reduce both depredation and by-catch mortality have received far less attention in the case of ringed seals than for grey seals. The present results suggest that selective removal may not be an effective method of reducing depredation by ringed seals, as no indication was found of the existence of problem individuals among the ringed seal population (II). The results also imply that the by-catch of ringed seals in coastal fyke nets affects juveniles in particular and may be significant in its extent, but the seal sock would seem to offer

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one method for reducing it (III). In addition, the planning of spatial management measures for protection of the key habitats of ringed seals may prove problematic due to their low haul-out and foraging site fidelity by comparison with grey seals. The foraging areas of the ringed seals were clustered to some extent, however, and the directing of conservation measures towards these “hot spots” could help in creating refuges where exposure to anthropogenic influences is limited.

Climate change may have more negative effects on the ringed seal population than on that of the grey seal, as ringed seals are in general more dependent on ice conditions. Climate change may also further intensify the conflict between seals and coastal fisheries, as a decrease in the extent of the ice cover could bring the seals closer to the shore in winter and spring. Measures to safeguard the coexistence of seal populations and the coastal fishing will therefore remain essential in the future as well.

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60

61

6 Appendices

App

endi

x 1.

Det

ails

of t

he B

altic

seal

s equ

ippe

d w

ith G

PS p

hone

tags

. Cap

ture

site

: BS

= th

e Bot

hnia

n Se

a,

GF

= th

e Gul

f of F

inla

nd, B

B =

the B

othn

ian

Bay.

a : ju

veni

le a

ccor

ding

to b

ody

weig

ht. *

: In

divi

dual

s tag

ged

addi

tiona

lly w

ith S

POT5

flip

per t

ags.

Valu

es in

the b

rack

ets d

escr

ibe t

he d

ate o

f las

t loc

atio

n an

d nu

mbe

r of

loca

tions

obt

aine

d w

ith fl

ippe

r tag

s. N

o.lo

cati

ons

907

201

6 1346

167

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2607

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4223

5683

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cati

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l

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sea

l

grey

sea

l

grey

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l

grey

sea

l

grey

sea

l

grey

sea

l

61

6 Appendices A

ppen

dix

1. D

etai

ls o

f the

Bal

tic se

als e

quip

ped

with

GPS

pho

ne ta

gs. C

aptu

re si

te: B

S =

the B

othn

ian

Sea,

G

F =

the G

ulf o

f Fin

land

, BB

= th

e Bot

hnia

n Ba

y. a :

juve

nile

acc

ordi

ng to

bod

y w

eight

. * :

Indi

vidu

als t

agge

d ad

ditio

nally

with

SPO

T5 fl

ippe

r tag

s. Va

lues

in th

e bra

cket

s des

crib

e the

dat

e of l

ast l

ocat

ion

and

num

ber o

f lo

catio

ns o

btai

ned

with

flip

per t

ags.

No.

loca

tion

s

907

201

6 1346

167

2757

2607

104

92 796

229

0 2391

4223

5683

Du

r.

(d)

119

94 76 105

37 230

211

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80 0 124

140

162

Last

GP

S lo

cati

on

03.0

2.09

18.0

2.09

26.0

1.09

17.1

2.09

18.1

0.09

08.0

5.10

30.0

4.10

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0.09

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1.09

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2.10

12.0

2.10

02.1

2.09

02.1

1.10

14.0

3.11

24.0

4.11

Cap

ture

dat

e

07.1

0.08

16.1

1.08

11.1

1.08

03.0

9.09

11.0

9.09

20.0

9.09

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0.09

02.1

0.09

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7.10

25.1

0.10

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Sex

mal

e

mal

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mal

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e

mal

e

mal

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Mas

s (k

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193

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124

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Cap

ture

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oon

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Cap

ture

si

te

BS

BS

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BS

BS

BS

BS

BS

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GF

GF

Sea

l ID

BR08

AA08

PA08

AR09

SA09

HE0

9

RA09

VA09

MI0

9

KU

09

CH

09

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KA10

CR10

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0

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ecie

s

grey

sea

l

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62

App

edix

1. C

ontin

ues f

rom

pre

viou

s pag

e. N

o.

loca

tion

s

99

6030

781

479

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2013

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63

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I

Using movement data of Baltic grey seals to examine foraging-site fidelity: implications for seal-fishery

conflict mitigation

Oksanen S M, Ahola M P, Lehtonen E, Kunnasranta M (2014)

Marine Ecology Progress Series 507: 297-308

Reprinted with kind permission of Inter-Research

Reproduced under license from the copyright holder with the following limitation: The whole article is not to be further copied and distributed separately from the thesis. This limitation ends 5 years after the original publication date of the article in Marine Ecology Progress Series.

MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 507: 297–308, 2014doi: 10.3354/meps10846

Published July 17

INTRODUCTION

Many large aquatic predators that move over exten-sive spatial scales restrict their movement to smallerspecific areas during certain periods of time. Site fi-delity, the tendency of an individual to return to a cer-tain area over time, reflects areas important in satisfy-ing different life-history priorities during the annualcycle of many species. Numerous marine species suchas seabirds, large predatory fish, whales and sealsshow site fidelity to breeding and foraging grounds(Hamer et al. 2001, Bradshaw et al. 2004, Vincent et

al. 2005, Foote et al. 2010, Kelly et al. 2010, Barnett etal. 2011, Russell et al. 2013, Augé et al. 2014). Forag-ing-site fidelity can cause intense spatial overlap be-tween large aquatic predators and fisheries, and canlead to increased resource competition between thetwo (Hyrenbach & Dotson 2003, Hückstädt & Krautz2004, Karpouzi et al. 2007, Pichegru et al. 2009, Augéet al. 2014). This conflict is complex as it includes bothnegative impacts on fisheries by the predators and in-creased predator mortality due to incidental by-catchand removal of individuals as a conflict mitigationstrategy (Read 2008). In particular, there has been

© Inter-Research 2014 · www.int-res.com*Corresponding author: sari.oksanen@uef.fi

Using movement data of Baltic grey seals to examine foraging-site fidelity: implications for

seal−fishery conflict mitigation

Sari M. Oksanen1,*, Markus P. Ahola2, Esa Lehtonen3, Mervi Kunnasranta1

1Department of Biology, University of Eastern Finland, PO Box 111, 80101 Joensuu, Finland2Finnish Game and Fisheries Research Institute, Itäinen Pitkäkatu 3, 20520 Turku, Finland

3Finnish Game and Fisheries Research Institute, PO Box 2, 00791 Helsinki, Finland

ABSTRACT: Knowledge of the intensity of spatial overlap between aquatic top predators and fish-eries is required to efficiently alleviate the negative effects of marine mammal−fishery interactions.We used satellite telemetry to study the movements and habitat use of Baltic grey seals Halichoerusgrypus, with special focus on the degree of site fidelity to foraging and haul-out areas and spatio-temporal overlap with coastal fisheries. Most of the tracked seals (14/16 individuals) were ‘resi-dents’, which remained within 120 ± 62 km (mean ± SD) of their capture sites during the open-waterseason, whereas 2 ‘transient’ seals occupied much larger areas, over 400 km from their capture sites.Residents used on average 4.3 ± 2.5 haul-out sites, indicating high haul-out site fidelity during theopen-water season. Residents had active core areas (58 ± 35 km2, 50% local nearest-neighbour con-vex hull [LoCoH], excluding haul-out locations) near river estuaries or at other shallow water areas,indicating foraging-site fidelity to these foraging grounds. They overlapped both spatially and tem-porally with trap-net fishing. The high site fidelity of grey seals indicates that foraging and haul-outareas should be taken into account in population management. Selective removal of seals over -lapping with fishery could be a locally focused method to mitigate seal−fishery interactions. How-ever, removal of individuals in foraging areas may also compromise the conservation needs of thepopulation by targeting the same animals that are hauling out in seal sanctuaries.

KEY WORDS: Baltic Sea · GPS phone tag · Habitat preference · Halichoerus grypus · Homerange · Seal−fishery overlap · Trap-net fishery

Resale or republication not permitted without written consent of the publisher

Mar Ecol Prog Ser 507: 297–308, 2014

considerable debate globally over the interactions be-tween many seal species and commercial fisheries(e.g. Linnell 2011, Bowen & Lidgard 2013). As a result,seals have been historically hunted worldwide to alle-viate the conflict (Bowen & Lidgard 2013).

Studying foraging-site fidelity of pinnipeds in rela-tion to the overlap with fisheries around the worldcan help to assess actions to mitigate both sides of theseal−fishery interaction. On the one hand, marinemanagement areas have been found to be an effec-tive measure in limiting interactions between aquaticpredators and fisheries (Pichegru et al. 2010, Gorm-ley et al. 2012). However, despite the fact that manypinniped species spend the majority of their time inthe water, shaping protected areas to reduce theimpacts of human activities has mainly been focusedon terrestrial haul-out sites. Until recently, littleattention has been paid to foraging areas, which areharder to identify (Cordes et al. 2011, Augé et al.2014). For example, with the strong foraging-sitefidelity of the New Zealand sea lion Phocarctos hook-eri, protecting key foraging areas helps reduce theinteractions with fisheries such as resource competi-tion, by-catch mortality and reduction in habitatquality due to bottom trawling (Augé et al. 2014). Onthe other hand, to reduce the seal-induced damageto fisheries, spatial overlap between foraging areas ofseals and fisheries indicates that selective removal ofindividuals in the fishing areas can be a more fo -cused mitigation method than random hunting nearhaul-out sites (Graham et al. 2011). Selective removalis effective especially when individuals exhibit forag-ing-site fidelity and only a small fraction of the popu-lation is specialised and strongly overlaps with fish-eries (Graham et al. 2011).

The Baltic grey seal Halichoerus grypus is a typicalexample of a seal species that strongly interacts withcoastal fisheries. Although the present population(census size: 30 000 seals) is still less than half of itshistorical abundance at the beginning of the 20thcentury (Harding & Härkönen 1999, Kokko et al.1999, Harding et al. 2007, FGFRI 2014), the conflictbetween seals and fisheries has aggravated substan-tially in the areas of high seal abundance (Jounela etal. 2006, Bruckmeier & Larsen 2008, Varjopuro 2011).In the main distribution area of the species, i.e. Fin-land and Sweden, grey seals are largely consideredas game or even a pest species (Storm et al. 2007,Ministry of Agriculture and Forestry 2007). In con-trast, in low-abundance areas of the southern BalticSea, i.e. Poland and Germany, the grey seal is consid-ered to be a species that needs more focused conser-vation measures (Schwarz et al. 2003). In the main

distribution area, the main policy instruments forconflict mitigation are hunting, compensation pay-ments and financial support for the acquisition ofseal-safe fishing gear (Ministry of Agriculture andForestry 2007, Storm et al. 2007, Bruckmeier &Larsen 2008, Varjo puro 2011).

Considering the conflicting interests of seal conser-vation and fisheries management, information on thespatial ecology of Baltic grey seals is crucial for plan-ning conservation and management strategies (Min-istry of Agriculture and Forestry 2007). Telemetrystudies have shown that juvenile grey seals move inlarge areas (Sjöberg et al. 1995), whereas adults con-centrate their activities near haul-outs and showhabitat preference in relation to bathymetry (Sjöberg& Ball 2000). In general, research on the movementsof Baltic grey seals has mainly concentrated on youngindividuals (Sjöberg et al. 1995, Sjöberg & Ball 2000).More information on the spatial ecology of adults andtheir overlap with fisheries is needed, as studies ondiet indicate that older grey seals, in particular, feedon large and economically important fish species(Lundström et al. 2010, Kauhala et al. 2011). Seasonaland inter-annual haul-out site fidelity has beenshown for Baltic grey seals (Karlsson et al. 2005).However, there is little knowledge on foraging-site fi-delity and potential overlap with coastal fisheries.

Studying the spatial ecology of free-ranging sealsallows the level of interaction between seals andcommercial coastal fisheries to be estimated, which isessential for effective seal and fishery managementmeasures. The magnitude of the interactions de -pends partly on the degree of spatial and temporalresource overlap between seals and the fisheries(Matthiopoulos et al. 2008). In addition, an improvedunderstanding of spatial ecology is critical in deter-mining the ecological factors affecting the conserva-tion needs of seal populations. Therefore, in thisstudy, we examined the movements and habitat useof Baltic grey seals with special focus on the degreeof foraging and haul-out site fidelity. In addition, westudied the spatial and temporal overlap of grey sealsand coastal fisheries on several spatial scales.

MATERIALS AND METHODS

Animal handling and data recording

From 2008 to 2011, grey seals were captured withcommercial pontoon traps (Hemmingsson et al. 2008)in the Bothnian Sea and the Gulf of Finland, whichare both important areas for commercial coastal fish-

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eries (see Fig. 1, see Table 1). The pontoon traps(3−4 traps annually) were equipped with ‘non-return’gates attached in front of the fish chamber betweenJune and November (see details in Lehtonen &Suuronen 2010). An alarm system sent a GSM(mobile phone) message when a seal was caught,guaranteeing a quick removal. After capture, theseals were guided to swim into a shuttle attached tothe pontoon trap by sinking the fish chamber underthe water. The seals were then transported to land inthe shuttle and sedated using medetomidine (0.02−0.04 mg kg−1, Zalopine®) and butorphanol (0.08 mgkg−1, Butador®) by a veterinarian. The sedation wasreversed with atipamezole (0.2 mg kg−1, Anti-sedan®). A different anaesthetising protocol, mede -to midine and butorphanol combined with midazolam(Dor micum®) or ketamine (Ketalar®), had been usedat the beginning of the study, but this protocol wasabandoned due to the death of 2 individuals.

GPS phone tags (Sea Mammal Research Unit, Uni-versity of St. Andrews, UK) were attached to the dor-sal fur above the scapulas with 2-component epoxyglue (Super Epoxy, 15 min). The phone tags wereprogrammed to attempt to determine the GPS loca-tion 2−3 times per hour (see Table 1). To ensure lateridentification, a uniquely numbered plastic ID-tag(Jumbo tag, Dalton) was attached to the hind flipperand a plastic ‘hat’ (Kuggom Metal, Finland) wasglued to the fur on the top of the head. Individualsthat were small (≤70 kg) or in poor condition wereequipped with only a plastic ID-tag and releasedwithout sedation. Sex and body weight were re -corded for all captured seals. All captured seals weremales and they were classified as adults or juvenilesaccording to body weight on the basis of age−weightdata (database of grey seals, Finnish Game and Fish-eries Research Institute). Males with body weight<90 kg were categorised as juveniles (estimated age:<5 yr) and others (with body weight ≥90 kg) as adults.The research was permitted by the local gameauthorities (permit no. 2011/00087) and the AnimalExperiment Board of Finland (permit nos. ESLH-2008-04828/YM-23 and ESAVI-2010-05432/ Ym-23).

Temporal and behavioural classification of movement data

Grey seals are known to avoid areas with fast ice(Hook & Johnels 1972), and this phenomenon islikely to create differences in habitat usage and pref-erence between open-water and ice-cover seasons.Also, coastal fishing with passive gear occurs mostly

during the open-water season (highest effortbetween May and October; FGFRI 2009, 2010, 2011,2012). Therefore, tracking data were divided intoopen-water and ice-cover seasons. The open-waterseason designation started when the tags weredeployed (see Table 1) and ended with the seals’transition period (usually in mid-December). Thetransition period started when the seals left theiropen-water home ranges without permanently re -turning to this area during the study period andended when the locations were again clustered inlate December− early January. The transition periodwas left out of the spatial analyses, as migrations arenot typically included in home range estimates (Ken-ward 2001). The ice-cover season started after thetransition and ended when the tags stopped trans-mitting (see Table 1). Four studied seals did not makea transition, and the data were divided into open-water and ice-cover seasons by the average date ofthe beginning of transition obtained from other indi-viduals (20 December). This coincided well with theaverage date (22 December) when the first ice-coverappeared in satellite pictures of the study areas (FMI2014).

Location data on the seals were first filtered ac -cording to McConnell et al. (1992) and then dividedinto 2 behavioural classes: haul-out and active. Ahaul-out event began when the tag was continuouslydry for 10 min and ended when wet for 40 s. Activelocations (i.e. movements) were categorised as alllocations when a seal was outside a 1 km threshold,based on the Euclidean distance from the haul-outsite (Cronin et al. 2012).

Identification of home ranges and core areas

Individual total home ranges (for the whole track-ing period) and seasonal home ranges (for open-water and ice-cover seasons) were estimated forseals with a tracking of over 30 d using the 95% minimum convex polygon (MCP; Worton 1987) and95% adaptive local nearest-neighbour convex hull(a-LoCoH; Getz et al. 2007) methods. Main foragingareas for individuals were determined by construct-ing active core areas (50% LoCoH of the active loca-tions). In a-LoCoH, the parameter a was set by takingthe maximum distance between any 2 locations inthe individual data set (Getz et al. 2007). The discov-ery curves from the 95% MCP and 100% LoCoHincremental analysis (Kenward 2001) were visuallyexamined to ensure that the location data was suffi-cient for home range analysis.

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Individual home range sizes between open-waterand ice-cover seasons were compared using theWilcoxon signed rank test in SigmaPlot for Windows12.3 (Systat Software). The spatial overlap percentbetween seasonal home ranges was calculatedaccording to Bernstein et al. (2007). In addition toindividual home ranges, active group home ranges(all active locations) during the open-water seasonwere estimated for all seals using 95% a-LoCoH. Allhome ranges were constructed using the packageadehabitatHR (Calenge 2006) in R 2.15.3 (R Develop-ment Core Team 2013). Land areas were subtractedfrom the MCP home range estimates.

Overlap with fisheries

Habitat preference of grey seals was investigatedduring the open-water season, when the greatestoverlap with coastal fisheries is expected. Manly’sselection ratios (± Bonferroni adjusted confidencelimits) were calculated according to Manly et al.(2002) using the adehabitatHS package (Calenge2006) in R. Selection ratios were calculated for eachseal by relating areas used (i.e. 95% LoCoH homerange) to the available study area in 7 water-depthclasses (0−10, 10−20, 20−30, 30−40, 40−50, 50−100,100−200 m) defined from bathymetric raster maps(grid size: 250 × 250 m, Helsinki Commission HEL-COM). The study area was defined separately for theBothnian Sea and the Gulf of Finland by creatingMCPs (95%) around all the locations of the seals cap-tured in each area. Manly’s selection ratios (Wij) indi-cate preference when Wij > 1 and avoidance whenWij < 1. Preference for specific habitat (depth class)was considered statistically significant when the con-fidence interval for the selection ratios was >1 andavoidance when the value was <1 (Manly et al.2002).

We used 2 different fishery data sets to examinethe spatial and temporal overlap between fisheriesand tracked seals. First, catch report data basedon EU fishing logbooks and coastal fishery formsfrom Finnish marine waters during 2008−2011(FGFRI 2009, 2010, 2011, 2012) were used to examinethe overlap be tween grey-seal tracking data, trap-net fishing effort and seal-induced damages. Thedata represent monthly trap-net fishing effort, over-all catch and seal-induced catch losses of commercialfisheries in ICES statistical rectangles (50 × 50 kmgrids; ICES 2014). Second, a spatially limited, butmore fine-scaled data set of trap-nets (location, typeand fishing season of each trap-net) was used to

examine spatial overlap of tracked grey seals andtrap-nets on a weekly basis during 2010 and 2011.The trap-net location data were collected by inter-viewing commercial fishermen from areas overlap-ping with tracked seals in the Gulf of Finland. Thetrap-net location data set was limited to Finnish mar-ine waters in the east and south, while the westernlimit was set at longitude 25° 35’ E (in the WGS84coordinate system) (see Fig. 3B). In order to quantifythe fine-scaled spatio-temporal overlap betweentracked seals and trap-net fisheries, the percent oftracking days when each seal was within close proxi -mity (<250 m) to trap-nets was calculated.

RESULTS

Grey-seal movements

All 28 grey seals captured with modified pontoontraps were males and most of them were adults(82%). The mean weight of the captured seals was133 ± 36 kg (mean ± SD; range: 70−193 kg). All cap-tured seals were marked with plastic ID tags and 21of them were equipped with GPS phone tags. Four ofthe marked seals (3 with a GPS phone tag and 1 withan ID tag) were recaptured 1−3 times after initialrelease later in the year (Table 1). Overall, 16 tagsprovided data for at least 20 d (on average for 136 ±69 d) and >50 locations, and these data sets wereincluded in the analyses (Table 1). The average num-ber of locations per day was 13 ± 9 per individual.The mean number of received locations per individ-ual differed between study areas (1126 ± 1045 in theBothnian Sea and 2779 ± 2236 in the Gulf of Finland).The results of the 95% MCP and 100% LoCoH incre-mental analysis reflected the small sample sizes inthe Bothnian Sea, as the discovery curves for 4 indi-viduals (AA08, SA09, CH09, KU09) did not reach theslow increase phase. In addition, this could be partlydue to the less concentrated movements of juveniles(CH09 and KU09).

The total home range estimates varied consider-ably between individuals (33 675 ± 38 936 km2 withMCP and 6858 ± 7936 with LoCoH; Table 2). Most ofthe studied seals (14/16 individuals) were cate-gorised as ‘residents’, i.e. they remained within a dis-tance of 120 ± 62 km from the capture site during theopen-water season (Figs. 1 & 2). The mean size of theindividual home range of residents during the open-water season was 4443 ± 6941 km2 with the MCPestimator and 886 ± 764 km2 with the LoCoH estima-tor (Table 2). Two seals (HE09 and VO11) were ‘tran-

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sients’ and were mainly located outside Finnishcoastal waters, within a distance of 402 and 818 kmfrom the capture site, respectively. They occupiedMCP home ranges of 50 142 km2 and 132 761 km2

and LoCoH home ranges of 1284 km2 and 15 847km2, respectively (Table 2). During the open-waterseason, most of the residents (11/14 individuals) hadactive core areas (58 ± 35 km2) near river estuaries ata mean distance of 17 ± 9 km from their capture sites(Fig. 1C,D). For 3 residents (adult AA08 and juvenilesCH09 and KU09), the active core areas could not bereliably estimated with the given set of locations andlack of concentrated movements.

In general, home ranges for the open-water andice-cover periods (LoCoH 95%) did not differ in size(Wilcoxon signed rank test, Z = 3.54 × 10−316, p =1.00). Most resident seals that were tracked duringboth seasons (7/9 individuals) had separate open-water and ice-cover home ranges (overlap betweenhome ranges: 0−1%; Table 2). These seals made aclear transition to ice-cover home ranges between 10December and 4 January. The mean duration of thetransition was 5 ± 2 d and the length varied from107−492 km. However, the 2 residents (BR09, AA08)that did not make a transition occupied approxi-mately the same areas during the whole study period

(overlap ca. 20%; Table 2). The transients did nothave a clear transition period. One transient seal(HE09) mainly occupied the same area in the easternBaltic Proper as during the open-water season. Theother transient (VO11) continued to move in largeareas. Home ranges during the ice-cover period weremostly situated in the northern and eastern BalticProper or in the Gulf of Riga.

Resident seals made successive trips from activecore areas to haul-out sites during the open-waterseason (Figs. 1 & 2). On average, residents from theGulf of Finland and the Bothnian Sea used haul-outsites 64 ± 33 km from their capture sites (Fig. 2). Res-idents used on average 4.3 ± 2.5 haul-out sites duringthe open-water season. In the Gulf of Finland, 39% ofthe total (299 recorded events) haul-out events of res-idents were located in a seal sanctuary (5/7 individu-als used this haul-out site) and 56% in the haul-outsite (Hallikarti; easternmost haul-out in Fig. 1D) onthe border area between Finland and Russia (4/7 in -dividuals used this site) during the open-water sea-son. Only a small fraction (5%) of haul-out events oc -curred at other haul-out sites. In the Bothnian Sea,8% of haul-out events of residents were located in aseal sanctuary (4/7 individuals) and 31% in knownhaul-out sites in the southern Bothnian Sea (6/7 indi-

301

Seal ID Capture Body weight GPS No. of Tracking period duration (d) Start Endsite (kg) attempts locations Open-water Ice-cover Total

h−1 season season

BR08 BS 193 2 907 73 45 119 07/10/08 03/02/09AA08 BS 98 2 201 33 60 94 16/11/08 18/02/09PA08 BS 177 2 6 nd nd 76 11/11/08 26/01/09AR09 BS 147 2 1346 105 nd 105 03/09/09 17/12/09SA09 (3) BS 171 2 167 37 nd 37 11/09/09 18/10/09HE09 BS 121 2 2757 93 139 230 20/09/09 08/05/10RA09 BS 124 2 2607 81 123 211 01/10/09 30/04/10VA09 (1) BS 147 2 104 17 nd 17 02/10/09 19/10/09MI09 BS 188 2 92 13 nd 13 26/10/09 08/11/09KU09 BS 77a 2 796 57 43 101 06/11/09 15/02/10CH09 BS 76a 3 229 30 42 80 24/11/09 12/02/10LA09 BS 111 3 0 nd nd 0 02/12/09 02/12/09KA10 GF 125 3 2391 124 nd 124 01/07/10 02/11/10CR10 (2) GF 88a 3 4223 51 84 140 25/10/10 14/03/11OT10 GF 123 3 5683 39 118 162 13/11/10 24/04/11ST11 GF 133 3 99 7 nd 7 05/06/11 12/06/11AH11 GF 113 3 6030 172 125 301 22/06/11 18/04/12AR11 GF 156 3 781 81 nd 81 07/08/11 27/10/11TY11 GF 173 3 479 51 nd 51 24/08/11 14/10/11BE11 GF 121 3 979 99 51 158 30/08/11 04/02/12VO11 GF 128 3 1662 94 81 176 16/09/11 10/03/12

aJuvenile according to weight

Table 1. Details of male Baltic grey seals Halichoerus grypus equipped with GPS phone tags. Parentheses: number of recap-tures (in addition, 1 flipper-tagged male without a GPS phone tag was recaptured 2 times). Dates are dd/mm/yy. BS: Bothnian

Sea, GF: Gulf of Finland, nd: no data available

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viduals) (Fig. 1C). However, 54% of haul-out eventswere near (within 20 km) the active core areas.

Spatial and temporal overlap with fisheries

In the study area, both trap-net fisheries and resi-dent seals preferred coastal waters during the open-water season (Fig. 3). Trap-net fishing effort wasmainly situated in shallow water areas, and the meandepth of trap-net locations in the Gulf of Finland was8.5 ± 7.6 m (Fig. 3). Resident seals also showed habi-tat preference for shallow water in their home ranges(Fig. 4). Manly’s selection ratios showed that resi-dents generally preferred areas with depths < 30 mand avoided areas > 50 m deep, although for theBothnian Sea the selection for the 10−20 m depthclass was not statistically significant. In contrast,

transients did not show as clear a preference for shal-low waters (Fig. 4).

The active core areas of residents overlapped withtrap-net fishing areas where fishing effort was higherthan the median (1710 trap-net days; Fig. 3A). Theyalso overlapped with the greatest annual seal-induced catch losses (>6.5 tons, i.e. >50% of themaximum value) during the open-water season(Fig. 3A). The resident seals in the Gulf of Finland(n = 7) visited within close proximity of trap-nets(≤250 m) on average 30 ± 20% of all tracking daysduring open-water season. All of the most importantprey categories of grey seals, Baltic herring Clupeaharengus membras, whitefish Coregonus lavaretus,Salmo sp. and cypri nids (Lundström et al. 2010,Kauhala et al. 2011, Suuronen & Lehtonen 2012),were present in the catches of overlapping fishingareas.

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Seal ID Open-water season Ice-cover season Total tracking period OverlapMCP LoCoH LoCoH Distance MCP LoCoH LoCoH Distance MCP LoCoH (%)(95%) (95%) (50%) (km) (95%) (95%) (50%) (km) (95%) (95%)

ResidentsBR08 2040 1296 56 26 849 378 16 28 2039 1340 21AA08 2470 605 na nd 388 205 47 23 2031 605 19AR09 26878 3012 65 5 nd nd nd nd 26878 3012 ndSA09 2760 629 99 12 nd nd nd nd 2760 629 ndRA09 2600 161 4 10 42737 10203 2503 (3) 346 61476 10349 0KU09 8264 1195 na nd 1454 733 151 (2) 213 18679 3291 0CH09 8567 1806 na nd 13662 3897 536 336 33076 7632 0KA10 1026 556 95 (2) 28 nd nd nd nd 1026 556 ndCR10 1148 639 64 29 17663 8000 2328 (3) 420 50718 13433 0OT10 1795 686 106 (3) 25 40360 12010 3167 (6) 370 75414 19455 0AH11 798 271 13 (2) 10 48152 12414 1079 (3) 178 43943 9861 1AR11 2620 936 72 4 nd nd nd nd 2620 936 ndTY11 811 432 36 24 nd nd nd nd 811 432 ndBE11 425 187 27 18 328 172 57 (2) 345 14052 759 0

Mean 4443 886 58 17 18399 5335 1098 251 23966 5164 4SD 6941 764 35 9 20090 5318 1326 149 25292 6030 9Median 2255 634 64 18 13662 3897 536 336 16365 2176 0

TransientsHE09 50142 1284 na nd 22568 6698 535 (2) 416 57668 10050 3VO11 132761 15847 5269 (2) 679 73736 9861 1021 (2) 106 145606 27391 3

Mean 91452 8566 – – 48152 8279 778 261 101637 18720 –SD 58420 10298 – – 36181 2237 344 219 62182 12262 –

TotalMean 15319 1846 492 72 23809 5870 1040 253 33675 6858 4SD 33949 3802 1505 191 24467 4954 1124 150 38936 7936 8Median 2535 662 64 21 17663 6698 536 336 22779 3152 0

Table 2. Estimated sizes (km2) for total and seasonal home ranges (95%) and active core areas (50%) of Baltic grey seals Hali-choerus grypus, distances from the capture site to core areas, and overlap between seasonal home ranges. Parentheses: num-ber of core areas if >1. Residents: occupied areas near the capture site during open-water season; transients: left the capturearea; MCP: minimum convex polygon; LoCoH: local nearest-neighbour convex hull; na: not applicable, as the LoCoH 50%could not be reliably estimated with the combination of the chosen a-parameter and given amount of locations; nd: no data

available

Oksanen et al: Site fidelity of Baltic grey seals

Seal tracking and trap-net fishing effort also over-lapped temporally during the open-water season.However, fishing effort in the overlapping statisticalrectangles was highest in June (mean: 6037 ± 3336trap-net days), declining towards autumn, whereasthe grey seals were mostly captured between Octo-ber and November (75% of individuals). The overallfisheries catches in the overlapping statistical rec -tangles had 2 peaks, one in May−June and the otherin September−October. Residents usually left theiropen-water season foraging area in December(range between 10 December and 2 January), whiletrap-net fishing effort in the overlapping statisticalrectangles declined after October to <20% of the

maximum effort in June (1077 ± 489trap-net days), and remained at thislevel for the winter months (untilApril, range: 378−1286 trap-net days).

DISCUSSION

Our study confirms that, while greyseals are capable of travelling longdistances (Sjöberg et al. 1995, Mc -Connell et al. 1999), they also oftenconcentrate their movements in rela-tively small areas near haul-out sitesfor extended periods (McConnell et al.1999, Sjöberg & Ball 2000, Austin et al.2004). In our study, most of the trackedseals were residents (88%), stayingnear their capture sites on relativelysmall home ranges during the open-water season. Only 2 seals left thearea of capture during the open-waterseason and were therefore catego -rised as transients. The open-waterhome ranges of resident seals(4443 km2) were similar to summerhome ranges previously reported inthe Baltic Sea (6293 km2; Sjöberg &Ball 2000) and in the western Atlantic(8900 km2; Harvey et al. 2008). Tran-sients had larger MCP home ranges(>50 000 km2). On average, all homeranges for the total tracking periodwere large (33 675 km2; 95% MCP).All our study seals were males, andthis could have had an effect on thisresult, as males often move over largerareas than females (Austin et al. 2004,Breed et al. 2009). Home-range esti-

mates also typically increase with increasing track-ing period and sample size (Kenward 2001). Al -though residents remained on small home rangesduring the open-water season, most of them alsomade a 100−500 km long transition to the winteringareas in the Baltic Proper or the Gulf of Riga, whichinclude drift-ice breeding areas. Most seals also vis-ited the land breeding areas in the southern Archi-pelago Sea of Finland and coastal areas of Saaremaain Estonia (Jüssi et al. 2008). In addition to the longtracking period, we used GPS phone tags that oftenprovide larger sample sizes compared to the Argossystem (Vincent et al. 2010). Although a few individ-uals of our data set had relatively few locations, the

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Fig. 1. (A) Baltic Sea study area. (B) Active group home ranges (95% adaptivelocal nearest-neighbour convex hull [a-LoCoH]) of grey seals Halichoerusgrypus captured in the Bothnian Sea (BS; n = 8) and Gulf of Finland (GF; n = 8)during the open-water season. The parts of the home ranges occupied bytransient seals (n = 2) are indicated with fill colour and seal ID. (C,D) Individ-ual active core areas of residents (50% a-LoCoH) and tracks (grey lines) in theBothnian Sea (n = 4) and the Gulf of Finland (n = 7). Core areas of 3 individu-

als (AA08, KU09, CH09) could not be reliably estimated

Mar Ecol Prog Ser 507: 297–308, 2014

overall sampling frequency (13 locations per day)was higher than in several earlier grey-seal studiesusing the Argos system (2.1 locations per day inSjöberg & Ball 2000, 3.9 locations in Austin et al. 2004and 6.6 locations in Vincent et al. 2005).

Most adult grey seals showed strong foraging andhaul-out site fidelity during the open-water season,as has also been shown in earlier studies (Karlsson etal. 2005, Vincent et al. 2005, Russell et al. 2013). Wedetermined the foraging areas with active locationpoints (locations at or near haul-out sites were ex -cluded), although these locations contained seal be -haviour other than foraging (i.e. transits, socialisingand resting). However, foraging constitutes about50% of the total time budget of seals (McClintock etal. 2013) and it is likely to dominate at-sea behaviour.We therefore assumed that the active core areasreflect the important foraging grounds. Residentseals preferred the vicinity of river estuaries andother shallow-water areas for foraging. They also

made repeated trips between the inshore foragingareas and off-shore haul-out sites, which were smallislets approximately 15−50 km from the mainland.Haul-out site fidelity of residents was to the generalarea rather than 1 specific haul-out site, as has beenobserved before in the Baltic Sea (Karlsson et al.2005). The typical foraging areas found in our studywere similar to those seen with several other sealspecies, which generally make foraging trips to rela-tively shallow areas on the continental shelves thatare often associated with sediment type or slope(McConnell et al. 1999, Breed et al. 2006, Aarts et al.2008, Heerah et al. 2013, Muelbert et al. 2013) orproximity to river estuaries, for example (Wright et al.2010, Graham et al. 2011, Bajzak et al. 2013).

Resident grey seals showed a preference for shal-low (<30 m deep) coastal areas and avoided deeper(>50 m) areas during the open-water season, whichis relatively similar to the earlier findings for Balticgrey seals (Sjöberg & Ball 2000). The preference for

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Fig. 2. Distances between capture sites and GPS locations of Baltic grey seals Halichoerus grypus during the open-water sea-son, for resident seals occupying (A) the Bothnian Sea and (B) Gulf of Finland, and (C) transient seals occupying other areas

(note different scale or the y-axis). Solid lines: distance to active locations, dots: distance to haul-out locations

Oksanen et al: Site fidelity of Baltic grey seals

shallow water areas leads to an inevitable overlapwith coastal fisheries. However, scale is a critical fac-tor in determining spatial overlap between top pred-ators and fisheries, and results may differ when a dif-ferent scale is chosen (Reid et al. 2004, Pichegru et al.2009). The residents of our study overlapped bothtemporally and spatially with commercial trap-netfisheries on many spatial scales. On a relativelycoarse spatial scale (50 × 50 km), resident sealsshowed foraging-site fidelity to their capture areasthat had a high fishing effort with trap-nets and highseal-induced damage. Also, on a finer scale (accu-racy likely <200 m), residents visited the vicinity

(≤250 m) of trap-nets on 30% of tracking days, indi-cating strong spatial overlap. Although the coexis-tence of seals with fisheries does not necessarily indi-cate depredation, the seals whose foraging areasoverlap with coastal fishing areas might share thesame resources as the fisheries, causing both directand indirect impacts to the fisheries. The seals andfisheries potentially use the same resources since themost important prey species of grey seals (Baltic her-ring, whitefish, Salmo sp. and cyprinids; Lundströmet al. 2010, Kauhala et al. 2011, Suuronen & Lehtonen2012) were also present in fisheries catches in over-lapping areas.

The temporal scope of our movement data ismainly within the open-water season when the pas-sive gear effort is also the highest. Although the trap-nets for capturing seals were set out in June, most ofthe seals were captured in autumn (from Septemberonwards). Our results coincided with previous find-ings that seal-induced damages tend to increase dur-ing autumn in the Baltic Sea (Fjälling 2005, Fjällinget al. 2006). In our study, most trap-net fishing ceasedafter October due to harsh weather conditions, whilethe grey seals left the area in December. This couldsuggest that the seals continue foraging even afterthe trap-net fishing season is over. Many residentsforaged close to half of the year without substantiallyoverlapping with the same fishing areas as duringthe open-water season, due to the low overlap be -tween seasonal home ranges and the decline of trap-net fishing after October. However, the degree ofoverlap between grey seals and coastal fisheries dur-ing the winter may increase due to climate change(Meier et al. 2004). Grey seals generally prefer open-water areas as they do not keep breathing holes inthe ice (Hook & Johnels 1972). Therefore, grey sealscan be expected to stay closer to the coasts if theperiod and extent of fast-ice declines.

The grey seals captured with pontoon traps wereall males and most of them were adults. This, com-bined with foraging-site fidelity and spatio-temporaloverlap with trap-net fisheries, indicates a high levelof interaction between grey-seal males and fisheries.Previous studies also suggest that males might causemore direct catch losses to the fishery than females,as males visit inside the pontoon traps more often(Lehtonen & Suuronen 2010, Königson et al. 2013),and the sex ratio of seals shot near fishing gear isbiased towards males (K. Kauhala et al. unpubl.). Inaddition, while Baltic herring is the most importantprey species for both sexes and all age groups, sex-related differences in diet have been reported. Inparticular, adult males also feed on bigger fish, such

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Fig. 3. (A) Spatial distribution of the active core areas of res-ident seals Halichoerus grypus (n = 11), the mean trap-netfishing effort and the most seal-induced damage (black rect -angles, >6.5 tons) along the Finnish coast in 2008−2011. (B)Movements (n = 7) of the resident seals and trap-net loca-tions in 2010−2011 during the open-water season. The ex-tent of trap-net location data is indicated with a dashed line

Mar Ecol Prog Ser 507: 297–308, 2014

as Salmo sp. (Lundström et al. 2010, Kauhala et al.2011, Suuronen & Lehtonen 2012). These results givean indication that some males might be so-calledproblem individuals (sensu Linnell et al. 1999), whichare responsible for a disproportional impact on trap-net fisheries. Hunting (annual quota: approx. 1730seals) has become a key policy instrument for themitigation of seal-induced losses to coastal fisheriesin the northern Baltic Sea (Ministry of Agricultureand Forestry 2007, Bruckmeier & Larsen 2008, Var-jopuro 2011, Kauhala et al. 2012, Naturvårdsverket2013). However, most of the hunting takes place onspring ice (Kauhala et al. 2012) and is not targeted atindividuals that strongly overlap with fisheries.Selective removal of individuals occurring near thefishing gear could be a more locally focused conflictmitigation method. In the future, comparative tele -metry data from potential problem seals and non-problem seals would further our understanding onthe seal−fishery conflict. Also, more fine-scaled ana -lyses on foraging behaviour and habitat preferenceduring foraging, based on diving behaviour, forexample, are encouraged.

Haul-out site fidelity (Karlsson et al. 2005, Vincentet al. 2005, present study) combined with foraging-site fidelity (Russell et al. 2013, present study) illus-trates the connectivity between these 2 areas in thecontext of conservation needs. In our study, 64% ofresident seals used seal sanctuaries for hauling outbut the same individuals also strongly overlappedwith coastal fisheries, which is likely to increase theirmortality risk (by mitigation actions or incidental by-

catch). Our study, therefore, confirms theimportance for the recognition of foragingareas when planning conservation meas-ures, which has also been previously sug-gested (Augé et al. 2014). Additionally, de -tailed knowledge on seasonal movementpatterns is important in planning conserva-tion measures. For example, in our study,due to extensive movements of grey sealsespecially after the open-water season, localmitigation actions may actually target someindividuals that breed in the southern BalticSea where the local abundance of grey sealsis low. This in turn may compromise conser-vation goals in these areas (Schwarz et al.2003). The conflicting interests of severalstakeholders necessitates that these factorsmust be taken into account when settingmanagement policy.

Acknowledgements. Our special thanks are extended tofisher men O. Välisalo and J. Välisalo, H. Salokangas,Mikael Lindholm, Magnus Lindholm and Y. Thomasson fortheir efforts in catching the seals. We also thank the wholefieldwork team, especially veterinarian N. Aalto and T.Mäkelä for their close cooperation. Thanks are due to P.Söderkultalahti for her help with the fisheries data. Finally,we thank M. Auttila, M. Niemi, J. Syväranta, M. Valtonenand the anonymous referees for their valuable commentson the manuscript. This project was funded by the Euro-pean Fisheries Fund (EFF), the ESKO Fishery ActionGroup, Sandmans stiftelse and the Maj and Tor NesslingFoundation.

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Editorial responsibility: Scott Shaffer, San Jose, California, USA

Submitted: October 14, 2013; Accepted: May 6, 2014Proofs received from author(s): July 10, 2014

II Identifying foraging habitats of Baltic ringed seals using

movement data

Oksanen S M, Niemi M, Ahola M P, Kunnasranta M (2015)

Movement Ecology 3: 33

This article is reproduced under the terms of the Creative Commons

Attribution License (http://creativecommons.org/licenses/by/4.0/)

RESEARCH Open Access

Identifying foraging habitats of Balticringed seals using movement dataSari M. Oksanen1*, Marja Niemi1, Markus P. Ahola2 and Mervi Kunnasranta1

Abstract

Background: Identification of key foraging habitats of aquatic top predators is essential for designing effectivemanagement and conservation strategies. The Baltic ringed seal (Phoca hispida botnica) interacts with anthropogenicactivities and knowledge of its spatial ecology is needed for planning population management and mitigatinginteractions with coastal fisheries. We investigated habitat use and foraging habitats of ringed seals (n = 26)with satellite telemetry in the northern Baltic Sea during autumn, which is important time for foraging forringed seals. We used first passage time (FPT) approach to identify the areas of high residency correspondingto foraging areas.

Results: Tracked seals showed considerable movement; mean (±SD) home ranges (95 % adaptive localnearest-neighbour convex hull, a-LoCoH) were 8030 ± 4796 km2. Two seals moved randomly and foragingareas could not be identified for them. The majority (24/26) of the studied seals occupied 1–6 main foragingareas, where they spent 47 ± 22 % of their total time. Typically the foraging areas of individuals had a meandistance of 254 ± 194 km. Most of the seals (n = 17) were “long-range foragers” which occupied several spatially remoteforaging areas (mean distance 328 ± 180 km) or, in the case of two individuals, did not concentrate foraging to anyparticular area. The other seals (n = 9) were “local foragers” having only one foraging area or the mean distancebetween several areas was shorter (67 ± 26 km). Foraging areas of all seals were characterised by shallow bathymetry(median ± SD: 13 ± 49 m) and proximity to the mainland (10 ± 14 km), partly overlapping with protected areas andcoastal fisheries.

Conclusions: Our results indicate that in general the ringed seals range over large areas and concentrate feeding todifferent—often remote—areas during the open water season. Therefore, removal of individuals near the fishing gearmay not be a locally effective method to mitigate seal depredation. Overlap of foraging areas with protected areasindicate that management of key foraging and resting habitats could to some extent be implemented within theexisting network of marine protected areas.

Keywords: Baltic Sea, First passage time, GPS phone tag, Habitat use, Home range, Pusa hispida botnica, Seal-fisheryinteraction

BackgroundIdentifying areas that are important in fulfilling differentlife history priorities, such as breeding and foraging hab-itats, is often an initial step in understanding habitat useof mobile aquatic predators, and thereby in designing ef-fective management and conservation strategies [1, 2].Many seal species interact with fisheries while feeding[3–5], therefore studying foraging habitats may help to

assess actions to mitigate seal − fishery interactions [6, 7].For example, marine protected areas (MPA) targeting toconserve the important feeding grounds of mobile preda-tors have successfully mitigated negative interactions, suchas by-catch and resource competition [8, 9]. Also thenegative effects that pinnipeds can have on fisheries, suchas damaging catches and fishing gear, could be reducedwith locally focused removal when seals show strong for-aging site fidelity [3, 10].Although Arctic ringed seal (Phoca hispida) in general

inhabits remote locations and interacts relatively little withhumans, the Baltic subspecies (P. h. botnica) inhabits areas

* Correspondence: sari.oksanen@uef.fi1Department of Biology, University of Eastern Finland, PO Box 111, FI-80101Joensuu, FinlandFull list of author information is available at the end of the article

© 2015 Oksanen et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Oksanen et al. Movement Ecology (2015) 3:33 DOI 10.1186/s40462-015-0058-1

where human activities range over their entire distribution[11]. Hunting and reproductive problems due to environ-mental pollution caused the population to collapse from~ 200 000 to only about 5000 individuals during the 20thcentury [12, 13]. Due to the protection of the seals and de-crease in organochlorine concentrations [12, 14], thepopulation has now recovered to circa 13 000 seals [15],and the most recent estimates indicate even larger popula-tion (census size 17 600 seals, T. Härkönen, personal com-munication). Ringed seals, as many other phocid seals,have three key elements during their annual cycle, i.e.breeding, moulting and foraging [16]. Ringed seals givebirth, rear pups and mate during the ice-covered time andexhibit site fidelity to breeding sites [16–19]. Moultingtakes place later in spring and is characterized by extendedhaul-out periods [20–22]. Although ringed seals do not fastduring breeding or moulting, foraging is limited duringbreeding and extensive haul out [16, 23]. Open water sea-son after the moult, on the other hand, is an important for-aging period, and seals gain weight for the next winter[23–25]. While the Arctic ringed seal is considered quitenomadic during the open water season [16, 26–28], its landlocked subspecies inhabiting Lake Saimaa (P. h. saimensis)is relatively sedentary throughout the year [29, 30]. Alsothe Baltic ringed seal are suggested to be sedentary [25],but detailed studies on its spatial ecology are lacking.Approximately 75 % of the current Baltic ringed seal

population inhabits the northernmost part of the BalticSea—the Bothnian Bay [15]. Other subpopulations in thesouthern breeding areas in the Gulf of Riga and Gulf ofFinland (Fig. 1) are suggested to suffer from lack of suit-able ice cover for breeding, and the relative importance ofthe Bothnian Bay as the main distribution area is expectedto increase due to climate change [15, 31, 32]. The grow-ing numbers of ringed seals in the Bothnian Bay report-edly cause substantial catch losses to coastal fisheries andmeans to mitigate depredation, such as removal of sealsnear the fishing gear, have been proposed [33–35]. De-tailed knowledge of the spatial ecology of ringed sealsinhabiting the Bothnian Bay is therefore needed for plan-ning strategies for conservation and mitigation of seal-fishery conflict. Predators concentrate foraging effort inareas with the highest probability of capturing prey [36].Therefore, identifying high residency areas of seals allowidentification of key foraging habitats and thereby estimat-ing the degree of spatial overlap between seals and coastalfisheries. In this study, we examined the habitat use of theBaltic ringed seal in the Bothnian Bay with a special focuson identifying important foraging habitats.

MethodsStudy areaThe Baltic Sea (surface area 400 000 km2) is a semi-enclosed brackish water system consisting of several

basins (Fig. 1) and characterised by shallow bathymetry(mean depth 54 m and maximum depth 459 m) [37].The study was mainly conducted in the Gulf of Bothnia(surface area 115 500 km2), which comprises the Both-nian Bay, the Quark and the Bothnian Sea (Fig. 1). Themean depth of the Gulf of Bothnia is 55 m and max-imum 293 m [37].

Animal handling and data collectionRinged seals were captured during autumn in 2011–2013from important coastal fishing areas in the Bothnian Bay(Fig. 1). Fyke nets (n = 4) were equipped with “seal socks”allowing the seals to access the surface to breathe [38] andwere set for fishing by commercial fishermen from May toOctober-November. In addition, floating seal nets (meshsize 180 mm, height 4 m, length 80 m, net material 0.7monofil, Hvalpsund net A/S) were used for capturing sealsduring October and November. The seal nets were usuallyanchored from both ends in areas with water depthof 5–8 m.Seals were manually restrained, while GPS phone tags

(Sea Mammal Research Unit, University of St Andrews,UK) were attached to the dorsal fur above the scapulaswith two-component epoxy glue (Loctite Power Epoxy,5 min). Only seals weighing ≥ 40 kg received tags. Toensure later identification, a uniquely numbered plasticID-tag (Jumbo tag, Dalton, UK) was attached to the hindflipper. Sex, weight, girth, and length were recorded andindividuals were divided into two age classes (juvenilesand adults) according to the weight on the basis of age-weight database (Natural Resources Institute Finland).Seals with body weight over 50 kg were classified asadults (estimated age ≥ 4 years). Capturing and taggingprotocol was approved by the Finnish Wildlife Agency(permit no. 2011/00082 and 2013/00197) and theAnimal Experiment Board of Finland (no. ESAVI/1114/04.10.03/2011). All efforts were made to minimize thehandling times and thereby the stress of the studyanimals.The phone tags were programmed to attempt GPS

location 2 to 3 times per hour. Tags separated be-tween at-sea locations and haul out locations and ahaul-out event began when the tag was continuouslydry for 10 min and ended when wet for 40 s. The lo-cation data of the seals (n = 26) were filtered follow-ing McConnell et al. [39] and as a result, on average(± SD) 2.0 ± 2.9 % of individual’s locations were re-moved. Data of individual KU13 contained 4 outlierlocations even after filtering and they were removed.To complement the GPS data, additional Argos flip-per tags (SPOT5, Wildlife Computers Inc.) were de-ployed to four seals. Flipper tags were duty cycled totransmit 2 h during daytime and 2 h during night in2 to 8 days per month.

Oksanen et al. Movement Ecology (2015) 3:33 Page 2 of 11

Home range analysisHome ranges were investigated with minimum convexpolygon (MCP) [40] and adaptive local nearest neigh-bour convex hull (a-LoCoH) analyses [41]. Home ranges(95 % of the locations in MCP and 95 % isopleths of theutilisation distribution in the LoCoH) were estimated forseals with a tracking period of over 20 days (Additionalfile 1: Table S1). In a-LoCoH, parameter a was set bytaking the maximum distance between any 2 locationsin each individuals’ data set [41]. For an individual MI12utilisation distribution could not be constructed with a-LoCoH with that a-parameter and set of locations. Asthe a-LoCoH estimator is not very sensitive for changesin a [41], we changed it to the nearest value allowing usto estimate the utilisation distribution (from 178 144to 178 010). Land areas were subtracted from theMCP home range estimates. Effect of age and sex on

the a-LoCoH home range size was tested with univar-iate general linear model (size = intercept + sex + age)in SPSS Statistics 19 (IBM). Two-way interactionterms were insignificant (p < 0.05) and therefore excluded.Variances of model residuals were not equal between theage classes and log-transformation was therefore used.

First passage time analysesWe investigated important foraging habitats of trackedseals between August and January. This largely coincideswith the period (Jun – Dec), when Baltic ringed sealsforage and gain weight more intensively than at othertimes of the year [25]. We hereafter refer to this mostlyopen water period as foraging season, with the recogni-tion that ringed seals also forage throughout the year[42, 43]. The foraging habitats were detected with thefirst passage time (FPT) analyses [36]. FPT, defined as

Fig. 1 Movements of Baltic ringed seals during the whole tracking period (a) and during breeding time (b). The whole tracking period:August-May in years 2011–2014. Breeding time: February-March (number of tracked seals during breeding time is in the brackets). Meanice concentration is for period 17.2.-2.3.2014 (data source: [71])

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the time required for a tracked individual to cross acircle of a given radius, is a measure of animals’ searchefforts along the track [36, 44]. FPT can also be used todetect any movement patterns leading to increased resi-dency [45].The analyses were done using the AdehabitatLT package

[46] in R 2.15.3 [47]. Haul out locations were included inthe FPT analyses. Before the analyses, we removed pos-sible gaps in the location data of each individual by divid-ing the data into several tracks when time between twoconsecutive locations was > 1 d. As the quality of FPT ana-lyses depends on tracking duration [48], we droppedshortest tracking records (<15 locations, mean duration ±SD: 8.8 ± 12.3 h) from the analyses. We received on aver-age 17 ± 8 daily locations and to ensure that points alongtracks were equally represented [36], we generated loca-tions in 1.2 km intervals (corresponding to the mean dis-tance between consecutive GPS locations) along thetracks, assuming that animals travelled linearly and withconstant speed between obtained GPS-locations. FPTvalues were calculated for every location with radii of thecircle changing from 1.5 to 80 km (in 0.5 km increments).The optimal radius for each track were then estimated byplotting the variances of the log-transformed FPTs as afunction of radius. The peak in the variance (var-max) in-dicates a scale at which an individual increased its searchefforts [36] and the FPTs corresponding to this radiuswere selected (see Fig. 2a and b for an example).

Defining foraging areas and haul out sitesTo separate locations with high FPT values (high resi-dency locations) from low, a threshold value was ob-tained from a histogram of FPT values for each track[49]. FPTs had multimodal distribution, where low FPTsformed one mode of the histogram and high FPTs oneor several modes (see Fig. 2c for an example). The highresidency locations were then used to detect one or sev-eral foraging areas within each track following themethod in Lefebvre et al. [45]; first foraging area wasconstructed by assigning the highest FPT value as acentre of the circle with radius corresponding to var-max. Other areas were formed when the next highestFPTs with the associated circle did not overlap with an-other foraging area. According to the number and loca-tions of these areas, the seals were then classified to“local foragers” and “long-range foragers”. Local foragershad only one foraging area or the maximum distance be-tween centroids of different areas was ≤ 121 km (corre-sponding to the two adjacent foraging areas with thelargest observed var-max of 60.5 km). Long-range for-agers either occupied several separate foraging areaswith a maximum distance of >121 km or did not showincreasing search effort (no var-max detected) and,therefore, foraging areas could not be identified.

Haul out sites were defined from the GPS locations.Location error and small scale changes in the haul outplace were taken into account by defining all locationsthat were within 50 m of each other as one haul out site.Time budget and diurnal rhythm of haul out were con-structed on the basis of summary data provided by GPSphone tag, which reports percent of haul out, diving andbeing near the surface (threshold 1.5 m) in two hoursbouts.

Foraging habitat characteristicsTo investigate the characteristics of foraging habitat, thedepth and distance to the coastline of high residency lo-cations were calculated using bathymetric raster data(grid size 250 × 250 m) and catchment area data [50].To examine the overlap of the foraging habitats withprotected areas, we calculated the percentage of highresidency locations of the seals within the MPAs desig-nated by the Helsinki Comission (HELCOM [50]) andNatura 2000 sites [51] that are protected under theEuropean Union’s Habitats Directive [52]. OverlappingMPAs and Natura 2000 sites can be of different shapeand size depending on the targets of protection, as theNatura 2000 network protects habitats and species atEU level and the HELCOM MPAs network at the levelof the Baltic Sea. To get an overview of the overlap ofseals and important coastal fishing areas, we used a data-set of annual catches (in tons of kg) of commercialcoastal fisheries in year 2007 [50]. We calculated thepercentage of high residency locations within 50 × 50 kmgrids (corresponding to ICES statistical rectangles) inwhich the annual catch were above the median value forthe Baltic Sea.

ResultsTelemetry performance and home range sizeIn total, 26 out of the 61 live-captured ringed seals wereheavy enough (≥40 kg) to be equipped with GPS phonetags. Tagged seals captured with fyke nets (in Aug-Nov)were mainly young (9/10 individuals) whereas sealscaptured with nets (in Oct-Nov) were mostly adults(13/16, Additional file 1: Table S1). Juveniles were onaverage (±SD) tracked for longer periods than adults(156 ± 31 days and 86 ± 33 days, respectively; Table 1).Two tags (for adults EL11 and PI12) only functioned< 20 days and these data sets were therefore excludedfrom the home range analyses. The average numberof GPS locations per tracking day was 17 ± 8. Threeout of four flipper tags functioned and provided data(21–97 total locations) from tagging until May ex-tending the overall tracking period by two to threemonths (Additional file 1: Table S1).During the whole tracking period (August-May), tracked

seals ranged over large areas in the Bothnian Bay and the

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Bothnian Sea (Fig. 1a); mean maximum distance fromcapture sites being 392 ± 195 km (measured as great-circledistance between the capture site and the utmost locationpoint). Mean a-LoCoH home range size for juveniles was8721 ± 6177 km2 and for adults 7339 ± 2983 km2 (Table 2).Juveniles had considerably greater individual variationamong their home range sizes than adults (Levene’s test,F = 7.742, p = 0.011). However, we did not detect any ageor sex dependent differences on the a-LoCoH home rangesizes (for age p = 0.900 and for sex p = 0.513, R2 = 0.021).Two adult females (HE11 and II11) migrated to the Gulfof Riga (maximum distance from capture site 888 and798 km, respectively) in late November—early December

and were located there until the end of tracking inFebruary.Tracking of many adults ended likely when they

moved to the ice-covered areas, and the locationsdata of adults are therefore scarce during the breed-ing season in February-March (Table 1). The last ob-tained locations from GPS phone tags and additionallocations from flipper tags indicate that adults weremostly located in the ice-covered areas in the Both-nian Bay and two also in the Gulf of Riga (Fig. 1b),which are also important breeding areas. The juve-niles were moving mostly in open-water areas andnear the ice-edge (Fig. 1b).

Table 1 Summary of the tag performance of the Baltic ringed seals equipped with GPS phone tags. Dur = duration of trackingperiod (d). Locs = number of obtained GPS locations

Whole tracking period Foraging season (Aug-Jan) Breeding season (Feb-Mar)

Weight (kg) dur locs locs/d dur locs dur locs

Juveniles Mean 43 156 2524 16 112 1959 43 608

SD 3 31 1571 8 26 1293 22 521

n 12 12 10

Adults Mean 91 86 1346 17 68 1305 14 57

SD 19 33 771 9 22 686 16 161

n 14 14 10

Fig. 2 Examples of FPT analyses and foraging areas of individual AA13. a: variance in first passage time (FPT) as a function of radius (r). b: Changeof FPT in time. c: Classification of high residency locations on the basis of the histogram (red line indicates the division). d: Movements, foragingareas and haul out sites. e: Closer look to the foraging area with the highest FPT values

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Foraging areas and haul out sitesDuring the foraging season (Aug–Jan), 41 out of 79tracks had a peak in the variance of log(FPT), indicatingincreased search effort at scales varying from 2.5 to60.5 km (mean 13.5 ± 14.7 km). Foraging areas could notbe identified for two individuals (ME11, PI12), which didnot show increasing search effort at any scale and were,therefore, moving randomly. The other 24 seals hadfrom 1 to 6 foraging areas (mean 3.1 ± 1.6, Fig. 3) andthey spent 47 ± 22 % of time inside these zones. Typic-ally foraging areas of individuals had a mean distance of254 ± 194 km. However, the distance between foragingareas had large variation among individuals: 9 seals wererelatively local foragers having only one foraging area orthe mean distance between several foraging areas was67 ± 26 (range 35–100) km. The other 17 seals were“long-range foragers”, which had either several separ-ate foraging areas (mean distance 328 ± 180 km, range150–825 km) or no main foraging areas could be de-tected. Each tracked ringed seal used 26 ± 16 haul outsites (range 0–55), 59 ± 30 % of which were inside theforaging areas. Haul out consisted 7.5 % of the timebudget during the foraging season and was mainlynocturnal (Fig. 4).Despite the high number of long-range foragers among

the tracked seals, two clusters of foraging “hot spots”were identified; one in the northern Bothnian Bay andanother in the northern Bothnian Sea and the Quark(Figs. 3 and 5). The foraging areas were characterized bya shallow bathymetry (median depth of high residencylocations 13 ± 49 m [mean 38 m]) and proximity to theshore (median distance from the mainland 10 ± 14 km[mean 15 km]). Overall, 22 % of high residency locationswere situated within the existing protected areas (19 %to MPAs and 15 % to Natura 2000 sites) and 47 % over-lapped with areas where annual catch of coastal fisherieswere over the median value (63.8 tons of kg) (Fig. 5).

DiscussionThe present study is the first to document extensivemovements of Baltic ringed seals. The tracked seals uti-lised on average 27 % (MCP home ranges 31 565 ± 16640 km2) of the surface area of the Gulf of Bothnia (115

500 km2, [37]). The distances that Baltic ringed sealsranged from the tagging site (mean 392 km) were similarto Arctic ringed seals that range over distances of severalhundreds of kilometres during the post-moulting season[16, 27, 28, 53–55]. However, Arctic ringed seals report-edly travel a couple of thousand kilometres from the tag-ging site [16, 26, 56]. The estimated home ranges of thepresent study (8030 km2, 95 % a-LoCoH) were similar tothose reported for ringed seals in the eastern Canada(“locals” 2281 and “long rangers” 11 854 km2, [57]). Incontrast, ringed seals in Lake Saimaa have very modesthome ranges (92 km2, [30]), likely due to the complexstructure of the small lake habitat (area 4400 km2, [58]).The home ranges reported here match the average homeranges of the Baltic grey seals (Halichoerus grypus,6294 km2 [59] and 6858 km2 [10]), which are known tomove long distances over the whole Baltic Sea. Althoughthe home range sizes for Baltic ringed seals have notbeen previously reported, they have been consideredquite sedentary due to the limited movements observedin the previous study [25]. However, our observations in-dicate that the movements of ringed seals in the BalticSea are similar order of magnitude to those in the ArcticSea. In addition, also genetic results [28, 60] have indi-cated that Baltic ringed seals may be more mobile thanearlier suggested.The results of the present study suggest that during

breeding season adults are mostly associated with goodice conditions whereas juveniles are near the ice edge orin the open-water areas. Baltic ringed seals may there-fore exhibit similar habitat partitioning between adultsand juveniles during the breeding season as reported inthe Arctic [61]. Whereas the GPS phone tags of juvenileswere mostly working well during breeding season, tagsof adults ceased to work or only transmitted very few lo-cations when they moved to ice-covered areas inJanuary-February. However, the last obtained locationsfrom the breeding season indicate that most adults occu-pied the ice-covered areas in the northern Bothnian Bayand the Gulf of Riga, which are the main breeding areasfor the Baltic ringed seals and characterised by the pres-ence of pack and stable ice during most winters [62].Two adult females migrated from the Bothnian Bay to

Table 2 Estimated home range sizes (km2) of the Baltic ringed seals

Home range (MCP 95 %) Home range (a-LoCoH 95 %)

N Mean SD Range Mean SD Range

Juveniles 12 31664 18777 5289–66937 8721 6177 727–18899

Adults 12 31466 15045 12852–61882 7339 2983 1132–12280

Males 9 28601 18415 5289–66937 7297 5220 727–18899

Females 15 33343 15878 6431–61882 8470 4654 1132–17565

Total 24 31565 16640 5289–66937 8030 4796 727–18899

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Fig. 4 Time budget (left panel) and times of haul out (right panel) for Baltic ringed seals. Time frame: August-January, years 2011–2014. Trackedseals: 26 individuals. Time is local time (UTC + 2)

Fig. 3 Foraging areas for juvenile (a) and adult (b) Baltic ringed seals

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the Gulf of Riga in November-December, suggesting thatsome individuals move between different subpopula-tions. Ringed seals show breeding site fidelity [16, 19]and it is likely that these individuals were feeding in theBothnian Bay and returned to breed to the Gulf of Riga.The frequency of the movements between breedingareas on the population level remains unclear.Our results confirm the previous observations of noc-

turnal haul out behaviour during the post-moulting forthe Baltic ringed seal [25]. The Saimaa seal also hassimilar nocturnal haul out rhythm [21, 29, 63]. In con-trast, ringed seals in Greenland have not shown any cir-cadian rhythm in their haul out behaviour [20, 53].Tracked ringed seals hauled out only 8 % of their totaltime, which is quite similar to the 10 to 17 % previouslyreported for ringed seals during the post-moulting sea-son [16, 25, 63]. The observed low proportion of time

spent hauling out indicates that haul out contributesrelatively little to the high residency areas (referred to asforaging areas) estimated with the FPT approach. Ringedseals can also sleep in the water [64], and at-sea activitiesmay include some of this resting behaviour as well.However, as the open-water season is the most import-ant foraging time when ringed seals gain considerableweight [23–25], the high residency areas very likely referto the areas of increased foraging effort.Baltic ringed seals used large regions for foraging.

Most (65 %) of the tracked ringed seals were “long-range” foragers that used spatially remote foraging areasor did not concentrate foraging efforts to any particulararea. Foraging near the mainland (median distance10 km) in areas with shallow bathymetry (depth 13 m)indicates potential overlap and interactions with coastalfisheries. Ringed seals are suggested to cause substantial

Fig. 5 Overlap of high residency locations of Baltic ringed seals with marine protected areas (a) and coastal fisheries (b). Count of high residency(HR) locations in 5 × 5 km grids for tracked ringed seals (n = 26). Time frame: August-January, years 2011–2014. Annual catch of coastal fisheries isin tons of kg for year 2007

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catch losses to the coastal fisheries in the Bothnian Bay,although grey seals induce most damage at the scale ofthe Baltic Sea [33, 34, 65]. Removal of ringed seals nearthe fishing gear in the Bothnian Bay has been proposedto mitigate the depredation [35]. As most of the ringedseal individuals seem to feed on relatively large areaswithin the foraging season, our results indicate that re-moval of the individuals near the fishing gear may notbe locally effective method to mitigate the ringed seal-induced damages to coastal fishery. Furthermore, due tothe extensive movement capacities, local mitigation ac-tions may target individuals from the southern subpopu-lations and therefore compromise conservation goals inthese areas, further complicating the management of theconflict.Despite the extensive movements and large proportion

of long range foragers, two clusters of ringed seal for-aging “hot spots” were identified, one in the Quark andthe other in the northern Bothnian Sea. According toold bounty statistics, ringed seals gather to the northernBothnian Bay in the late fall [66], when we also capturedmostly adults with the seal nets. Their foraging areaswere more clearly clustered to the northern BothnianBay compared to juveniles. The juveniles were mainlycaptured in fyke nets earlier in fall, which is in line withthe by-catch records [38]. The foraging areas of thetracked seals partly overlapped with MPAs and Natura2000 sites especially in the identified foraging hot spots.Both protected area networks aim to conserve importantspecies and habitats, ringed seal being one of those spe-cies [52, 67]. However, ringed seal was listed as criteriafor protection in 7 out of 15 MPAs and in only 5 out of30 Natura 2000 sites that overlapped with high ringedseal residency [67, 68]. Our results therefore indicatethat safeguarding of the important resting and feedinghabitats could to some extent be implemented in andadjacent to the existing protected area networks. Conse-quently, identified foraging areas of ringed seals shouldbe taken into account when updating the managementplans for overlapping protected areas. Importance of theBothnian Bay as the main distribution and breeding areaof the Baltic ringed seal may be emphasized in the fu-ture, as the warming climate reduces ice cover andthereby the breeding success of the southern subpopula-tions [15, 31]. Therefore, the future conservation mea-sures may need to be directed more strongly towardsthe subpopulation of the Bothnian Bay. In general, mar-ine mammals rely on healthy ecosystems for theirsurvival and they are indicators of marine ecosystemchange and biodiversity [69]. The foraging distributionof ringed seals might therefore be utilised also as indica-tors for identifying important areas for protection.The chosen analytical approach, including position fil-

tering, linear interpolation of the tracks and first passage

time analyses, was heuristic rather than statistical [70].However, our results and conclusions should be quiterobust to the weaknesses of these approaches, given theaccuracy of the GPS positions, large number of dailyfixes (17 ± 8 locations/d) and the study questions relatedto the broad-scale habitat use. In the future, however,more fine-scaled analyses on foraging behaviour andhabitat preference of the Baltic ringed seal, based onstate-space methods, for example, are encouraged.

ConclusionsThe foraging of Baltic ringed seals is mostly concentratedto relatively shallow areas near the mainland, indicatingpotential overlap with coastal fisheries. The conflict be-tween ringed seals and coastal fisheries has intensified inthe Bothnian Bay as the seal population has been recover-ing. The mitigation of the conflict is complex, as ringedseals range over large areas and concentrate to forage todifferent—often remote—areas. Selective removal of sealsnear the fishing gear may not therefore be the most suit-able method to mitigate the depredation. On the otherhand, clusters of foraging effort hot spots were identified.The hot spots overlapped partly with the existing pro-tected areas. The importance of Bothnian Bay as the maindistribution area may further increase due to changing cli-mate, and the management of key foraging and restinghabitats of ringed seals could to some extent be estab-lished within the existing network of protected areas.

Additional file

Additional file 1: Table S1. Details of the Baltic ringed seals equippedwith GPS phone tags. * : Individuals tagged additionally with SPOT5flipper tags. Values in the brackets describe the date of last location andnumber of locations obtained with flipper tags. (PDF 86 kb)

Abbreviationsa-LoCoH: Adaptive local nearest neighbour convex cull; FPT: First passagetime; HELCOM: Helsinki comission; MCP: Minimum convex polygon;MPA: Marine protected area designated by HELCOM; Natura 2000site: Network of protected areas, protection is based on the EU’s habitatsdirective; var-max: The maximum variance of log-transformed first passagetime values.

Competing interestsThe authors declare that they have no competing interests.

Authors’ contributionsSMO, MA and MK participated in the design of the study. SMO, MA and MKparticipated in the fieldwork. SMO and MN performed the data analyses.SMO drafted the manuscript. MN, MA and MK helped to draft and finalizethe manuscript. All authors read and approved the final manuscript.

AcknowledgementsThis study was funded by European Fisheries Fund (EFF) and Maj andTor Nessling Foundation. We would like to thank fishermen S. Kehus,E. Pirkola, M. Posti, T. Matinlassi, J. Vierimaa and M. Viitanen and for theclose co-operation during the study. We thank also E. Helle, P. Hepolaand M. Lahti for sharing the old local know-how about capturing sealswith nets. We wish to thank J. Oikarinen and P. Timonen for all the help

Oksanen et al. Movement Ecology (2015) 3:33 Page 9 of 11

during the project. Our thanks are also due to J. Aspi, M. Auttila, P. Kauppinen,T. Laitinen, R. Levänen, J. Taskinen, M. Vehmas and J. Ylönen for their help in thefield work. We would like to thank J. London and P. Boveng from NOAA for theflipper tags and all the help with them. We also thank J. Heikkinen forthe statistical consulting. Finally, we thank J. Syväranta, M. Valtonen andtwo anonymous reviewers for valuable comments on the manuscript.

Author details1Department of Biology, University of Eastern Finland, PO Box 111, FI-80101Joensuu, Finland. 2The Natural Resources Institute Finland, Itäinen Pitkäkatu 3,FI-20520 Turku, Finland.

Received: 29 January 2015 Accepted: 6 September 2015

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A Novel tool to mitigate by-catch mortality of Baltic seals in coastal fyke net fishery

Oksanen S M, Ahola M P, Oikarinen J, Kunnasranta M (2015)

PLoS ONE 10: e0127510

This article is reproduced under the terms of the Creative Commons

Attribution License (http://creativecommons.org/licenses/by/4.0/)

RESEARCH ARTICLE

A Novel Tool to Mitigate By-Catch Mortalityof Baltic Seals in Coastal Fyke Net FisherySari M. Oksanen1*, Markus P. Ahola2, Jyrki Oikarinen3, Mervi Kunnasranta1

1 Department of Biology, University of Eastern Finland, Joensuu, Finland, 2 Natural Resources InstituteFinland, Turku, Finland, 3 Perämeren Kalatalousyhteisöjen Liitto ry, Oulu, Finland

* sari.oksanen@uef.fi

AbstractDeveloping methods to reduce the incidental catch of non-target species is important, as

by-catch mortality poses threats especially to large aquatic predators. We examined the ef-

fectiveness of a novel device, a “seal sock”, in mitigating the by-catch mortality of seals in

coastal fyke net fisheries in the Baltic Sea. The seal sock developed and tested in this study

was a cylindrical net attached to the fyke net, allowing the seals access to the surface to

breathe while trapped inside fishing gear. The number of dead and live seals caught in fyke

nets without a seal sock (years 2008–2010) and with a sock (years 2011–2013) was re-

corded. The seals caught in fyke nets were mainly juveniles. Of ringed seals (Phoca hispidabotnica) both sexes were equally represented, while of grey seals (Halichoerus grypus) theratio was biased (71%) towards males. All the by-caught seals were dead in the fyke nets

without a seal sock, whereas 70% of ringed seals and 11% of grey seals survived when the

seal sock was used. The seal sock proved to be effective in reducing the by-catch mortality

of ringed seals, but did not perform as well with grey seals.

IntroductionFisheries worldwide capture several non-target species as incidental by-catch. Populations ofmany large aquatic predators, such as marine mammals and sharks are especially vulnerable toby-catch mortality in consequence of their slow reproductive rates [1]. Mitigation of by-catchmortality has therefore become increasingly important in both fisheries management [2] andwildlife conservation [3]. However, the interactions between aquatic predators and fisheriescan be complex, and in particular, many seal species cause losses to fish catches and damage tofishing gear [4,5]. Developing fishing methods can provide means to mitigate both aspects ofthe seal-fisheries conflict. For example, physical barriers mounted on the fish traps, such aswire grids, prevent seals from entering the trap [6–8]. Alternatively, exclusion devices that facil-itate the seals’ exit from the fishing gear [9] and techniques that trap seals alive inside the gearhave also been developed [10].

The Baltic grey seal (Halichoerus grypus) and ringed seal (Phoca hispida botnica) have hadsignificant interactions with human activities. Both populations declined drastically to onlyabout 5000 seals in the 1970s as a result of excessive hunting and reproductive disorders caused

PLOSONE | DOI:10.1371/journal.pone.0127510 May 18, 2015 1 / 9

OPEN ACCESS

Citation: Oksanen SM, Ahola MP, Oikarinen J,Kunnasranta M (2015) A Novel Tool to Mitigate By-Catch Mortality of Baltic Seals in Coastal Fyke NetFishery. PLoS ONE 10(5): e0127510. doi:10.1371/journal.pone.0127510

Academic Editor: Jan Geert Hiddink, BangorUniversity, UNITED KINGDOM

Received: December 30, 2014

Accepted: April 16, 2015

Published: May 18, 2015

Copyright: © 2015 Oksanen et al. This is an openaccess article distributed under the terms of theCreative Commons Attribution License, which permitsunrestricted use, distribution, and reproduction in anymedium, provided the original author and source arecredited.

Data Availability Statement: All relevant data arewithin the paper and its Supporting Information files.

Funding: This study was partly funded from theEuropean Fisheries Fund (http://ec.europa.eu/fisheries/cfp/eff/index_en.htm) awarded to the FinnishGame and Fisheries Research Institute by the ELY-Centre of Kainuu (#1000862, #1001767) and the Majand Tor Nessling Foundation (http://www.nessling.fi/en/, #201200171, 201300044, 201400081,201500146 to SMO, MK). The funders had no role instudy design, data collection and analysis, decision topublish, or preparation of the manuscript.

by environmental pollution. The seal populations have been recovering, but the present num-ber of grey seals (census size: 30 000 seals) is still less than half and ringed seals (13 000 seals)less than one tenth of the estimated historical abundance at the beginning of the 20th century[11–13]. The recovery of these populations has led to an increase in seal-induced losses tocoastal fisheries, which has had negative effects on attitudes towards the conservation of sealpopulations [14]. The Baltic grey seal in general causes more losses to fisheries than the ringedseal [6,15]. However, increasing numbers of ringed seals are assumed to cause substantial catchlosses in the Bothnian Bay, the northernmost part of the Baltic Sea (Fig 1) [16]. The hunting ofseals has become a key action for the mitigation of damage in the northern Baltic Sea [14,17].In addition to hunting, by-catch mortality may be another significant component of the an-thropogenic mortality of seal populations. Estimated annual by-catch of the Baltic grey seal isin the order of 2000 animals or more [18] but for ringed seals the magnitude is unknown. Aninformation gap regarding by-catch and its mitigation methods in coastal and small scale fish-eries has been recognized globally [19]. Also in the Baltic Sea broader knowledge of by-catch isof great importance for sustainable fishery and population management [20].

Considerable efforts have been put into research on interaction between Baltic grey seal andfisheries [10,14,17,18,21,22] and on the development of seal-proof fishing gear. For example, trapnets with modified fish chambers, i.e. pontoon traps, are reported to reduce grey seal-inducedlosses to fisheries and simultaneously to reduce by-catch [8,23]. However, relatively little attentionhas been paid to estimating and reducing the by-catch mortality of seals in other stationary gears.In addition, there is little information on the interaction between ringed seals and fisheries, notonly in the Baltic Sea [24], but also globally. Methods for reducing interactions of Baltic ringedseals and coastal fishery are of great importance, as climate change can be expected to furtherthreaten the population growth of this ice-breeding species, especially in the southern breedingareas, where the stocks have recovered slowly, if at all [12]. In this study we tested the effectivenessof a new type of pinniped by-catch reduction device for fyke nets, called a seal sock. We also ex-amined the demographic properties, such as age and sex distribution, of the by-caught seals.

MethodsAround 70% of the Baltic ringed seal population is found in the Bothnian Bay (Fig 1), and greyseal is also encountered in this region. Coastal fyke net fishery in the area targets whitefish(Coregonus lavaretus), vendace (C. albula), Atlantic salmon (Salmo salar) and brown trout (S.trutta) in particular. Various modifications of fyke nets are used, but in general they are large,bottom-anchored fishing gear that consists of a leader net and wings guiding the fish throughthe middle chamber into the fish chamber. Seals entering the chambers may not find their wayout and drown, as the chambers are usually submerged (approximately 2 m from the surface).The seal sock is a by-catch reduction device that is designed to enable seals to have access tothe surface to breathe while trapped inside the fyke net. The sock was initially developed andconstructed by a Finnish fish trap manufacturer (Ab Scandi Net Oy).

We developed a modification of the seal sock, comprising a cylindrical net (diameter 0.7 m,length 2.5–4.0 m) made of strong seal-proof netting (Dyneema, mesh size 30 mm). It was at-tached to the roof of the fish chamber in a fyke net (Fig 2A). The sock was fitted with one ortwo hoops, to keep its shape, and a small float. To examine the effectiveness of the seal sock inreducing by-catch, we compared the number of seals caught alive in a set of fyke nets betweenyears when the socks were not used (2008–2010) and when they were used (2011–2013). An-nually, between May and October–November, a commercial fisherman fished with 4–6 fykenets within the same fishing area in the Bothnian Bay (64°32N, 24°19E, Fig 1). The fyke netscomprised a leader net (mesh size 200–300 mm), wings (60–100 mm) and two cylindrical

Mitigating By-Catch of Baltic Seals

PLOS ONE | DOI:10.1371/journal.pone.0127510 May 18, 2015 2 / 9

Competing Interests: The authors have declaredthat no competing interests exist.

chambers (30–35 mm, Dyneema, Fig 2B). Fishing effort was determined based on the fisher-man’s logbooks and was relatively same between both periods (2106 days for years 2008–2010and 1767 days for years 2011–2013).

The fisherman kept a logbook on the number, species and capture date of by-caught seals.During the testing of the sock in the years 2011–2013, the fisherman also recorded the sex andweight (kg) or age class (juveniles and adults) of the by-caught seals. Researchers took part tothe field work (tagging live animals and taking samples from dead ones) during years 2011–

Fig 1. The Baltic Sea.

doi:10.1371/journal.pone.0127510.g001

Mitigating By-Catch of Baltic Seals

PLOS ONE | DOI:10.1371/journal.pone.0127510 May 18, 2015 3 / 9

2013 and they often had possibility to verify the reliability of the data. Although the fishermen’smotivation to report by-catch has generally been low in Finland [18], the volume of by-catch re-ported in this study is high and comparable between the two periods (2008–2010 and 2011–2013). It is therefore likely that the volumes of by-catch detected in our study are not under-rated. Weighed seals were further divided into age classes on the basis of an age-weight database(Natural Resources Institute Finland). Weighed ringed seals with a body weight> 50 kg wereclassified as adults (estimated age� 4 years). Grey seal males with a body weight> 92 kg andfemales> 65 kg were categorized as adults (estimated age� 5 years). We also used the sealsocks for live-capturing ringed seals for satellite tagging (Oksanen et al., unpublished). We testedthe effect of the seal sock on survival with Fisher’s exact test. The effect of weight and capturemonth on survival of ringed seals when the sock was in use (2011–2013) was tested with binarylogistic regression. Effects of weight and month on the grey seal survival were not tested due tothe small sample size. We conducted statistical testing with IBM SPSS Statistics 20 software.

Ethics StatementThe use of a seal sock and animal handling was permitted by the game authorities (permit no.2011/00082 and 2013/00197) and the Animal Experiment Board of Finland (no. ES AVI/1114/04.10.03/2011).

Fig 2. A type of by-catch reduction device, a seal sock, is attached to the fish chamber of a fyke net allowing the seal to have access to the surface.(A) A side view of a seal sock and chambers. (B) A fyke net (seen from above) consists of a leader net, wings and chambers.

doi:10.1371/journal.pone.0127510.g002

Mitigating By-Catch of Baltic Seals

PLOS ONE | DOI:10.1371/journal.pone.0127510 May 18, 2015 4 / 9

ResultsA total of 135 seals (30 live and 105 dead) were caught (4–6 fyke nets annually) during thestudy years in 2008–2013 (S1 Dataset). Of all the by-caught seals, 103 (76%) were ringed and32 (24%) grey seals. During the years 2011–2013, 95% of the ringed (n = 40) and 67% of thegrey seals (n = 18) were juveniles (Table 1). The mean weight of the by-caught and weighedringed seals (n = 33) was 36 kg (SD 11). The weight of the ringed seals visiting fyke nets in-creased towards autumn (linear regression, R2 = 0.469, F = 27.3, p<0.001; Table 1): ringedseals by-caught and weighed in May (n = 4) had a mean weight of 19 kg (SD 9) (correspondingto young-of-the-year), whereas individuals by-caught in October–November (n = 7) weighedon average 44 kg (SD 10). The sex-ratio of the ringed seals was even (49% males and 51% fe-males), but for the grey seals it was biased towards males (71% males and 29% females;Table 1). In fact, while the sex distribution of juvenile grey seals (n = 12) was even and theywere caught mostly during summer (8 caught in May-July), the adults (n = 6) were mostlymales (5 males, 1 unknown) and mostly captured during the autumn (5 caught in August-No-vember: Table 1).

Overall, the survival of seals in the fyke nets increased when the sock was used (Fisher’sexact test, p<0.001). Altogether 77 dead seals were by-caught during the years when the sealsock was not used (2008–2010), whereas 30 live and 28 dead seals were caught when the sealsock was used (2011–2013) (Fig 3). The seal sock increased survival of ringed seals (p<0.001):of all caught ringed seals (n = 40), 83% found their way to the sock and 70% remained alive.However, the survival of grey seals was not increased (p = 0.308): only 17% of the grey (n = 18)seals found way to the sock, and 11% remained alive. The ringed seals that did not find theirway into the sock were mostly small pups (5/7) having a mean weight of 16 kg (SD 9) and theywere mostly captured in early summer. In fact, capture month was the only statistically signifi-cant predictor of ringed seal survival in logistic regression (model summary: χ2(1) = 4.210,p = 0.040, Nagelkerke R2 = 0.142; parametermonth: p = 0.048, coefficient = –0,405). By con-trast, all the by-caught adult grey seals (n = 6) were caught in the middle chamber, while 8 outof 12 of juveniles were either in the fish chamber or in the sock. However, only 3 out of 8 greyseals reaching the fish chamber found their way into the sock.

Table 1. Monthly variation in numbers, age and sex distribution and survival of the by-caught seals in fyke nets equipped with the seal sock.

Month Ringed seals Grey seals Total

Total Juveniles/ adults Males/ females Alive/ dead Mean weight Total Juveniles/ adults Males/ females Alive/ dead

May 6 6/0 3/3 2/4 19 ± 9 3 3/0 1/2 0/3 9

June 1 1/0 1/0 0/1 na 3 2/1 3/0 0/3 4

July 3 3/0 2/1 2/1 24 ± 8 3 3/0 1/2 0/3 6

Aug 4 4/0 2/2 4/0 34 ±8 2 1/1 2/0 0/2 6

Sept 16 15/1 7/9 13/3 38 ± 8 4 2/2 4/0 1/3 20

Oct 8 7/1 3/4a 5/3 45 ± 12 1 1/0 0/1 1/0 9

Nov 2 2/0 1/1 2/0 43 ± 1 2 0/2 1/0a 0/2 4

Total 40 38/2 19/20 28/12 36 ± 11 18 12/6 12/5 2/16 58

In years 2011–2013, annually 4–6 fyke nets equipped with the seal sock were set out for fishing in the Bothnian Bay. Monthly mean weight (kg) ± SD for

ringed seals is reported for a total of 33 weighed individuals.a Gender for one seal not recorded

doi:10.1371/journal.pone.0127510.t001

Mitigating By-Catch of Baltic Seals

PLOS ONE | DOI:10.1371/journal.pone.0127510 May 18, 2015 5 / 9

DiscussionThe seal sock proved to be effective in reducing the by-catch mortality of ringed seals in fykenets, but it did not perform as well with grey seals. According to our pilot test, the seal sockalso seemed to work well in a so-called bottom fyke net (Flex R, Ab Scandi Net Oy), which wetested during the summer of 2013, and all of the captured seals (7 ringed seals and 1 grey seal)survived in the sock. Although the use of the seal sock increased survival of ringed seals in gen-eral, small pups caught during early summer survived poorly despite the sock. On the contrary,adult grey seals were by-caught only in the middle chamber, and it is likely that they could notreach the fish chamber through the narrow passage between the chambers (diameter 0.3 m).To overcome this problem, a sock could be inserted also into the middle chamber. However,only 3 out of 8 grey seals that reached the fish chamber found their way into the sock, whichmay indicate behavioral differences between the species. Ringed seals keep open breathingholes in the sea ice, which may be several meters thick in the Arctic [25], and swimming up-wards in a narrow funnel might, therefore, be more typical behavior for them. In addition to re-ducing by-catch, the seal sock has potential applications in capturing seals alive and removingindividuals repeatedly visiting the fishing gear. When the seal sock is used only for by-catchmitigation, it could have an opening at the surface enabling seals to climb out of the gear, as ex-clusion devices are usually designed to enable rapid exit of the animal from the fishing gear.

Our results illustrate that the majority of seals by-caught in fyke nets are young seals of bothgenders. This was especially evident in ringed seals, of which 95% were juveniles and equally ofboth genders, but also among grey seals most of the by-caught individuals were juveniles(67%). The dominance of juveniles in by-catch could to some extent be explained by their na-ivety, as juveniles of many seal species are more vulnerable to being by-caught [26,27]. Ringedseal pups are especially prone to get caught in fishing gear just after weaning in the early sum-mer [28]. We also observed lower survival of ringed seals during early summer compared toautumn, and our results indicated that survival in the seal sock was more dependent on naivetyof the young-of-the-year than weight of the seals. Although the overall sex ratio of grey sealswas biased towards males, young grey seals of both sexes were by-caught especially in spring,and adult males in autumn. A similar temporal trend in by-caught grey seals has been reported

Fig 3. Monthly variation in the number of by-caught seals in fyke nets (4–6 fyke nets annually). Firstbars: fyke nets without a seal sock (years 2008–2010). Second bars: fyke nets with a seal sock (2011–2013).

doi:10.1371/journal.pone.0127510.g003

Mitigating By-Catch of Baltic Seals

PLOS ONE | DOI:10.1371/journal.pone.0127510 May 18, 2015 6 / 9

in previous studies [29,30]. A strong male dominance has also been observed in grey seals visit-ing pontoon traps [10,21,22,31].

Our study indicates that by-catch particularly increases juvenile mortality, which may notreduce population growth and viability as much as a similar increase in adult—especially fe-male—mortality would [32]. Nevertheless, by-catch mortality may constitute another signifi-cant source of anthropogenic mortality in the grey seal population, whose annual huntingpressure in the Finnish sea area was around 5% of the total population [33], and should there-fore be taken into account in the population management. By-catch mortality also increasestotal mortality of the ringed seal population and mitigating it may become increasingly impor-tant for population management, as the population growth of this ice-dependent species is pre-dicted to decrease due to climate change [12,34].

Knowledge of the by-catch mortality and its mitigation methods are important aspects ofthe sustainable management of seal populations. Reducing incidental mortality is also becom-ing increasingly important measure for the fisheries management to respond to growing de-mands for sustainable seafood. Several ecolabels take by-catch reduction into account [2]. Forexample ecolabel of the Marine Stewardship Council (MSC) is granted to fisheries that followthe sustainable fishery standards of the MSC. Our study introduces a novel and inexpensivetool, a seal sock, for mitigating by-catch of seals in coastal fyke net fisheries. In addition, theseal sock provides a practical and ethical method for the selective removal of seals repeatedlyvisiting fyke nets in areas where high seal abundance causes substantial losses to fisheries. Itcan also be used in scientific studies where seals need to be captured alive, for example teleme-try studies.

Supporting InformationS1 Dataset. Details of seals by-caught in fyke nets (4–6 fyke nets annually) during the years2008–2013. The fyke nets were set out for fishing without a seal sock in the years 2008–2010and with a seal sock in 2011–2013 in the Bothnian Bay. Species: Phb = Baltic ringed seal, Phocahispida botnica; Hg = grey seal,Halichoerus grypus. Location of the seal in a fyke net: sock/middle chamber/ fish chamber/ wings. na = not available(PDF)

AcknowledgmentsWe would like to thank fishermen M. Viitanen, T. Matinlassi and M. Posti for their close co-operation during the study. We also wish to thank K. Kyyrönen for the fyke net graphics and L.Mehtätalo for the statistical consulting. Thanks are also due to M. Valtonen and anonymousreviewers for their valuable comments on the manuscript.

Author ContributionsConceived and designed the experiments: SMOMPA JOMK. Performed the experiments:SMOMPA JOMK. Analyzed the data: SMO. Contributed reagents/materials/analysis tools:SMOMPA JOMK. Wrote the paper: SMOMPA JOMK.

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Year Month DateSpecies

(Phb/Hg)Gender (f/m)

Weight (kg) or age class (juvenile/adult)

Status (dead/alive)

Location in the fyke net

2008 5 20.5.2008 Phb na na dead na2008 5 30.5.2008 Hg na na dead na2008 6 16.6.2008 Phb na na dead na2008 6 20.6.2008 Phb na na dead na2008 6 20.6.2008 Phb na na dead na2008 6 27.6.2008 Phb na na dead na2008 6 28.6.2008 Phb na na dead na2008 7 25.7.2008 Hg na na dead na2008 7 29.7.2008 Phb na na dead na2008 7 29.7.2008 Hg na na dead na2008 8 11.8.2008 Phb na na dead na2008 8 11.8.2008 Phb na na dead na2008 8 16.8.2008 Hg na na dead na2008 8 21.8.2008 Phb na na dead na2008 8 27.8.2008 Phb na na dead na2008 8 27.8.2008 Hg na na dead na2008 8 29.8.2008 Phb na na dead na2008 8 29.8.2008 Phb na na dead na2008 9 5.9.2008 Phb na na dead na2008 9 5.9.2008 Phb na na dead na2008 9 11.9.2008 Phb na na dead na2008 9 11.9.2008 Phb na na dead na2008 9 11.9.2008 Phb na na dead na2008 9 11.9.2008 Phb na na dead na2008 9 11.9.2008 Phb na na dead na2008 9 14.9.2008 Phb na na dead na2008 9 19.9.2008 Phb na na dead na2008 10 9.10.2008 Hg na na dead na2008 10 9.10.2008 Hg na na dead na2008 10 11.10.2008 Phb na na dead na2008 10 11.10.2008 Phb na na dead na2008 10 15.10.2008 Phb na na dead na2009 7 3.7.2009 Hg na na dead na2009 7 19.7.2009 Phb na na dead na2009 8 25.8.2009 Phb na na dead na2009 8 25.8.2009 Phb na na dead na2009 9 3.9.2009 Phb na na dead na2009 9 3.9.2009 Hg na na dead na2009 9 6.9.2009 Phb na na dead na2009 9 6.9.2009 Hg na na dead na2009 9 13.9.2009 Hg na na dead na2009 9 18.9.2009 Phb na na dead na

S1 Dataset. Details of seals by-caught in fyke nets (4–6 fyke nets annually) during the years 2008–2013. The fyke nets were set out for fishing without a seal sock in the years 2008–2010 and with a seal sock in 2011–2013 in the Bothnian Bay. Species: Phb = Baltic ringed seal, Phoca hispida botnica ; Hg = grey seal, Halichoerus grypus . Location of the seal in a fyke net: sock/ middle chamber/ fish chamber/ wings. na = not available

2009 9 25.9.2009 Phb na na dead na2009 9 25.9.2009 Hg na na dead na2009 10 7.10.2009 Phb na na dead na2009 10 7.10.2009 Phb na na dead na2009 10 11.10.2009 Phb na na dead na2009 10 18.10.2009 Phb na na dead na2009 10 20.10.2009 Phb na na dead na2009 10 24.10.2009 Phb na na dead na2009 10 27.10.2009 Phb na na dead na2009 10 29.10.2009 Phb na na dead na2009 11 4.11.2009 Phb na na dead na2009 11 4.11.2009 Phb na na dead na2009 11 5.11.2009 Phb na na dead na2010 7 2.7.2010 Phb na na dead na2010 8 13.8.2010 Phb na na dead na2010 8 13.8.2010 Phb na na dead na2010 8 16.8.2010 Phb na na dead na2010 9 10.9.2010 Hg na na dead na2010 9 12.9.2010 Phb na na dead na2010 9 12.9.2010 Phb na na dead na2010 9 14.9.2010 Phb na na dead na2010 9 14.9.2010 Phb na na dead na2010 9 14.9.2010 Phb na na dead na2010 9 17.9.2010 Phb na na dead na2010 9 17.9.2010 Phb na na dead na2010 9 17.9.2010 Phb na na dead na2010 9 21.9.2010 Phb na na dead na2010 9 25.9.2010 Phb na na dead na2010 9 28.9.2010 Phb na na dead na2010 10 5.10.2010 Hg na na dead na2010 10 7.10.2010 Phb na na dead na2010 10 18.10.2010 Phb na na dead na2010 10 19.10.2010 Phb na na dead na2010 10 21.10.2010 Phb na na dead na2010 10 21.10.2010 Phb na na dead na2011 5 26.5.2011 Hg f juvenile dead middle chamber2011 6 4.6.2011 Hg m juvenile dead wings2011 8 20.8.2011 Phb m 40 alive sock2011 9 2.9.2011 Phb f 38 alive sock2011 9 8.9.2011 Hg m juvenile dead fish chamber2011 9 8.9.2011 Hg m juvenile alive sock2011 9 9.9.2011 Phb f 33 alive sock2011 9 22.9.2011 Phb f 50 alive sock2011 9 25.9.2011 Phb m 35 alive sock2011 10 1.10.2011 Phb m juvenile dead sock2011 10 1.10.2011 Phb m juvenile dead fish chamber2012 5 17.5.2012 Phb f 17 dead fish chamber2012 5 22.5.2012 Phb f 25 alive sock2012 5 23.5.2012 Phb m 25 alive sock2012 5 29.5.2012 Hg f juvenile dead sock

2012 5 31.5.2012 Hg m juvenile dead fish chamber2012 5 25.5.2012 Phb f 7 dead fish chamber2012 6 29.6.2012 Phb m juvenile dead sock2012 6 29.6.2012 Hg m juvenile dead fish chamber2012 6 29.6.2012 Hg m adult dead middle chamber2012 7 6.7.2012 Phb m 30 alive sock2012 7 10.7.2012 Phb m 18 dead sock2012 7 13.7.2012 Hg f juvenile dead fish chamber2012 7 14.7.2012 Phb f juvenile alive sock2012 7 14.7.2012 Hg f juvenile dead fish chamber2012 8 2.8.2012 Phb f 28 alive sock2012 8 12.8.2012 Phb m 27 alive sock2012 8 29.8.2012 Phb f 42 alive sock2012 9 3.9.2012 Phb m 29 alive sock2012 9 3.9.2012 Phb f 32 dead sock2012 9 16.9.2012 Phb f 54 dead fish chamber2012 9 22.9.2012 Phb f 47 alive sock2012 9 24.9.2012 Phb m 33 alive sock2012 9 28.9.2012 Phb m 24 dead fish chamber2012 10 3.10.2012 Phb f 46 dead sock2012 10 6.10.2012 Phb f 65 alive sock2012 10 10.10.2012 Hg f juvenile alive sock2012 10 11.10.2012 Phb f 34 alive sock2012 10 15.10.2012 Phb f 37 alive sock2013 5 27.5.2013 Phb m juvenile dead fish chamber2013 5 27.5.2013 Phb m juvenile dead fish chamber2013 7 11.7.2013 Hg m 64 dead middle chamber2013 7 or 8 na Hg m juvenile dead middle chamber2013 8 16.8.2013 Hg m 154 dead middle chamber2013 9 11.9.2013 Phb f 37 alive sock2013 9 19.9.2013 Phb m 45 alive sock2013 9 19.9.2013 Phb m 33 alive sock2013 9 19.9.2013 Hg m 112 dead middle chamber2013 9 24.9.2013 Phb m 37 alive sock2013 9 24.9.2013 Phb f 45 alive sock2013 9 27.9.2013 Phb f 30 alive sock2013 9 28.9.2013 Hg m 98 dead middle chamber2013 10 1.10.2013 Phb na juvenile alive sock2013 10 12.10.2013 Phb m 43 alive sock2013 11 1.11.2013 Hg m 150 dead middle chamber2013 11 2.11.2013 Phb f 44 alive sock2013 11 5.11.2013 Phb m 42 alive sock2013 11 na Hg na adult dead middle chamber

Publications of the University of Eastern FinlandDissertations in Forestry and Natural Sciences No 188

Publications of the University of Eastern Finland

Dissertations in Forestry and Natural Sciences

isbn: 978-952-61-1893-2 (printed)

issn: 1798-5668

isbn: 978-952-61-1894-9 (pdf)

issn: 1798-5676

Sari Oksanen

Spatial ecology of the grey seal and ringed seal in the Baltic Sea– seeking solutions to the coexistence of seals

and fisheries

The Baltic seal populations have

recovered since the 1990s and the

conflicts between seals and fishery

interests have aggravated at the

same time. This thesis provides

new information on the spatial

ecology of the grey seal and ringed

seal in relation to coastal fisheries

in the Baltic Sea and discusses the

implications for conflict mitigation

and population management

measures.

disser

tation

s | 188 | S

ar

i Ok

san

en | S

pa

tial ecolo

gy of th

e grey seal a

nd

ringed

seal in

the B

altic S

ea

Sari OksanenSpatial ecology of the grey seal and ringed seal in the

Baltic Sea– seeking solutions to

the coexistence of seals

and fisheries