Movements and distribution of cod (Gadus morhua) in the...

J. Mar. Biol. Ass. U.K. (2007), 87, 599–613Printed in the United Kingdom

Journal of the Marine Biological Association of the United Kingdom (2007)

doi: 10.1017/S0025315407054641

Movements and distribution of cod (Gadus morhua) in the southernNorth Sea and English Channel: results from conventional and

electronic tagging experiments

The sub-structure of Atlantic cod (Gadus morhua) stocks in the North Sea has important consequences for fisheries management as the Common Fisheries Policy moves towards a more regional approach. We investigated the movements, distribution and behaviour of cod in the southern North Sea (ICES IVc) and English Channel (ICES VIId) by re-analysing historic data from conventional tagging experiments, and by conducting new experiments with electronic tags. Cod tagged and released in IVc showed a northwards shift in distribution during the feeding season consistent with a homing migration away from spawning grounds along the coasts of the UK and the Netherlands. In contrast, cod tagged and released in VIId did not exhibit a consistent pattern of seasonal movement. Many cod released in VIId were subsequently recaptured close to their release position, although some moved out of the Channel and into the southern North Sea. Overlap between the recapture areas of cod released in the different management areas was no more than 25% in either the spawning or feeding season. Behavioural data from electronic tags suggest that cod in IVc make use of tidal streams to migrate northwards and eastwards in spring, whereas selective tidal stream transport was rarely exhibited by cod tagged and released in VIId. Overall, the evidence suggests that there are behavioural differences between cod in IVc and VIId that limit the mixing of cod from these two areas during the feeding and spawning seasons.

INTRODUCTION

Atlantic cod (Gadus morhua L.) are distributed across the continental shelves of the North Atlantic, including the North Sea and English Channel (Graham, 1948; Daan, 1978; Brander, 1994). Similar to many other commercial marine fish in the North Sea, such as plaice (Pleuronectes platessa L.) and sole (Solea solea), seasonal changes in the distribution of cod are observed as a result of the migratory behaviour of individuals between feeding and spawning grounds (Graham, 1948; Bedford, 1966; Daan, 1978). Since the mid 1950s to the present day, many conventional tagging experiments have been carried out on cod in the North Sea to provide greater insight into these migratory movements and to estimate more accurately the geographical range of cod populations. This has yielded recapture data for thousands of individuals and some of these results have been reported in part (or summarized) by a number of authors (Bedford, 1966; Symons & Raitt, 1966; ICES, 1971; Daan, 1978; Easey, 1987; Pawson, 1995; Robichaud & Rose, 2004; Wright et al., 2006).

In 1971, the ICES North Sea Roundfish Working Group summarized the first 20 years of cod tagging experiments in an attempt to establish the inter-relationships and exchange of cod between the North Sea and surrounding areas (ICES, 1971). Overall, the conclusions of these early analyses

suggest that the dispersal of cod from the point of tagging and release in the North Sea is limited to a few hundred km (ICES, 1971; Daan, 1978). Indeed, a recent study of cod tagged in the northern North Sea suggested that average displacement was likely to be less than 100 km irrespective of the time at liberty after tagging (Wright et al., 2006). This, in turn, points towards spatial structuring of the North Sea stock because it is unlikely that cod living in the northern North Sea (above 57°N) would mix with cod below 55°N in the southern North Sea (Anon, 1971; Robichaud & Rose, 2004). ICES (1971) identified the following regional groupings of cod: (a) the Norwegian side of the Skaggerak; (b) the Danish side of the Skagerrak; (c) one or several coastal regions, from Flamborough to the Scottish east and north coasts; (d) the central North Sea; (e) the Southern Bight, from the Strait of Dover to latitude 54°N; (f) the English Channel, south and west of the Strait of Dover.

These conclusions are supported by the results of recent genetic studies (Hutchinson et al., 2001). Genetically distinct sub-groups were found in four areas: the northern North Sea, the Moray Firth, Flamborough Head and the Southern Bight, broadly corresponding to the ICES stock assessment and management areas IVa, IVb and IVc/VIId. These areas are large, however, and evidence is increasing that the movement of cod is much more restricted than previously

David Righton*, Victoria Anne Quayle, Stuart Hetherington and Gary Burt

Centre for Environment, Fisheries and Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road,Lowestoft, NR33 0HT, UK. *Corresponding author, e-mail: [email protected]

600 D. Righton et al. Movements and distribution of cod in the southern North Sea and English Channel


thought (in the North Sea, Righton et al., 2001; Turner et al., 2002; Wright et al., 2006; Metcalfe, 2006; more generally, Green & Wroblewski, 2000; Lawson & Rose, 2000; Morris & Green, 2002; Robichaud & Rose, 2004), which may lead to fine-scale structuring of populations. In addition, the role of spawning sites in maintaining population structuring and stock recovery is thought to be of critical importance (Robichaud & Rose, 2001, 2004; Wright et al., 2006), which requires a better understanding of seasonal changes in population distribution.

The southern North Sea is an important area for the exploitation of cod by a number of fisheries (ICES, 2005). Unlike plaice (Pleuronectes platessa), for which separate stock assessments are performed, quota management of cod in ICES areas IVc and VIId has been based upon a combined stock assessment since 1996. Pawson (1995), in re-summarizing published cod tagging studies conducted up to 1971, concluded that the data support the movement of cod

between the eastern English Channel and North Sea. This is due to a northwards and eastwards dispersal of juveniles from nursery grounds in the eastern English Channel. These fish, as adults, may then move back to spawning grounds in the Channel. However, previous analyses of cod movement in the southern North Sea and English Channel were based on limited sub-sets of the available data (Bedford, 1966; ICES, 1971) and did not distinguish between migrations away from or back to spawning sites.

In this study, we have re-evaluated 4336 recaptures (of cod tagged with simple marker tags) between 1964 and the present day to assess the seasonal changes in distribution, migration rates and exchange of juvenile and adult cod between the North Sea and eastern English Channel. In addition, we present the preliminary results of new tagging experiments using electronic data storage tags (DSTs) to investigate the extent of any spatial and temporal differences in the behaviour of cod in these areas.

Figure 1. Release positions of tagged cod and recapture numbers over time. (A) IVc release positions (hollow cirles), recaptures indicated from each release location, with revised numbers used for further analyses in brackets; (B) IVc total number of recaptures over time; (C) VIId release positions (hollow cirles), recaptures indicated from each release location, with revised numbers used for further analyses in brackets; (D) VIId total number of recaptures over time.


601Movements and distribution of cod in the southern North Sea and English Channel D. Righton et al.

MATERIALS AND METHODSConventional tagging

All records of cod (Gadus morhua) recaptures from cod released in ICES areas IVc (southern North Sea) and VIId (English Channel) were extracted from the Cefas ‘Tagfish’ database (archived data sets from1964 to the present day). The majority of cod to be tagged in areas IVc and VIId were caught in shallow coastal waters by commercial trawl (heavy otter, bottom pair or beam, 86%), however, some were also caught by Danish anchor seine (9%), and long-line (5%). All cod were then measured to the nearest cm and externally tagged with a conventional tag attached to the dorsal musculature at the base of the first dorsal fin. Cod tagged in IVc were released in three main coastal areas of the UK and Netherlands (Figure 1), while cod tagged in VIId were released from three main areas off the south coast of the UK. Most cod were released during autumn and winter (October to March).

In total, release and recapture information for 4336 tagged cod were available (3742 in IVc and 594 released in VIId respectively). Recapture data for tagged cod spanned the early-1960s to the mid-1980s with 70% of IVc cod recaptured during the 1980s and 66% of VIId cod recaptured during the 1970s. Any recaptures of cod that occurred within 90 days of release were excluded. This was to ensure that the movements we describe in this paper are those of cod with sufficient time to migrate to different areas between the spawning and feeding periods, and that any same quarter recaptures remaining are of cod that had been at liberty for over a year after their release. Recaptured cod were all released during quarters 1 and 4 (1785 & 255 in IVc and VIId respectively). Only 101 (68 and 33 in IVc and VIId respectively) of the remaining cod recaptures were made during the quarters 1 and 4 in the first year after release. Recapture data were then sub-divided into quarters of the year according to the recapture season; winter and autumn (Q1 & Q4), spring and summer (Q2 & Q3), (see also Bedford, 1966 and ICES, 1971). Finally recaptures were placed into size categories dependent on the length of cod at recapture: smaller than 50 cm (classed as ‘juvenile’) or larger than 50 cm (classed as ‘adult’). Maturity ogives derived from International Bottom Trawl Survey (ICES, 2005) data collected between 1977 and 2005 confirmed that this split was consistent with mean size at 50% maturity (ICES, unpublished data).

Recapture positions for cod were plotted for IVc and VIId release areas in ArcView 9.0. The Animal Movement

Analysis Extension to Arcview (AMAE: Hooge & Eichenlaub, 2000) was used to estimate the extent of geographical range for cod in each area by generating kernel probability density function surfaces (KPDF) for 95%, 70% and 50% volume estimates under the three-dimensional KPDF surface (see Worton, 1987, Seaman & Powell, 1996; Hooge et al., 2000). The KPDF method is more typically used in studies of territoriality and home range (Jones, 2005; Righton & Mills, 2006). However, because tag recapture locations are analogous to the density and distribution of the locations of single individuals over time, the technique is extremely applicable to population level mark–recapture data.

Electronic tagging

Data storage tags (DSTs)The following DSTs were used during the study: the Star-

Oddi (Marine Device Manufacturing, Iceland) ‘Milli’ and ‘Centi’ tags weighing 5 g and 12 g in water and capable of storing over 43,000 and 250,000 data respectively, and the LOTEK (Marine Technologies, Canada) 1200 and 1400 tags, (weighing 3 g in water and capable of storing over 750,000 and over 270,000 data samples respectively). For the purpose of the study, tags were set to record temperature and pressure at 10 min intervals, with the exception of the ‘Milli’ tags, which were programmed to record pressure at 20 min intervals and temperature at 1 h intervals. Data downloaded from ‘Milli’ tags were then linearly interpolated to 10 min intervals.

Tagging procedureBetween November 2004 and January 2006, a total of

133 cod were tagged and released off Lowestoft (in the vicinity of 50°30'N 00°22'E and 52°30'N 1°50'E), while a further 185 cod were tagged and released off Hastings (in the vicinity of 50°52'N 00°43' E; Table 1). Cod were caught by hook and line in the southern North Sea and by trawl and gill/trammel nets in the eastern Channel. Individuals were brought slowly to the surface to avoid rupture of the swimbladder and those in good condition were placed into 1000-l deck tanks. Fish suitable for tagging with a DST (length >45 cm) were then selected and tagged with a DST or, if length was between 35 cm and 45 cm, tagged externally with a conventional tag (‘Howitt’ type). Cod less than 35 cm were returned to the sea. Cod tagged with DSTs were anaesthetized in a shallow (20 cm) bath

Pre-spawning Post-spawning

Area DSTs Length Conventional Length DST Length Conventional Length Total

IVc 54 68 ±8 10 56 ±15 49 58 ±9 20 45 ±3 133VIId 75 59 ±13 44 45 ±3 53 66 ±13 13 50 ±7 185

Total 129 63 ±12 54 47 ±8 102 62 ±12 33 47 ±6 318

Table 1. Number and mean length (in cm ±SD) of cod tagged with DSTs and conventional tags in each ICES management area during pre-spawning (November/December) and post-spawning (March/April) periods. Mean length of tagged cod differed significantly between pre and post spawning tagging trips and area of capture (ANOVA, P<0.05).



containing 2-phenoxy-ethanol (0.5 ml l-1) before surgically implanting the DST into the peritoneal cavity (details in Righton et al., 2006). A spaghetti tag attached to the DST was then placed through the lateral body wall by a curved needle and a conventional tag was attached to the dorsal musculature at the base of the first dorsal fin to act as external identification markers (Righton et al., 2006). Cod tagged with conventional tags were not anaesthetized before tagging. All fish were placed into a recovery tank before release. In order to encourage the recovery of recaptured tags a reward scheme was set up throughout the commercial fisheries (as for Turner et al., 2002; Righton & Metcalfe 2002; Hunter et al., 2004; Righton et al., 2006).

Analysis of tag returnsOn the return of cod to the laboratory, the carcasses

were examined to assess the wounds caused by the tags and to measure length and weight. Data from all DSTs were downloaded and plotted to identify repeated behaviour patterns. Cross-tag/ area comparisons were made and classifications of distinct behavioural patterns were considered on a qualitative basis.

Geographic reconstructionsHydrostatic (tidal) data, derived from the sinusoidal

pressure cycle recorded in the depth data when a fish is at rest on the sea-f loor, was used to enable the geographical reconstruction of a cod’s movements (termed geolocations). This is referred to as the Tidal Location Method (TLM, as described in Metcalfe & Arnold 1997; Hunter et al., 2004). Tidal ranges were extracted using a wave-fitting algorithm. For each day, the best-fitting wave-form was used to calculate the times of high and low water, the tidal range, and an indication of the quality of fit (sum of squares). Estimates of time of high water and tidal range were filtered to obtain the most accurate daily estimate (based on least sum-of squares). These data were then used in the TLM to derive, by day, possible geographic locations where the individual may have been. During the winter and spring, when most of the North Sea and English Channel is vertically mixed, daily average temperatures recorded by the DSTs were then compared to averaged sea surface temperature (taken from Bundesamt für Seeschifffahrt und Hydrographie, Hamburg), and positions

at which DST temperature and SST differed by more than 2°C were excluded.

Analysis of fish movements

Movement parametersMovement parameters (derived from Jones, 1976), were

calculated using the release and recapture dates and positions for each recaptured cod from archived conventional tag and DST returns. Values of distance travelled in km (r), duration of time in days to recapture (t), bearing in degrees north θ, northwards movement r(N) in km (r cos θ), eastwards movement r(E) in km (r sin θ) and the average distance in km travelled by day (velocity (V)=r/t) were calculated for each recapture as follows:

Overall bearing north (degrees),

Y =åq

1

n

n

Total northwards movement (km),

R(N) =å r Nn

1

( )

Total eastwards movement (km),

R(E)=å r En

1

( )

Mean velocity (km/day),

V =å v

n

n

1

Mean square dispersion coefficient (km2/day),

a2 =æ

èçççç

ö

ø÷÷÷÷-

ìíïï

îïï

üýïï

þïïå1 2

2

nrt

v T

Correction of data for fishing effortWe were unable to source a fishing effort dataset for either

management area that would enable us to correct for the effect of effort distribution on population-level migration distances and redistribution (as for Rijnsdorp & Pastoors,

Area Lr (cm) Lc (cm) Q1 Q2 Q3 Q4 Total Q1 & Q4Autumn & Winter

Q2 & Q3Spring & Summer

IVc <50 <50 124 411 304 195 1034 319 715<50 >50 163 152 151 121 587 284 303>50 34 39 60 31 164 65 99

Total 321 602 515 347 1785 668 1117

VIId <50 <50 34 31 12 33 110 67 43<50 >50 34 24 23 38 119 72 47>50 6 7 5 8 26 14 12

Total 74 62 40 79 255 153 102

Table 2. The number of conventional tag recaptures in the stratified dataset after exclusion of recaptures of fish at liberty less than 90 days. Lr and Lc refer to length at release and recapture respectively. Q1 & Q4 are combined to show number of recaptures during the autumn and winter quarters; Q2 & Q3 are combined to show number of recaptures during the spring and summer quarters.



1995; Bolle et al., 2005; Wright et al., 2006). Landings data held by Cefas are largely unattributed to location (even at a course, ICES rectangle scale) before 1982. It was not possible, therefore, to correct the recapture data for any spatial or seasonal bias that may exist in fishing effort, and distances between release and recapture are therefore unweighted. However, trawling effort in the southern North Sea between the mid-1970s and 1980s has been consistently high and widely distributed within IVc and IVb (Jennings et al., 1999; Engelhard, 2005), although with some bias towards the coasts. These data are probably representative of the distribution of fishing effort, although not the overall effort, over the preceding decades. Hence, while we were not able to correct tag recapture density or distance between release and recapture for fishing effort, we can be reasonably confident that seasonal changes in the distribution of cod were well described by the recapture data.

RESULTSConventional tagging

RecapturesAlthough the details of 4336 cod recaptures were extracted

from the Tagfish database, this number fell to 2040 once recaptures of cod after less than 90 days were excluded (Table 2, in bold text). All subsequent references to recaptures are for cod at liberty for >90 days. Due to the similar distribution parameters exhibited by recaptured cod from the three main release areas in IVc and VIId respectively, data sets

for all release areas in each ICES area were combined. Cod remained at liberty on average for 296 days after tagging (±239 days, see Figure 1), travelling an average distance from release to recapture of 135 km (±135 km). The longest time at liberty was 2572 days (seven years), by a cod recaptured only 7 km away from its original release site in VIId during Q4. The greatest distance travelled was by a cod at liberty for 195 days, recaptured 1005 km from original release site in IVc during Q3 just inside the ICES management area of IVa (west of Scotland; 59°75'N 3°06'W).

In IVc, 58% of tagged cod (1034) recaptured throughout all quarters were classed as ‘juveniles’ (<50 cm) at the time of their recapture, whilst 33% (587) of recaptures were of cod that had been tagged as ‘juveniles’ and had reached the mean length of 50% maturity (50 cm) during their time at liberty and were therefore classed as ‘adult’ upon recapture. Only 9% of recaptures were of cod that were classed as ‘adult’ (>50 cm) upon release (Table 2). For VIId cod relatively small numbers per size category and quarter of the year were recaptured (n~20). In all 43% (110) of tagged cod in VIId were recaptured as ‘juveniles’, while 47% (119) were recaptured as ‘adults’. Only 26 (10%) cod were released as ‘adults’. Due to the limited number of returns for cod for VIId, particularly in Q2 & Q3, the recaptures of cod in Q2 & Q3 were combined, as were the recaptures for Q4 & Q1 (see Table 2). These sub-divisions correspond very broadly to the ‘spawning’ (Q4 & Q1) and ‘feeding’ (Q2 & Q3) seasons (as for Wright et al., 2006).

‘Juveniles’ ‘Adults’

IVc:- North Sea Q4 & Q1 Q2 & Q3 Q4 & Q1 Q2 & Q3

Total number of fish 319.0 715.0 349.0 402.0Average time at liberty 271.0 154.0 441.0 383.0Average distance (km) 82.0 125.0 114.0 222.0Average northwards movement 51.0 100.9 56.7 182.0Average eastwards movement -3.0 -9.0 3.0 24.0Bearing of movement (degrees) -3.0 -5.1 3.3 7.5Dispersion coefficient (a2) 28.4 71.6 47.1 150.550% KPDF area (km2) 6910 7060 16500 3950075% KPDF area (km2) 18300 14100 38500 8320095% KPDF area (km2) 68800 61300 122000 234000

VIId:- English Channel Q4 & Q1 Q2 & Q3 Q4 & Q1 Q2 & Q3

Total number of fish 67.0 43.0 86.0 59.0Average time at liberty 252.0 157.0 588.0 457.0Average distance (km) 53.7 57.0 151.0 213.0Average northwards movement 9.5 14.0 94.0 159.0Average eastwards movement -3.1 6.0 60.0 20.0Bearing of movement (degrees) -18.2 24.3 32.7 7.0Dispersion coefficient (a2) 19.3 44.3 85.7 99.850% KPDF area (km2) 5020 4460 12140 2140075% KPDF area (km2) 9290 9230 23020 4040095% KPDF area (km2) 24300 25100 95090 214000

Table 3. Population movement parameters for each area of release, split by season of recapture (autumn and winter Q1 & Q4; spring and summer Q2 & Q3) and length class (‘Juveniles’: recaptures <50 cm; ‘Adults’ recaptures >50 cm).



Figure 3. Recapture positions of cod released in ICES area VIId. Solid symbols show exact recapture locations, while shading shows the probability density surfaces for 50% (lightest grey), 75% (mid grey) and 95% (dark grey) of the recaptures. Data shown are for (A) ‘juve-niles’ recaptured during Q4 & Q1; (B) ‘adults’ recaptured during Q4 & Q1; (C) ‘juveniles’ recaptured during Q2 & Q3; and (D) ‘adults’ recaptured during Q2 & Q3.

Figure 2. Recapture positions of cod released in ICES area IVc. Solid symbols show exact recapture locations, while shading shows the probability density surfaces for 50% (lightest grey), 75% (mid grey) and 95% (dark grey) of the recaptures. Data shown are for (A) ‘juve-niles’ recaptured during Q4 & Q1 (autumn and winter); (B) ‘adults’ recaptured during Q4 & Q1; (C) ‘juveniles’ recaptured during Q2 & Q3 (spring and summer); and (D) ‘adults’ recaptured during the Q2 & Q3.



Spatial distribution of returnsIn IVc, ‘adult’ and ‘juvenile’ recaptures were restricted to

the coast close to their release sites during Q4 & Q1 (Figure 2A,B). In contrast, it was evident that both ‘juvenile’ and ‘adult’ cod mixed over a much wider area of the central North Sea during Q2 & Q3, after dispersing north away from the coast (Figure 2C,D; Table 3), with some ‘adult’ cod dispersing as far as the Danish coastline. The recapture area for ‘adult’ cod was therefore relatively large (Table 3). ‘Juve-nile’ cod were more restricted in their dispersal compared to ‘adult’ cod during Q2 & Q3 and the size of the recapture area of ‘juveniles’ was similar for Q4 & Q1 and Q2 & Q3 (Figure 2A&C; Table 3). The average distances travelled (in

km), and dispersion coefficients were both lower for ‘juve-nile’ cod compared to that of ‘adult’ cod in IVc and empha-sizes the difference in their seasonal movements.

In all observations for cod tagged in VIId it should be noted that the number of recaptures per quarter are relatively limited (Table 2). There was not a clear pattern of movement at any time of year (Figure 3). ‘Juvenile’ cod were generally caught close to their point of release within VIId at all times of the year with little eastwards or northwards movement (Figure 3A&C). Similarly ‘adult’ cod released in VIId were generally recaptured close to the area of release, but were also captured within the southern North Sea during Q4 & Q1 (Figure 3B) and throughout the central North Sea during Q2 & Q3 (Figure 3D). These recaptures had a large influence on the size of the recapture area, making it almost as large as the recapture area for ‘adult’ cod released in IVc. As for cod in IVc, ‘juvenile’ cod did not move or disperse as far as ‘adult’ cod at any time of the year (Table 3). The difference in northwards movement between the size-classes was particularly pronounced: ‘adult’ cod were recaptured approximately ten times further north than ‘juvenile’ cod.

While cod tagged in IVc moved north and were more dispersed during Q2 & Q3, recaptures were still made close to release sites. The distributions of tagged cod therefore overlapped when comparing Q4 & Q1 with Q2 & Q3 (Table 4; Figure 4A,B). In ‘juveniles’, the proportion of overlap remained the same for when comparing Q4 & Q1 with Q2 & Q3 (Table 4A), indicating that the recapture area changed shape and position, but was still centred on the areas of release (Figure 4A). In contrast, the area occupied by ‘adults’ during Q4 & Q1 was overlapped to a large extent by the area occupied by ‘adults’ during Q2 & Q3, but not vice versa (Table 4A), indicating range expansion (to the north) during Q2 & Q3 (Figure 4B). The recapture areas of ‘juvenile’ cod were overlapped considerably by ‘adult’ cod recapture areas when comparing Q4 & Q1 with Q2 & Q3 (Table 4B).

As for cod tagged in IVc, cod tagged in VIId were more dispersed during Q2 & Q3 but recaptures were still made close to release sites. Patterns of overlap in recapture areas were therefore similar, with proportional overlap for ‘juvenile’ being similar when comparing quarters Q4 & Q1 with Q2 & Q3 (Table 4A; Figure 4C), and with proportional overlap of the ‘adult’ recapture area being greater during Q4 & Q1 (Table 4A; Figure 4D). Proportional overlap of ‘juvenile’ recapture area by ‘adult’ recapture area was two to three times as large as those for cod tagged in IVc. The recapture areas of ‘juvenile’ cod tagged in VIId were overlapped considerably by ‘adult’ cod ranges when comparing Q4 & Q1 with Q2 & Q3 (Table 4B).

Seasonal migrationsIn both ICES areas, cod were recaptured further away

from the point of release during Q2 & Q3 than during Q4 & Q1 (Table 3). In IVc, a strong pattern of seasonal migration was evident (Figure 5), and a significant fit to a cyclical pattern was achieved without constraining the input parameters (cycle frequency, amplitude and offset) of the cycle (F1,20=23.9, P<0.001). A significant annual cycle did not emerge from the recapture distances of cod tagged in VIId,

Proportion of Q4 & Q1 area that

overlaps with Q2& Q3 area

Proportion of Q2& Q3 area that

overlaps with Q4& Q1 area

(a) KPDF IVc VIId IVc VIId

‘Juvenile’ 50% 0.58 0.34 0.57 0.3875% 0.29 0.23 0.38 0.2395% 0.22 0.22 0.24 0.22

‘Adult’ 50% 0.35 0.60 0.15 0.3475% 0.21 0.47 0.10 0.2795% 0.16 0.51 0.08 0.23

(b) Q4 & Q1 Q2 & Q3

‘Juv’ vs ‘Adult’ 50% 0.79 0.63 0.81 0.7775% 0.45 0.41 0.52 0.3995% 0.33 0.31 0.31 0.25

‘Adult’ vs ‘Juv’ 50% 0.33 0.26 0.15 0.1675% 0.21 0.16 0.09 0.0995% 0.18 0.08 0.08 0.29

(c) Q4 & Q1 Q2 & Q3

‘Juveniles’ 50% – –75% 324 (0.03) –95% 5537 (0.15) 2970 (0.08)

‘Adults’ 50% 2199 (0.16) –75% 3663 (0.13) 2436 (0.04)95% 23462 (0.22) 54316 (0.24)

Table 4. Overlap of recapture areas. (a) Proportional overlap of recapture areas of each size-class between seasons, calculated as the overlap (in km2) between recapture areas divided by the seasonal recapture area. Proportional overlap is therefore greater for the smallest of the two recapture areas; (b) proportional overlap of recapture areas of different size-classes within season, calculated as the overlap (in km2) between recapture areas divided by the recapture area for each size-class; (c) overlap (in km2) between recapture areas of cod tagged in IVc and VIId. Figures in parentheses are average values of proportional overlap calculated by dividing the area of overlap by the recapture area for each management area.



Figure 4. Overlap of recapture areas (75% KPDF surfaces) of cod tagged in ICES areas IVc and VIId. Proportion of overlap (dotted) evident between Q4 & Q1 (dark grey) and Q2 & Q3 (hollow, black outline) in (A) IVc ‘juveniles’; (B) IVc ‘adults’; (C) VIId ‘juveniles’; and (D) VIId ‘adults’ (overlap area covers whole of Q4 & Q1 area). Between ICES areas IVc (dark grey) and VIId (light grey) proportion of overlap (dotted) evident between (E) IVc and VIId ‘juveniles’ and (F) IVc and VIId ‘adults’ for Q4 & Q1 areas.



Figure 5. Mean distance between release and recapture positions of cod tagged and released in ICES area IVc in the spawning sea-son. Lines of best fit were calculated by co-varying the parameters of sinusoidal wave (frequency, amplitude and offset) and using a least-squares minimization routine (as for Bolle et al., 2005).

even after constraining cycle frequency to the same value as IVc (F1,20= 3.65, P>0.05, not shown). An unconstrained fit of a cycle to the data suggested that cod migrated further in the second year after release but not in the first (F1,20=12.5, P<0.01, not shown).

Movement of cod between IVc and VIId and overlap between recapture positions

The exchange of ‘juvenile’ and ‘adult’ cod between the different ICES management areas and the combined seasonal quarters is shown in Table 5. In IVc and VIId, the majority of ‘juvenile’ cod did not move into a different management area between Q4 & Q1 and Q2 & Q3. This was also true of ‘adult’ cod released in VIId. In contrast, while the majority of ‘adult’ cod released in IVc were recaptured within IVc during Q4 & Q1, recaptures during Q2 & Q3 strongly suggest a northwards movement into the neighbouring management area of IVb.

The overlap of recapture areas was relatively small for ‘juveniles’ and ‘adults’ (Table 4C; Figure 4E,F), and remained similar between seasons and size-classes. Overlap only reached a relatively large proportion (~25%) when considering the overlap in 95% KPDF ranges (Table 4C), which may have been heavily biased by the North Sea recapture positions of relatively few cod tagged in VIId.

‘Juveniles’ ‘Adults’

Q4 & Q1 season Recapture area Recapture area

Release area IVb IVc VIId VIIe # tags IVb IVc VIId VIIe # tagsIVc 0.29 0.69 0.03 0.00 319 0.29 0.65 0.05 0.01 349VIId 0.00 0.28 0.69 0.03 67 0.10 0.38 0.48 0.02 86

Q2 & Q3 season Recapture area Recapture area

Release area IVb IVc VIId VIIe # tags IVb IVc VIId VIIe # tagsIVc 0.44 0.54 0.03 0 715 0.58 0.38 0.03 0.01 402VIId 0.02 0.29 0.62 0.02 43 0.25 0.17 0.51 0.05 59

Table 5. Exchange of tagged cod between ICES management areas, based on area of recapture. Recapture proportions in area of release are shaded in grey.

Figure 6. Recapture positions of cod tagged in (A) ICES area IVc and (B) ICES area VIId. Hollow symbols show the recapture positions of DST-tagged cod, while solid symbols show the recap-ture positions of cod tagged with conventional tags.



Electronic taggingRecaptures

To date, 30 cod (13%) tagged with DSTs since 2004 have been recaptured: 9 from releases in IVc and 21 from VIId. Returned cod were at liberty between 3 and 374 days, and grew by up to 20 cm in length during this time. In addition 12 cod tagged in the same experiment with just a conventional tag have been recaptured: 1 from IVc and 11 from VIId which were at liberty between 3 and 142 days and grew by up to 8 cm between release and recapture (Table

6). Two DSTs were lost in the gutting process of the cod by fishermen. In addition, two of the recovered DSTs (one recovered from IVc; one recovered from VIId) failed to download any data upon return.

The spatial distribution of the recaptures was similar to the results of the conventional tagging data. In IVc, eight cod were recaptured <90 days at liberty between January and June, with an average distance from the release site to recapture of 60 km (1–152 km). However, two cod recaptured in May and July after more than 90 days at liberty (106

Location Tag Type ID. Release Date Recapture Date Days at Liberty Lr (cm) Lc (cm) Distance (km)

IVc L1400 12282 30/11/04 11/02/05 73 65 68 81CENTI 5203 02/12/04 12/01/05 41 66 67 18L1400 12235 08/12/04 01/02/05 55 55 55 11L1400 12237 13/12/04 04/05/05 142 67 75 569

CENTI 4717 13/12/04 19/01/05 37 68 - 28CENTI 4854 13/12/04 12/01/05 30 81 - 135CENTI 1976 30/03/05 31/05/05 62 56 59 1CENTI 1977 30/03/05 07/04/05 8 47 - 12CENTI 234 31/03/05 15/07/05 106 54 60 184

HOWITT 920051 29/03/05 20/06/05 83 49 - 152

VIId MILLI 6448 11/11/04 20/11/05 374 48 68 16L1400 12239 11/11/04 24/01/05 74 47 51 25MILLI 6437 11/11/04 08/02/05 89 44 50 7L1400 12274 11/11/04 14/11/04 3 47 47 5L1400 12285 11/11/04 04/12/04 23 45 46 8L1400 12263 11/11/04 08/02/05 89 50 54 25MILLI 6438 11/11/04 13/01/05 64 47 51 45MILLI 6421 11/11/04 21/12/04 40 44 46 1MILLI 6432 11/11/04 16/01/05 66 48 - 5MILLI 6434 11/11/04 14/01/05 64 51 - 65MILLI 6440 11/11/04 02/07/05 233 53 66 187MILLI 6441 11/11/04 11/02/05 92 51 54 5L1400 12297 12/11/04 20/01/05 69 45 49 7L1400 12273 12/11/04 14/04/05 153 47 - 20MILLI 6433 12/11/04 11/04/05 150 46 - 261MILLI 6423 12/11/04 11/09/05 303 45 62 1MILLI 6420 12/11/04 15/11/04 3 47 47 7MILLI 6444 12/11/04 19/11/04 7 52 52 6MILLI 6445 12/11/04 10/12/04 28 44 45 7CENTI 1180 09/12/05 12/01/06 34 74 74 2CENTI 1937 09/12/05 13/12/05 4 51 51 3

HOWITT 920017 11/11/04 22/11/04 11 44 44 34HOWITT 920006 11/11/04 02/12/04 21 50 51 5HOWITT 920009 11/11/04 14/11/04 3 44 44 9HOWITT 920008 11/11/04 16/11/04 5 43 43 6HOWITT 920016 11/11/04 05/03/05 114 43 - 8HOWITT 920015 11/11/04 07/02/05 88 43 50 64HOWITT 920033 12/11/04 26/11/04 14 52 52 42HOWITT 920023 12/11/04 03/04/05 142 42 50 44HOWITT 920022 12/11/04 14/11/04 2 40 40 6HOWITT 937403 06/12/05 02/01/06 27 43 43 9

Table 6. Details for recaptured cod tagged with DSTs or conventional tags in IVc and VIId from November 2004 to present. Lr and Lc refer to length at release and recapture respectively. Distance (km) relates to the distance between release and recapture positions. (-) indicates recaptured cod that were not returned to Cefas for further analysis.



and 142 days), dispersed considerable distances away from the release site before being recaptured within the central southern North Sea (184 km; Figure 6A) and just off the Denmark coastline (569 km; Figure 6A). In contrast 23 cod from VIId recaptured <90 days at liberty between November and February, showed more limited dispersal away from their release site with an average distance to recapture of 12 km (1–65 km). Sixteen of these recaptured cod were classed as ‘juveniles’ at release (Figure 6B). For the other eight VIId cod recaptured after more than 90 days at liberty between March and September, an average distance of 76 km (1–261

km) away from the release site was achieved. Of these seven cod the greatest distance travelled away from the release site (261 km), was by a cod at liberty for 150 days and captured in April 2005, in the neighbouring VIIe management area (Figure 6B). Overall movement parameters for cod tagged with DSTs is shown in Table 7. The average distance travelled in IVc recaptured cod was over four times higher than VIId cod with northwards and eastwards bearings for IVc movements, whereas any notable movement away from the release site in VIId was on a westward bearing (Table 7).

Migratory behaviourOf the nine cod returned after release in IVc, six could

be geolocated between release and recapture using the tidal location method (TLM). Four of these cod were at liberty <75 days. Two of these cod (ID’s 12282 & 4854; Table 6) migrated away from their release position on a north-eastwards bearing towards the Brown Bank. In their short time at liberty, these cod displayed evidence of selective tidal stream transport (STST; as described by Arnold et al., 1994). The other two cod (IDs 5203 & 1976; Table 6) stayed within 18 km of their release position and spent the majority of their time close to the seabed, and occasionally ascended into mid-water at night to feed. Two cod (IDs 12237 & 234; Table 6) were at liberty for longer than three months. Figure 7A,B shows the reconstructed migrations of these cod. Figure 7A shows the migration of cod 12237.

Figure 7. Migrations of cod tagged in ICES areas IVc and VIId. (A) Cod 12237; (B) cod 234; (C) cod 6448; and (D) cod 6433.

IVc VIId

Total number of tags returned 10 32DST returns 9 22Conventional tag returns 1 10Average time at liberty 64 76Average distance (km) 130 33Average northwards movement 51 -0.23Average eastwards movement 68 -12Bearing of movement (degrees) 46 89Dispersion coefficient (a2) 95 -12

Table 7. Overall movement parameters for recaptured cod tagged with DSTs and conventional tags in IVc and VIId.



After an initial post-tagging movement north-east the cod undertook a period of rest off Knoll Deep for 14 days before continuing its migration north-eastwards. For the week following 25 March 2005, the cod moved into mid-water at 13 h intervals, behaviour that is consistent with STST. On 5 April 2005 STST was abandoned for 18 days as the cod consistently swam at depths between 5 and 20 m. In the two weeks between 23 April and recapture on 4 May, the cod exhibited further evidence of STST as it moved towards the Horn North Ground off the coast of Denmark. Figure 7B shows the southwards migration of Cod 234. The cod displayed a moderate and continuous pattern of vertical movement activity as it migrated south within shallow coastal waters towards South Falls. From 1 June 2005 the cod increased its active movement with evidence of STST to aid south-east movement towards French coastal waters where it was recaptured on 15 July 2005.

Of the 21 cod returned after release in VIId, 12 could be geolocated using the TLM. Seven of these cod (IDs 6421, 6434, 6441, 1180, 12239, 12285 & 12297; Table 6) were at liberty less than 92 days and stayed within 10 km of their release position close to the seabed (<40 m). Two other cod (IDs 6423 & 6437; Table 6) moved just south-west of their release position into deeper waters (<60 m), where periods of high activity were observed during the hours of darkness. Both of these cod moved back into shallower waters before being recaptured within 7 km from their release position. One other cod (ID 6440; Table 6) undertook a south-west migration into deeper waters towards the edge of Hurd Deep (an area within the neighbouring ICES management area of VIIe), and was recaptured just to the east of the Hurd Deep after eight months at liberty. The final two cod (IDs 6448 & 6433) migrated considerable distances during

their time at liberty. Figure 7C,D shows the reconstructed migrations of these cod. Figure 7C shows a year-long return migration by cod 6448. The cod initially remained within shallow waters (<40 m) close to the seabed with occasional bouts of nocturnal activity until the end of April 2005. From 1 May 2005 the cod migrated south-west into deeper waters (60–70 m). On 7 November 2005 it began to migrate back towards coastal waters before being recaptured off Hastings on 20 November 2005, 16 km away from its original release position.

Figure 7D shows the south-west migration by cod 6433. The cod initially migrated south-west into deeper waters (<60 m) until the end of November then moved on towards shallower waters on 1 December (<30 m) until the end of December 2004. Throughout January 2005 the cod resided off the Cornish coastline. From 4 February 2005 the cod began to move back round the coast off Exmouth between 11 February and 25 March 2005. From 27 March 2005 the cod migrated away into deeper waters (<60 m) before being captured within the area of the Hurd Deep.

DISCUSSIONOur reanalysis of archived data has demonstrated that

clear differences in migratory pattern exist between cod tagged and released in the ICES management areas IVc and VIId. Conventional tagging data has enabled us to describe significant seasonal variations in the distribution of both IVc and VIId cod. Our analysis shows that cod tagged in VIId during the Q4 & Q1 autumn and winter quarters occasionally move into IVc and beyond. This is mediated by the movement of ‘juvenile’ cod that then remain in the southern North Sea until capture as ‘adults’. ‘Juvenile’

Figure 8. Schematic to illustrate proposed movements of cod that spawn in ICES areas IVc (black arrows) and VIId (grey arrows). (A) Movements of cod away from the coast in the summer (Q2 & Q3), and into adjacent management areas; and (B) return migrations to coastal spawning areas in Q4 & Q1. Movement of ‘juveniles’ is indicated by broken lines, ‘adults’ by solid lines. Shaded areas indicate areas important to cod during the spawning season.



cod also appear to move west into area VIIe. In contrast, the clear seasonal pattern of northward movement of cod tagged in IVc suggests that the most significant exchange of cod between management areas is between area IVc and IVb. The population of cod in VIId does not appear to be supplemented significantly by the movement of post-spawning cod from IVc during Q2 & Q3. The data retrieved from electronic tags demonstrated that cod in the southern North Sea show behaviour patterns consistent with tidal stream transport and relatively long-distance migration. Our results expand the understanding of stock structure and behaviour provided by previous studies (ICES, 1971; Pawson, 1995).

It must be acknowledged that the conventional tag data are limited in two ways. First, the number of recaptures for cod tagged in VIId was relatively small. Thus, while we can be confident that the patterns we describe for cod tagged in IVc are genuine and robust, the results for cod tagged in VIId are less so. Second, few tagging experiments have been conducted during summer on cod in the North Sea. Hence, while we are able to determine the movements of cod between Q4 & Q1 and Q2 & Q3, we were unable to determine, for example, whether cod tagged in IVc during summer might move into VIId to spawn during the autumn and winter.

Seasonal changes in distributionSeasonal changes in distribution of ‘juvenile’ and ‘adult’

cod tagged in both IVc and VIId were evident. In IVc, the centre of distribution of cod moved north during the spring and summer quarters, and cod were recaptured close to their release position in Q4 & Q1 (as for ICES, 1971). There was a much larger area of distribution for ‘adult’ cod in IVc than for ‘juveniles’ (as calculated from 50% and 75% KPDF surfaces). This is consistent with previous studies (Bedford, 1966; Symons & Raitt, 1966; ICES, 1971; Daan, 1978; Easey, 1987; Pawson, 1995). In VIId, there was no obvious pattern of seasonal redistribution, although cod were more likely to be recaptured close to their release position during the autumn and winter quarters. The distributions of cod in VIId appeared to be much more closely matched, both seasonally and ontogenetically. In both areas, however, the proportional overlap of ‘juvenile’ ranges with ‘adult’ ranges within each management area was always larger in the autumn and winter quarters than in the spring and summer quarters.

The seasonal pattern of redistribution was a consequence of migration of cod away from release sites. Our calculation of migration distance suggests that the migration was much more pronounced in cod tagged in IVc than in VIId. This pattern was detectable up to two years after release (beyond which low recapture numbers hampered the analysis) and is consistent with a homing pattern of migration. In contrast, cod released in area VIId did not show such a clear pattern of migration and conform more to a pattern of residency (Robichaud & Rose, 2004; Neat et al., 2005). Indeed, one cod was caught within 7 km of the release location during the winter quarter after seven years at liberty.

The data collected from the electronic tagging experiment shed some light on possible reasons for the different patterns

of migration in IVc and VIId. Cod tagged in IVc showed evidence of circatidal periodicity that is consistent with the use of tidal streams to migrate. The long distances over which these cod moved between release and recapture support this. In contrast, cod tagged in VIId did not often exhibit STST even though the strength of the tidal currents in an east–west direction in the English Channel is as strong as the north–south tidal currents southern North Sea. Both the data from the conventional tagging studies and also the electronic tags are, however, currently biased towards recaptures of juvenile (smaller) fish (Tables 2 & 6). The apparent difference in migratory strategy may therefore be a result of ontogenetic differences in migratory ability, feeding requirements and spawning status. The increased migration distance of adult cod in VIId suggests that adult cod may migrate further, but the low sample size compromises the detection of a clear pattern. Returns of more adult cod tagged with electronic tags will shed some light on this issue.

Exchange of cod between management areasThe movement of cod released in IVc resulted in a net

movement of cod into ICES area IVb during spring and summer quarters. In contrast, cod in VIId were nearly always recaught within VIId at all times of year, although some individuals strayed either north into IVc or west into VIIe. There were relatively small overlaps between both ‘juvenile’ and ‘adult’ areas of recapture. ‘Juvenile’ areas hardly overlapped at all, suggesting discrete nursery grounds in IVc and VIId, but there was some overlap between adult areas during the spawning season. The relative isolation of eastern Channel cod from that of the southern North Sea was supported by our preliminary DST experiments. Cod tagged in both areas showed fidelity to the area of their release during the autumn and winter quarters and to feeding grounds during the spring and summer quarters, with no cod from the southern North Sea moving down into the English Channel or vice versa. Cod tagged in IVc moved away from their release position on a predominantly north-east bearing, whilst cod tagged in VIId moved westwards from their release position into deeper water.

Our data suggests similar implications to that found for plaice in the North Sea and English Channel by Kell et al. (2004), in that eastern English Channel sub-stocks may be compromised by fisheries management measures imposed on larger neighbouring North Sea sub-stocks. In addition, the data show that management boundaries are frequently crossed by fish stocks, and that different sub-stocks are likely to differ in the extent to which they move. However, conventional tag recapture positions can only ever provide evidence of a single position that a tagged fish visited and estimates of exchange of cod between areas may be biased if cod make only brief visits to other management areas. Bolle et al. (2005) found that conventional tag experiments on plaice significantly underestimated the movement into the English Channel at spawning time because plaice only move into the English Channel for a brief spawning period (6 weeks) in spring. At present, while our electronic tag data have provided some useful information on migrations that cause cod to cross fisheries management boundaries, the data are still too limited to assess movement rates between



management areas. Furthermore, assessing movements and changes in the distribution of juvenile and adult fish is only one step towards understanding the population structure of fish stocks. Genetic and otolith studies would also be required for a full understanding of the relationship between different putative sub-stocks.

Behavioural differencesOur experiment was the first time that cod have been

tagged with DSTs in the English Channel. Cod tagged in IVc appear to be active migrators that undertake prolonged periods of swimming activity within a vertical range of <40 m during their time at liberty between December and May. Turner et al. (2002) also noted this behaviour, where cod tagged in IVc spent the vast majority of their time swimming mid-water between March and early June, before settling into close association with the seabed to feed from late June to August. This period of association with the seabed was not observed in cod in IVc as all but one of the returned cod were recaptured by the end of May. However, patterns of selective tidal stream transport (STST) were evident in our cod, as previously been observed by Arnold et al., (1994) in cod and by Metcalfe & Arnold (1997) in plaice. In contrast, cod tagged in VIId spent prolonged periods resting close to the seabed, with repeated bouts of nocturnal activity, probably related to feeding. This is supported by the limited stomach content data that feature prey items vulnerable by night (Hislop, 1997; Stensholt et al., 2000; Adlerstein & Welleman, 2000; Alderstein & Ehrich, 2001). Further returns of electronic tags will undoubtedly reveal more.

ConclusionsOur results show that re-analysing historical archives of

conventional tagging data can still provide an important insight into characteristic patterns of behaviour that are important to fisheries management. We have summarized our description of seasonal stock movements and important areas of cod congregation during the spawning season in Figure 8. By coupling such analyses with an insight into fine-scale behaviour and movements of individual cod tagged with electronic tags, it may be possible in the future to characterize behaviour quantitatively enough to provide information critical to the stock assessment and management process (as for Hunter et al., 2004). As such, our analysis provides important first steps towards the evaluation of the biological and management implications of cod movement between these two areas.

We would like to thank the many Cefas staff who have contributed to the Tagfish database over the years, both those who worked in the field and those who entered the recapture data. We would also like to than Hans Ratz for provision of the unpublished cod maturity data from ICES IBTS surveys. Julian Metcalfe, Mike Armstrong and Mike Pawson who all provided valuable comments on the manuscript. This work was funded by Defra under contract codes MF0158 and MF0154, and by the EU under contract code Q5RS-2002-00813 (CODYSSEY).

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Submitted 6 June 2006. Accepted 14 February 2007.

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