Review of potential impact to ecological receptors …...2016/05/27 · Pest control is an...

Review of potential impact to ecological receptors of the Stour Estuary Several Order Application

Wash Mussels Ltd

APEM Ref: 414003

June 2016

Rachel Antill, Dr Roger Buisson and Dr Rafael Pérez-Domínguez

Client: Wash Mussels Ltd

Address: 62 Burnham Road

Leigh-on-Sea

Essex, SS9 2JS

Project reference: 414003

Date of issue: May 2016

________________________

Project Director: Dr Rafael Pérez-Domínguez

Project Manager: Rachel Antill

Other:

________________________

APEM Ltd Riverview

A17 Embankment Business Park Heaton Mersey

Stockport SK4 3GN

Tel: 0161 442 8938 Fax: 0161 432 6083

Registered in England No. 2530851

This document should be cited as:

“Antill, R., Buisson, R. & Pérez-Domínguez, R. 2016. Review of potential impact to ecological receptors of the Stour Estuary Several Order Application. APEM Scientific Report for Wash Mussels Ltd. Ref. 414003., May 2016, Final, 45 pp.”

Registered in England No. 2530851, Registered Address Riverview A17 Embankment Business Park, Heaton Mersey, Stockport, SK4 3GN

Revision and Amendment Register

Version Number

Date Section(s) Summary of Changes Document by

Approved by

v1 22.09.15 ALL Creation of document RA RP-D

v2 27.10.15 ALL Review of comments and draft update

RP-D RP-D

v3 05.05.16 3.5 Addition of results of noise modelling

RA RB

Contents

1. Introduction ...................................................................................................................... 1

2. Project description ........................................................................................................... 1

2.1 Site Locations ........................................................................................................... 1

2.2 Activities associated with the proposed mussel relaying method ............................... 2

2.3 Timeline for the five year pilot study .......................................................................... 3

3. Response to stakeholder comments ................................................................................ 4

3.1 Effects on bass & mullet fishery ................................................................................ 4

3.2 Effects on carrying capacity (plankton and benthos) ................................................. 8

3.3 Effects on Peacock fan worm beds ........................................................................... 8

3.3.1 Site-specific information ................................................................................... 10

3.4 Effects of active predator and pest removal interventions on the general ecology of the estuary ......................................................................................................................... 10

3.5 Effects on birds, and baseline assessment of Special Protection Area (SPA) features 11

3.5.1 Updated and additional information on the proposed mussel fishery that affects the concerns expressed by the RSPB ............................................................................ 13

3.5.2 Baseline data on birds ..................................................................................... 13

3.5.3 The potential for the mussel fishery operations to cause disturbance .............. 16

3.5.4 The proposed mussel fishery operation in the context of other potentially disturbing activities in the Stour Estuary ......................................................................... 18

3.5.5 Effect levels for noise and vibration from mussel dredging activities ................ 19

3.5.6 Measurements of noise from mussel dredging activities .................................. 20

3.5.7 Scale of numbers of birds potentially disturbed by visual presence of the dredging activities .......................................................................................................... 21

3.5.8 Predicted spatial extent of disturbance within a WeBS sector / around a mussel bed: Visual presence ..................................................................................................... 22

3.5.9 Predicted spatial extent of disturbance within a WeBS sector / around an operating mussel dredger: Noise ................................................................................... 25

3.5.10 Conclusion ....................................................................................................... 26

4. References .................................................................................................................... 27

Appendices ........................................................................................................................... 30

Appendix 1 Stour Estuary WeBS ‘core’ count (high tide) 5 year mean peak counts for the winters 2009/10 to 2013/14 for the whole estuary and for each count sector ..................... 31

Appendix 2 Relative importance of the bird populations in WeBS count sectors compared to the whole Stour Estuary (for the winters 2009/10 to 2013/14) ............................................ 32

Appendix 3 Relative importance of the bird populations in WeBS count sectors compared to the whole Stour Estuary and the Stour and Orwell Estuaries SPA: For the Stour and Orwell Estuaries SPA (sector data from 2009/10 to 2013/14 compared to SPA populations at time of designation) ................................................................................................................... 32

Appendix 4 Literature reviewed to identify an appropriate ‘disturbance effect distance’. .... 34

Appendix 5 The length of the WeBS sector along the direction of the deep water channel in relation to the length (diameter) of the zone of potential disturbance. ................................ 36

Appendix 6 An alternative method of considering the proportion of estuarine habitat that is made temporarily unavailable through disturbance. ........................................................... 38

List of Figures

Figure 1 Proposed Mussel Relaying Sites in the River Stour................................................... 2

Figure 2 Environment Agency survey locations, 2009. ............................................................ 5

Figure 3 Length frequencies for fish caught during the Environment Agency surveys in the River Orwell and River Stour. Legends are given in the figure. ............................................... 6

Figure 4 Length frequencies for golden grey mullet caught during Environment Agency surveys in the River Orwell. ..................................................................................................... 7

Figure 5 Location of the 10 WeBS count sectors with the reference numbers used in this report. ................................................................................................................................... 14

List of Tables

Table 1 Location of seed mussel and half-grown mussel relaying sites. The area expected to be used for culture is restricted to 5 ha plot for all sites irrespective of the actual site size. All sites are subtidal. .................................................................................................................... 1

Table 2 Main activities required for mussel culture. The methods and equipment used for the different activities have been described in the in the TMSFO application. ............................... 2

Table 3 Timeline for the five year pilot study, W = winter, Sp = spring, Su = summer, A = autumn. Pest control is an exception measure that will only be used when required. .............. 3

Table 4 Length frequencies for sea bass caught during the Environment Agency surveys in the River Orwell, 2009. The stations are presented in order starting from the inner (left) to the outer Orwell estuary. ............................................................................................................... 5

Table 5 Length frequencies for sea bass caught during the Environment Agency surveys in the River Stour, 2009 .............................................................................................................. 6

Table 6 Bird species that are qualifying interest features of the Stour and Orwell Estuaries SPA and Ramsar site named in RSPB responses as requiring additional assessment. ........ 12

Table 7 WeBS core count sector names and reference numbers used in this report. ............ 14

Table 8 Proposed mussel beds matched to WeBS count sectors ......................................... 14

Table 9 WeBS ‘core’ count five year mean peak count 2009/10 to 2013/14 for the sectors where mussel beds are proposed and the whole Stour Estuary. ........................................... 15

Table 10 Relative importance of the populations of the relevant WeBS count sectors compared with the whole Stour Estuary using the most recent count data. ........................... 16

Table 11 Relative importance of the populations of the relevant WeBS count sectors compared with the Stour and Orwell Estuaries SPA populations at time of designation. ....... 16

Table 12 ‘Disturbance effect distance’ applied in the assessment and its derivation from the literature. ............................................................................................................................... 18

Table 13 Natural England guidance on noise affecting birds (Atkins, 2016). ......................... 19

Table 14 Guidance on effect levels for non-breeding waterbirds in estuaries (Cutts et al. 2013). ................................................................................................................................... 20

Table 15 Dredging noise levels at varying distances (from Table 5-1 Atkins 2016). .............. 21

Table 16 Species by species location for the worse case potential disturbance effect (largest and second largest peak counts). ......................................................................................... 21

Table 17 Estimated lengths of WeBS count sectors along the deep water channel. .............. 22

Table 18 Length of the potential disturbance zone in relation to the estimated lengths of WeBS count sectors along the deep water channel for each species. ................................... 23

Table 19 Assessment, based on worse case parameters, of the potential disturbance caused by the proposed boat activity. ................................................................................................ 23

Table 20 Bird numbers potentially affected in relation to the populations of the whole Stour Estuary and to the populations of the Stour and Orwell Estuaries SPA. ................................ 25

1. Introduction

This document has been prepared in response to consultation received in 2015 in relation to the application for a Trial Mussel Several Fishery Order (TMSFO) in the River Stour. An Environmental Statement (ES) was prepared to support this application in 2012 (Otto, 2012). Subsequently a Supplementary Environmental Statement was prepared in response to comments made on the information presented within the ES (Otto, 2013). This document provides a response to those comments made as part of the 2015 consultation that relate to ecological issues. A response to comments made in relation to the socioeconomic use of the area is provided separately (Franklin & Mowlam, 2015).

The main concerns raised during the 2015 consultation can be grouped into five main areas:

1. Effects on bass & mullet fishery; 2. Effects on plankton and carrying capacity; 3. Effects on Peacock fan worm beds; 4. Effects of active predator and pest removal interventions on the general ecology of

the estuary: crabs, starfish and slipper limpets; and 5. Effects on birds, and baseline assessment of Special Protection Area (SPA) features.

2. Project description

A description of the proposed mussel fishery was provided in the ES (Otto, 2012; Otto 2013). Since this time, further details regarding the fishery have been determined. This section sets out the details of the fishery that are relevant to the 2015 consultation and have not been previously presented within the ES. A more detailed description is given in the TMSFO application. The current application is for an initial five year pilot study period.

2.1 Site Locations

The application is for five plots covering a total of 84 hectares (ha) (Table 1 and Figure 1).

Table 1 Location of seed mussel and half-grown mussel relaying sites. The area expected to be used for culture is restricted to 5 ha plot for all sites irrespective of the actual site size. All sites are subtidal.

Site Name Site Size (ha)

Expected Lays cover

1: Erwarton Bay 34 Half-grown mussel: 50 tons Nett per hectare

2: Deep Fleet 12 Half-grown mussel: 50 tons Nett per hectare

3: Harkstead Point 18 Seed mussel: 25 tons Nett per hectare

4: Copperas Bay 10 Seed mussel: 25 tons Nett per hectare

5: North Shoal 10 Seed mussel: 25 tons Nett per hectare

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Figure 1 Proposed Mussel Relaying Sites in the River Stour

2.2 Activities associated with the proposed mussel relaying method

The application is based on best culture practices used by the operator and the industry. The full cycle of culture involves five basic activities (Table 2). Two plots, 1 and 2 have been identified as suitable for relaying half-grown mussels (see Table 1).

Table 2 Main activities required for mussel culture. The methods and equipment used for the different activities have been described in the in the TMSFO application.

Activity Description Expected scope & intensity

Site cleaning

A 5 ha plot within each of the five sites will be cleaned to meet the specific criteria set out within the environmental reports referred to in the order.

In the first year, initial site cleaning with take 1-5 days. Subsequent cleaning of harvested sites will take approximately 5-10 hour per hectare depending on the amount of pseudofaeces left after harvesting.

Relaying

Relaying occurs between August and December when there is seed mussel available and the weather conditions allow small boats to operate in the area. Relaying takes about 1 hour per load, each load is approximately 15-25 tonnes gross with the dredger moving at 3-5 knots. Each hectare requires approximately two loads of seed mussel and four loads of half-grown mussels

To relay seed mussels to all sites it will take approximately 15 visits with each visit lasting approximately an hour to relay approximately 300 tonnes of seed mussel*.

Relay two sites with half-grown mussels will require approximately 40 visits with each visit lasting approximately an hour. It is anticipated that approximately 500 tonnes of half-grown mussels may be available annually.

Monitoring

Every month, after relaying, onsite monitoring of growth and predation will be conducted taking approximately 0.5 hours per plot (2.5 hours in total)

During years 2 to 5 there will be approximately 20 monitoring visits a year which will sample using Day grabs for the Local Authority Environmental Health department. Each visit will take approximately 0.5 days.

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Predator Control

There will not be any control of crabs within the sites. If starfish predation significantly affects a trial site, starfish mopping will be conducted. This activity is only likely to be necessary during summer months.

This will involve daily dragging of mops across the plot (approximately 5 hours per plot travelling at 1.5 knots or less) repeated over 2-5 days depending on the quantity of starfish. If starfish mopping were required at all five plots, mopping operations would take between 50 and 125 hours in total.

Harvesting

Harvesting and selling of mussels takes place from September to the end of March. All mussels of marketable size will be harvested within this period each year.

Harvesting takes approximately 15 hours per hectare for fully grown mussels so a full 5 ha plot will require up to 75 hours of operations to harvest all stock with the dredger travelling at 2-3 knots.

* It is unlikely that more than 300 tonnes of seed mussel will be available in any one year

2.3 Timeline for the five year pilot study

The cycle for mussel pilot study is as follows:

• Year 1: seed laying and growing • Year 2: growing, possible harvest if started with half grown, possible relay of half

grown • Year 3: harvest, clean beds and relay – review data and alter strategy if necessary • Year 4: harvest, clean beds and relay – analyse key trends in different year classes • Year 5: harvest, clean beds and relay – review data and consider future operations

If enough seed is found and the trials progress well, then the operators plan to increase the number of 5 hectare plots used to seven in year 3 and to nine in year 5. A detailed timeline for each year of the pilot study is provided below with details of the likely activities during each period (Table 3).

Table 3 Timeline for the five year pilot study, W = winter, Sp = spring, Su = summer, A = autumn. Pest control is an exception measure that will only be used when required.

Year Year 2 Year 3 Year 4 Year 5 Season/ Task W Sp Su A W Sp Su A W Sp Su A W Sp Su A

Monitor Harvest

Clear and Relay

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3. Response to stakeholder comments

3.1 Effects on bass & mullet fishery

Concerns have been raised that fishermen will be excluded from fishing areas that overlap with the proposed mussel areas. The effects on the fishery are considered within a separate report (Franklin & Mowlam, 2015). The effects considered here are on the fishing resource: in particular the juvenile population of bass and mullet present within the River Stour.

In the Orwell Estuary, Colclough (2010) found bass nurseries along the estuary, with many nursery sites apparently present for most of the year. Data from six sites that have been fished in the Orwell Estuary since 2004 suggest that the whole estuary is currently a very important bass nursery ground, with the number of bass increasing significantly from none identified in a fish monitoring survey conducted in 1999 (Dyer, 2005) to many found in a survey conducted between December 2003 to 2004 (Ashelby, 2005) and a prevalent feature of the estuary in recent times (EA, unpublished data; Natural England, 2011). In addition, the Environment Agency considers the almost permanent presence of juvenile bass here to be unprecedented amongst British estuaries (EA, unpublished data; Natural England, 2011).

The data used to inform this assessment have been obtained from the Environment Agency surveys in the Rivers Orwell and Stour as part of the Water Framework Directive (WFD) compliance monitoring for biological quality element (BQE) fish in transitional and coastal (TraC) waters. These surveys use a multi gear sampling method based on fine mesh seine nets, fyke nets and beam trawls, therefore providing a robust cross section of the resident fish assemblage and in particular fish juveniles.

The locations sampled are shown in Figure 2. These survey data show high numbers of sea bass were caught within the River Orwell that are small (less than 20 cm long) indicating the fish are using the area as a nursery (Table 4 and Figure 3). The survey data for the River Stour recorded smaller numbers of larger fish, all of which were greater than 16 cm or longer (Table 5). These fish are slightly larger than those recorded in the River Orwell indicating the fish are probably using different areas as they grow older during the nursery phase before becoming fully mature. The data are consistent with the view that the Orwell supports a greater number of smaller young of the year (YOY) fish than the Stour (Figure 3). Only one fish caught during the Environment Agency survey was bigger than the current minimum landing size of 42 cm. All other fish caught during this survey were too small to be legally landed.

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Figure 2 Environment Agency survey locations, 2009.

Table 4 Length frequencies for sea bass caught during the Environment Agency surveys in the River Orwell, 2009. The stations are presented in order starting from the inner (left) to the outer Orwell estuary.

Length (mm) Pipers Vale & O4 Pinmill & O3 Colimer Trimley & O2 Total 80-100 22 18 40 101-120 12 33 45 121-140 1 1 141-160 2 5 2 4 13 161-180 3 15 7 9 34 181-200 2 8 1 11 201-220 1 2 3 221-240 1 1 241-260 1 1 2 261-280 2 2

Total 45 80 11 16 152

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Table 5 Length frequencies for sea bass caught during the Environment Agency surveys in the River Stour, 2009

Length (mm) Dovehouse Harkstead Point & S2 Erwarton Ness & S1 Total Close to Mussel Plot: 5: North Shoal 3: Harkstead Point 2: Deep Fleet

80-100 101-120 121-140 141-160 161-180 1 5 8 14 181-200 9 11 20 201-220

1 1

221-240 1 3 4 241-260 1 4 5 261-280 1 1 280-300 301-320 321-340 341-360 1 1 361-380 1 1 381-400 401-420 421-440 441-460 461-480 1 1

Total 1 17 30 48

Figure 3 Length frequencies for fish caught during the Environment Agency surveys in the River Orwell and River Stour. Legends are given in the figure.

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Whilst the majority of bass caught during surveys by the Environment Agency were juveniles, there are adult bass within the River Stour that are targeted by fishermen. The River Orwell appears to be the primary nursery location for YOY, particularly further upstream in the River Orwell (as evidenced by the high number of smaller fish recorded at Pipers Vale and Pinmill). However, the Stour and the proposed mussel plot areas may support small numbers of bass YOY and older juveniles.

The YOY are likely to be temporarily displaced from the mussel plot areas during some mussel operation activities such as site cleaning, however, the YOY are expected to return to the area once the activity ceases. Although displacement may have consequence for the local fish the mussel lays are likely to act as a reef, providing shelter for YOY and older juveniles and reducing predation mortality. This is assessed as beneficial for sea bass in the area and together with enhanced feeding opportunities within the mussel bed. Sea bass is an opportunistic predator that feeds of a wide range of potential prey items including benthic decapod and fish that are often associated with reef areas. The formation of a mussel ‘reef’ may enhance feeding opportunities for this species as both crab populations and populations of small fish associated with reef may increase as a result of the fishery.

Mullet is also caught within the River Stour and have been recorded within the River Orwell by the same WFD TraC monitoring surveys (Figure 4). Mullet feed on zoobenthos and detritus and will not be affected by changes in the benthic habitat to a mussel biotope. Furthermore, the species is likely to benefit from available shelter and food supply.

Figure 4 Length frequencies for golden grey mullet caught during Environment Agency surveys in the River Orwell.

We anticipate that any direct effects on bass or mullet juvenile populations within the River Stour will be minor. Large adult bass and mullet, as targeted by commercial fishermen and prized by anglers, may be indirectly affected by the mussel fishery as the fish may be temporarily displaced from mussel plots during mussel fishery operations and by disturbance to the supporting habitat. However, the effect of displacement will be temporary and localised, affecting a small proportion of the River Stour habitat. It is unlikely that these effects will cause a measurable change in the population of sea bass and mullet within the

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River Stour. Moreover, beneficial effect of the increased complexity and biodiversity associated with mussel biotopes will add to the nursery-type ecological services of the Orwel-Stour systems.

3.2 Effects on carrying capacity (plankton and benthos)

There have been some concerns raised that the addition of new mussel populations to the River Stour might exceed the carrying capacity of the river to support the filter feeding invertebrate community elsewhere in the estuary. However, as set out within the Environmental Statement (Otto, 2012), the river can support 84 ha of additional mussels without affecting other filter feeding species. The current application is initially for 25 ha, increasing to a total of 45 ha which is well within the available carrying capacity of the estuary. The proposed initial relay density will not exceed 2.5 kg/m2 (25 tonnes/ha), giving a theoretical annual yield of 1 kg/m2 (10 tonnes/ha). Assuming a transfer efficiency of 10% (only 10% of the seston1 consumed is converted into mussel biomass), each hectare of mussel bed will consume approximately 10 kg/m2 (100 tonnes/ha) of seston in a year. Estimates of primary production in estuaries vary but for temperate areas it is likely to be in the range of 165 to 350 gr C m2 year (Underwood and Kromkamp 1999). Considering that the carbon content of organic matter is about 50% by weight (a conservative figure for aquatic systems), the natural production per hectare would approximately be between 0.33 and 0.70 kg/m2 (3.3 to 7.0 tonnes/ha). Given this level of productivity and the effect of the tide in estuarine environments and in the Stour in particular, it is likely that the level of consumption calculated above will not deplete the seston within the area of influence of the mussel beds as initially concluded by Otto (2012).

Any filter feeding invertebrates within the mussel beds itself may be outcompeted by the mussels. However, some species may settle and grow on the beds that are tolerant of this effect. As stated within the Environmental Statement (Otto, 2012) the effects of mussel beds have not been recorded beyond 10 m of the beds (Beadman et al, 2004) other studies report localised and variable effects according to type of gear, type of substrata (Kaiser et al 2003) and sensitivity of the species affected (MacDonald et al 1996). Due to the tidal nature of the system and density of the lays it should not be any significant effects on the River Stour’s invertebrate community and effects if any will be localised to the immediate boundary, typically 10 m or less of each mussel plot.

3.3 Effects on Peacock fan worm beds

Sessile filter feeding organisms are part of the Stour benthic community. There are particularly sensitive to direct effects such as abrasion caused by towed fishing gear and smothering (Kaiser et al 2003). These effects were evaluated in the original ES and assessed as unlikely to cause a significant effect on Peacock Fan worm Sabella pavonina beds. The assessment was based on recorded distribution of existing beds and survey data

1 Seston is minute material moving in water and including both living organisms (such as plankton, resuspended microphytobenthos and nekton) and non-living matter (such as plant debris or suspended soil particles).

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available at the time of the ES. The scoping identified peacock fan worm beds within the possible area of influence in plots 1, 2 and 3 but with low confidence resulting from the age of existing records (Worsfold 2005). This paucity of contemporary data makes difficult to assess potential effect with precision and a dedicated survey has been recommended.

Natural England advice of 2nd April 2015 states:

In relation to monitoring we would like to stress the need to conduct an initial drop-down video prior to any operations affecting the plots, to discuss implications of any. This is particularly important as the report used to inform locations of areas devoid of any SSSI features dates back to 2005. If there are indications of features being present, the area should be excluded as stated in the Supplementary ES (p.17). Natural England would welcome further discussions with the applicant should features be present. Please also note our comments regarding the timing of receiving survey results.

Natural England’s advice of 14th July 2015 further states:

Additionally, we would like to clarify that pre-works surveys should not only serve to identify presence and to avoid features of the SSSI but also any habitats and species of principal importance for the purpose of conserving biodiversity as listed in Section 41 of the NERC Act (2006). Specifically, this includes both the Peacock Fan worm Sabella pavonina, a typical species of the habitat Sheltered muddy gravels, and native oyster Ostrea edulis. Provisions under the NERC Act (2006) require that every public authority must, in exercising its functions, have regard to the purpose of conserving biodiversity and we therefore advise that habitats and species of conservation importance are avoided where possible.

Concerns have also been raised by the Eastern and Kent and Essex IFCAs and Natural England that the Trial Mussel Several Fishery Order (TMSFO) may have an impact on the peacock fan worm Sabella pavonina. Effects (direct and indirect) on the peacock fan worm may include:

• direct mortality caused by abrasion • increased competition for food – this effect is discussed above with regard to carrying

capacity • smothering of any peacock fan worms immediately adjacent to the plots – during

dredging or from pseudofaeces • turbidity/increased suspended sediments • nutrient levels increased but competition for nutrients increased • lower oxygen levels

Peacock fan worms immediately adjacent to but not within mussel plots have the potential to be smothered by settling sediment following dredging of plots and by fine sediments and/or increased organic loads produced by the mussels (faeces and pseudofaeces). This effect may occur within 10 m of the mussel bed with effects at larger scales (10-100 m) difficult to detect against natural variability (Beadman et al, 2004). It should be noted that peacock fan worms may already be affected by smothering as a result of regular maintenance dredging of the channels, marinas and ports within the River Stour and its estuary. Cumulative effects are likely. Dredging at Harwich Navyard, Quay and International Port takes place every 10-12 weeks whereas dredging further up the river occurs much less frequently (Harwich Haven Authority, 2012). As the dredged material is primarily silt, the plumes created during dredging can be large and is likely to have a much greater effect on peacock fan worms than the plume caused by working the mussel beds. The increase in turbidity and suspended

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sediments over normal background levels as a result of dredging to clear the beds, and from the production of fine sediments by the mussels is also likely to be limited to within 10 m of the mussel bed. Existing effects due to port dredging may be act cumulatively with sediment plumes originating from the mussel bed dredging within 10 m of the mussel beds. As mussel plots will not be cover more than 5 hectare per site and will be sited more than 10 m from any identified peacock fan worms, the potential impact from an increase in turbidity and suspended sediments is likely to be minimal.

Dredging of the mussel bed will release nutrients from the mussel mud into the water column. The nutrient-rich waters will mix with surrounding water and quickly disperse. The effect will be temporary and is unlikely to cause an effect on peacock fan worms.

During initial site prospecting, mussel plots will be sited so that each plot is more than 10 m from any peacock fan worms where possible. If a 10 m separation cannot be achieved, the size of the bed will be reduced. A buffer zone could be agreed with the Regulator after an initial characterisation survey.

3.3.1 Site-specific information

The assessment will benefit from contemporary data on the distribution of the peacock fan worm beds in or around proposed lays. The initial survey will be targeted using historical data from the intertidal zone provided by the Eastern IFCA (from 2003 onwards), earlier evidence (Worsfold 2005), and a preliminary proposal for the location of the mussel lays. It is advised to use side scan sonar to map the beds and the use drop down video (DDV) to ground truth the side scan sonar data and identify the presence of any peacock fan worms within the possible area of influence in plots 1, 2 and 3. A video-based survey may be hampered by low visibility in the estuary but it is proposed due to its non destructive nature. It will also provide, if successful, a permanent record of the biotope present on infralittoral mixed sediment adjacent to the proposed lays.

In addition, a limited towed dredge survey is recommended within the proposed plots and an agreed buffer zone to complement the video data and provide additional evidence on native oysters (Ostrea edulis) present. To prevent further damage to the seabed and to sensitive habitats the towed dredge survey should be combined with the exploratory works planned to assess the grounds for suitable lays. After the initial characterisation, it is not advisable to collect further dredge samples from outside lays 1, 2 and 3 during the routine on-going monitoring of mussel growth or by-catch monitoring. This approach will prevent direct effects on peacock fan worms present in areas expected to be outside the influence of any mussel plots.

The need, scope and timing of surveys will be discussed with Natural England and the IFCAs in advance of the commencement of mussel fishery operations.

3.4 Effects of active predator and pest removal interventions on the general ecology of the estuary

Removal of pest animals could result in directly effect on the ecology of the area through effects on non-pest organisms (i.e. by-catch by pest control methods) or effect on the supporting habitats (abrasion and alteration of benthic habitats). Naturally occurring mussel biotopes are characterised by a diverse assemblage and therefore of biodiversity value. The structure provided by stable mussel shell aggregations increases protection, available

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substrate for attachments to a diversity of flora and fauna, and feeding opportunities. Increased biodiversity in the area depends in large part on the resilience of the mussels themselves to natural levels of predation. Seed and half-grown mussels in monospecific culture lays are vulnerable to a number of predators. Sea stars/starfish and crabs as well as some fish and other molluscs are likely to be attracted to mussel lays. A degree of natural resilience is expected but in particular during the early stages of the newly laid seed plots, mussels are very vulnerable to predators (Elner 1978, Saier 2001). High numbers of mussels combined with a high vulnerability during the initial culture could, if not properly managed, result in the increase of normal predation levels to unsustainable pest-like levels, thereby upsetting a healthy predator-prey balance and destroying the entire mussel bed.

Otto (2012) identified starfish (Asterias rubens), brown crab (Cancer pagurus) and velvet crab (Necora puber) as potential pest organisms of relevance to this application and identified suitable pest control measures. Concerns have been raised with regard to predation controls for the TMSFO and potential impacts on the general ecology of the estuary. Since the publication of the Environmental Statement (Otto, 2012), the subsequent addendum (Otto 2013), and initial application for the TMSFO, the applicant has decided not to conduct predation control for crabs and so there will not be any effects from this activity. Removal of starfish will only occur if the starfish population levels within the mussel plots are sufficiently elevated to cause predation problems to the mussel fishery i.e. starfish removal will only take place when the overall ecology is out of a natural balance. This removal and extraordinary measure in itself, is expected to occur infrequently and only within the mussel plots (five 5 ha areas).

Concerns have been raised that the removal of starfish will removal prey species for birds. Starfish are not a key prey species for birds and are only likely to be scavenged by gulls at the strandline, not taken from within the deeper areas where the mussel beds will be located.

Concerns have been raised that regular harvesting of mussels may increase opportunities for more individuals of opportunistic and invasive species, such as slipper limpets Crepidula fornicata, to colonise the cleared areas. Slipper limpets will be cleared from the mussel plots are part of the site cleaning process. They will be separated from the mussels dredged from the area and taken to landfill. The intention is for mussel seed or half-grown mussels to be relayed to cleaned plots as soon as possible after cleaning to allow mussel beds to form before competitors such as slipper limpets become established. The intensity (five 5 ha plots) and frequency (usually every 12-18 months) of this activity mean that that it is unlikely that the population of invasive slipper limpets will increase.

Each mussel plot area is expected to be cleaned every 12-18 months depending on how fast the mussels grow and are harvested from the site. Husbandry of the site is not expected to be regularly required, however, if during monitoring of mussel beds the population of slipper limpets becomes a problem, additional cleaning may be required to remove mussels of epiphytes and to remove the beds of predators.

3.5 Effects on birds, and baseline assessment of Special Protection Area (SPA) features

This section of the report supplements technical work already carried out (Otto, 2012; Otto, 2013) to asses the potential effects on birds of the proposed trial mussel fishery in the Stour Estuary. It addresses, in response to consultation feedback from the RSPB (2015a, b & c),


potential impacts on a number of ‘open water’ and fish-eating bird species that were not assessed in the technical work that has already been carried out.

The responses made by the RSPB (2015a, b & c) following consultation over the proposed trial mussel fishery identified concern over potential impacts on a number of bird species that are qualifying interest features of the Stour and Orwell Estuaries SPA and the Stour and Orwell Estuaries Ramsar site (Table 6). The RSPB described these as either:

‘open water’ species - dark-bellied brent goose Branta bernicla bernicla, shelduck Tadorna tadorna, wigeon Anas penelope, goldeneye Bucephala clangula, great crested grebe Podiceps cristatus and cormorant Phalacrocorax carbo (Paragraph 4.6 of RSPB, 2015b), or

‘fish-eating’ species - red-breasted merganser Mergus serrator, great crested grebe and Slavonian grebe Podiceps auritus (Paragraph 1 of RSPB, 2015c).

The RSPB considered that these species could potentially be disturbed by boats operating the proposed mussel fishery.

Table 6 Bird species that are qualifying interest features of the Stour and Orwell Estuaries SPA and Ramsar site named in RSPB responses as requiring additional assessment.

Species Status in SPA designation

(English Nature, 2005)

Status in Ramsar site designation

(JNCC, 2008) Dark-bellied brent goose

Qualifies under Article 4.2 as occurring at levels of international importance

Qualifies under Criterion 6 as occurring at levels of international importance

Shelduck Qualifies under Article 4.2 as part of an assemblage of international importance

Qualifies under Criterion 5 as part of an assemblage of international importance and at levels of national importance

Wigeon Qualifies under Article 4.2 as part of an assemblage of international importance

Qualifies under Criterion 5 as part of an assemblage of international importance

Goldeneye Qualifies under Article 4.2 as part of an assemblage of international importance


Red-breasted merganser

Qualifies under Article 4.2 as part of an assemblage of international importance


Great crested grebe Qualifies under Article 4.2 as part of an assemblage of international importance


Slavonian grebe Qualifies under Article 4.2 as part of an assemblage of international importance


Cormorant Qualifies under Article 4.2 as part of an assemblage of international importance



3.5.1 Updated and additional information on the proposed mussel fishery that affects the concerns expressed by the RSPB

Since the RSPB received information about the proposed mussel fishery and its potential environmental impacts, further information has been assembled by the applicant to define the operations that will be carried out in the Stour Estuary. This includes the following elements that all have a bearing on assessing the potential of the proposed operation to affect bird populations about which the RSPB has expressed concern, including the group of ‘open water’ and fish-eating birds:

• Boat operations will only take place around high water and as a result disturbance to intertidal feeding waders and wildfowl when the estuarine flats are exposed will be avoided.

• The primary, regular boat operations over and around the mussel beds will take place at low speed: Relaying at less than 5 knots Harvesting at less than 3 knots Starfish ‘mopping’ at less than 2 knots

• A rigid inflatable boat (RIB) that travels at higher speed will be used for the shellfish monitoring work. This takes place only once a month, around a single high tide and with a duration of no more 30 minutes per plot.

• No more than two boats will be operating over and around the mussel beds on any one day and these will operate on two separate mussel beds.

• In the ‘winter’ months of October to March inclusive the number of days per month that there will be one or two boats operating (excluding the once per month use of a RIB for monitoring)

• The number and area of plots, if the trial progresses well, will be: 5 x 5 ha in years 1 and 2 7 x 5 ha in years 3 and 4 9 x 5 ha in year 5

3.5.2 Baseline data on birds

Baseline data on non-breeding birds using the Stour Estuary over high tide periods has been obtained from the British Trust for Ornithology (BTO) in the form of Wetland Bird Survey2 (WeBS) ‘core’ counts. These ‘core’ counts are monthly surveys carried out by volunteers at high tide, with larger sites such as the Stour Estuary divided in to a series of count sectors.

2 The Wetland Bird Survey (WeBS) is a partnership between the British Trust for Ornithology, the Royal Society for the Protection of Birds and the Joint Nature Conservation Committee (the last on behalf of the statutory nature conservation bodies: Natural England, Natural Resources Wales and Scottish Natural Heritage and the Department of the Environment Northern Ireland) in association with the Wildfowl and Wetlands Trust.


The Stour Estuary is divided in to 10 count sectors and WeBS ‘core’ count data was obtained for all 10 sectors for the five most recent winters available - 2009/10 to 2013/14. The sectors are named as set out Table 7, with a sector number applied for the purposes of reference within this report. The location of the 10 sectors is illustrated in Figure 5.

Table 7 WeBS core count sector names and reference numbers used in this report.

WeBS count sector name Reference number Bathside Bay 1 Erwarton Bay 2 Deep Fleet 3 Holbrook Bay East 4 Copperas Bay 5 Stutton to Holbrook Bay West 6 Jaques Bay to Mistley East 7 Seafield Bay 8 Manningtree 9 Cattawade Marshes 10

Figure 5 Location of the 10 WeBS count sectors with the reference numbers used in this report.

The locations of the WeBS count sectors can be related to the location of the proposed mussel beds (illustrated on Figure 1). This is set out in Table 8.

Table 8 Proposed mussel beds matched to WeBS count sectors

Proposed mussel bed WeBS count sector name Reference number Site 1: Erwarton Bay Erwarton Bay 2 Site 2: Deep Fleet Deep Fleet 3 Site 3: Harkstead Point Holbrook Bay East 4 Site 4: Copperas Bay Copperas Bay 5 Site 5: North Shoal Stutton to Holbrook Bay

West 6


For each of these WeBS count sectors a five year mean peak count has been calculated for each of the ‘open water’ and fish-eating species identified by the RSPB. This five year mean peak is based on the data for the winters 2009/10 to 2013/14, the most recent that is available. The five year mean peak has been used as the key statistic in this assessment because it is a good measure of the relative importance of sectors within a site, providing a good indication of how many individuals of a given species a count sector can support (Austin and Ross-Smith, 2014). The five year mean peak for each of the relevant species and for the WeBS count sectors where mussel beds are proposed is presented in Table 9 along with the five year mean peak for the whole of the Stour Estuary. The five year mean peak for each of the relevant species for all sectors in the Stour Estuary is provided in Appendix 1.

Table 9 WeBS ‘core’ count five year mean peak count 2009/10 to 2013/14 for the sectors where mussel beds are proposed and the whole Stour Estuary.

Species Stour Estuary

Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Proposed mussel bed Site 1 Site 2 Site 3 Site 4 Site 5 Brent Goose 2,276 629 827 292 198 627 Shelduck 1,816 85 273 52 100 164 Wigeon 3,280 608 627 123 153 360 Goldeneye 191 0 1 0 17 61 Red-breasted Merganser

130 4 6 15 42 58

Great Crested Grebe 349 11 4 36 15 77 Slavonian Grebe 8 0 0 0 1 7 Cormorant 121 12 13 12 11 5

The relative importance of each WeBS count sector where mussel beds are proposed can be identified by comparing the five year mean peak of a particular sector with the whole site (either Stour Estuary alone or Stour and Orwell Estuaries SPA) and expressing this as a percentage. This measure is presented in Table 10 for the relevant WeBS count sectors compared with the whole Stour Estuary using the most recent count data and in Table 11 for the relevant sector compared with the populations for the Stour and Orwell Estuaries SPA as stated on the relevant designation document (English Nature, 2005). The same comparisons for all WeBS count sectors in the Stour Estuary are provided in Appendix 2. The information in Table 10 and Table 11 is presented using the method recommended by the BTO for the analysis of WeBS data (Austin and Ross-Smith 2014) – a comparison of sector five-winter mean of peaks with whole site five-winter mean of peaks with the relative importance highlighted based on whether a sector contains >20% of the whole site population or 10-20% of the whole site population (shaded dark blue or light blue respectively).


Table 10 Relative importance of the populations of the relevant WeBS count sectors compared with the whole Stour Estuary using the most recent count data.

Species Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Proposed mussel bed Site 1 Site 2 Site 3 Site 4 Site 5 Brent Goose 28% 36% 13% 9% 28% Shelduck 5% 15% 3% 6% 9% Wigeon 19% 19% 4% 5% 11% Goldeneye 0% 1% 0% 9% 32% Red-breasted Merganser 3% 5% 12% 32% 45% Great Crested Grebe 3% 1% 10% 4% 22% Slavonian Grebe 0% 0% 0% 13% 88% Cormorant 10% 11% 10% 9% 4%

Table 11 Relative importance of the populations of the relevant WeBS count sectors compared with the Stour and Orwell Estuaries SPA populations at time of designation.

Species Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Proposed mussel bed Site 1 Site 2 Site 3 Site 4 Site 5 Brent Goose 24% 31% 11% 8% 24% Shelduck 3% 9% 2% 3% 6% Wigeon 15% 16% 3% 4% 9% Goldeneye 0% 0% 0% 8% 29% Red-breasted Merganser3 Great Crested Grebe 4% 2% 15% 6% 31% Slavonian Grebe4 Cormorant 5% 6% 5% 5% 2%

3.5.3 The potential for the mussel fishery operations to cause disturbance

The main operation of the mussel fishery that can give rise to disturbance is the boat activity related to the relaying, maintenance and harvest of the mussels. The once a month monitoring of the mussel beds is considered too infrequent and of too short duration to give rise to potentially significant disturbance effects.

The nature and scale of the proposed boat activity is described in the Project description above. The key factors in relation to the potential for disturbance to the ‘open water’ and fish eating bird species are:

• Operations occur over the period October to March when peak numbers of the ‘open water’ and fish-eating species are present.

3 The population of red-breasted merganser, an assemblage species, is not stated in English Nature 2005. 4 The population of Slavonian grebe, an assemblage species, is not stated in English Nature 2005.


• Operations only occur around the high tide period. • Operations are carried out by a boat that moves at no more than 5 knots within and

between mussel beds. • No more than two boats will operate at any one time and each will operate on a

separate mussel bed.

A review has been undertaken of the literature to identify evidence to support an assessment of the potential impacts of the proposed boat activity including existing guidance on disturbance management (e.g. Cutts et al., 2013; Kirby et al., 2004), existing reviews of waterbird disturbance (e.g. Cutts et al., 2009; Ruddock and Whitfield, 2007; Smit and Visser 1993) and individual technical studies, including those undertaken on the Stour Estuary (e.g. Ravenscroft et al., 2007).

This has identified that an assessment using a ‘disturbance effect distance’ is the most appropriate technique given the information currently available about the proposed operation and the birds that are the focus of the assessment. It is recognised that applying a measure of the distance at which disturbance is observed, expressed as the ‘escape flight distance’ (EFD) or ‘flight initiation distance’ (FID) has its limitations. In particular the meaning of the distance at which a bird responds can be interpreted in a contradictory fashion (Gill et al., 2001). One interpretation being that birds which allow a close approach are less affected than those which fly when the source of disturbance is at a greater distance. An opposite interpretation is that birds which allow a close approach are already under pressure to continue feeding or conserve energy by remaining at that location and are forced to tolerate a greater predation risk (the source of disturbance being equated to a predator). A further interpretation is that a bird which shows a response when a potential source of disturbance is distant may do so because it has alternative feeding or roosting sites to move to that are of equal utility, albeit it expends energy to fly there. It is considered reasonable to assume though that causing a bird to cease feeding and take flight or swim rapidly away does result in the expenditure of additional energy and that this is not of benefit to the bird compared to it remaining feeding on site.

The ‘disturbance effect distance’ that has been identified for each of the species is listed in Table 12. That process of identification of the distance has included accounting for relevant circumstances when assessing the utility of the study (was it boat or land based activities that were studied, was it estuarine or freshwater habitats that were studied) and accounting for where the study identified a range with confidence intervals (rather than a simple mean or maximum), in which case the upper 95% confidence level (the greater distance) has been selected in order to take a precautionary approach to effects on birds that are SPA interest features. Appendix 4 lists the individual studies that were examined and the ‘disturbance effect distance’ that they identified. It is also recognised that not all species respond by flying away, grebes in particular usually swim away and for these species the identification of the ‘disturbance effect distance’ accounts for such a behavioural response.


Table 12 ‘Disturbance effect distance’ applied in the assessment and its derivation from the literature.

Species Disturbance effect

distance (m)

Primary source Other relevant literature

Dark-bellied brent Goose

400 Laursen et al. 2005 Smit & Visser 1993; Liley & Fearnley 2011; Liley et al. 2011; Cutts et al. 2013

Shelduck 250 Laursen et al. 2005 Smit & Visser 1993; Cutts et al. 2009; Liley & Fearnley 2011; Liley et al. 2011; Cutts et al. 2013

Wigeon 300 Laursen et al. 2005 Madsen 1998; Bregnballe et al. 2009; Liley & Fearnley 2011; Liley et al. 2011

Goldeneye 300 Nowers pers. obs. in litt.

Hume 1976


65 Holloway 1997 Liley et al. 2011

Great crested grebe

150 Cooke 1987 Tydeman 1978

Slavonian grebe 50 Liley et al. 2011 Ruddock & Whitfield 2007 Cormorant 200 Bregnballe et al. 2009

The review has also identified that fast and unpredictably moving vessels such as speedboats, windsurfers and jet-skis (personal watercraft) often result in a greater response from waterbirds than slowly and steadily moving vessels (e.g. Platteeuw and Henkens 1997).

3.5.4 The proposed mussel fishery operation in the context of other potentially disturbing activities in the Stour Estuary

An overview of all potentially disturbing activities in the Stour and Orwell estuaries and the related management measures that could be taken to manage them is provided in the Scheme of Management for the two estuaries (SCHU, 2010).

The detailed study of disturbing activities in the Stour and Orwell estuaries (Ravenscroft et al., 2007) identified that walkers and those with dogs caused the greatest proportion of recorded disturbance to birds and that loud noises and fast moving objects had a disproportionate effect on birds. Specifically on boats, it was noted that powerboats caused a relatively large amount of disturbance for the combined reason of their speed, noise and that with a shallow draft they could approach the shore closely at high tide when they were most disruptive. The largest proportion of disturbance records noted as produced by boats was caused by the wash from large vessels as it ran over the mudflats at low tide. Slow moving and quiet vessels (e.g. sailing boats) caused very little disturbance. In this study the main species reported on were waders and wildfowl and of the eight species of interest to this assessment, records related to dark-bellied brent goose, shelduck and wigeon (i.e. no reported disturbance relating to goldeneye, grebes, red-breasted merganser and cormorant, although this is potentially an artefact of the study method).


3.5.5 Effect levels for noise and vibration from mussel dredging activities

A sound and noise assessment was undertaken by Atkins Acoustics, Noise and Vibration in relation to the current proposal to dredge mussels in the Stour Estuary (Atkins 2016). This study was commissioned following concerns raised by stakeholders about the effects of noise from dredging activity on non-breeding waterbirds in the estuary. Birds are considered to have a similar hearing range to humans and so a weighted sound level to represent the response of the human hearing mechanism to sound was used for all calculations (A-weighting). Atkins (2016) applied an assessment criteria for non-breeding waterbirds in the estuary that was based upon criteria developed to assess effects on humans (National Planning Practices Guidance: Noise 2013) and adapted for observed bird behaviour. The main criteria used were:

• Lowest Observable Adverse Effect Level (LOAEL) defined as the point at which noise causes birds to change behaviour, e.g. birds look up and look around for the source of a sound; and

• Significant Observed Adverse Effect Level (SOAEL) defined as the threshold at which noise either causes birds to take flight or move to alternative wading grounds.

Atkins used advice provided by Natural England and the Environment Agency based on documentary evidence from other similar projects to determine noise level criteria. These are reproduced in Table 13.

Table 13 Natural England guidance on noise affecting birds (Atkins, 2016).

Noise Level (dB) Response in Birds < 55 No effect in flight response in waterfowl

55 - 84 ‘Heads-up’ anxiety displays by wildfowl >84 Flight response in waterfowl

Natural England did not define the parameters for the noise levels and they were assumed in Atkins (2016) to relate to the LAmax

5 or similar measurement.

Threshold criteria for effect levels on non-breeding waterbirds have been identified in a study initially focused on the Humber Estuary (Cutts et al., 2009) and then developed for wider application to estuaries used by non-breeding waterbirds (Cutts et al., 2013) as the Waterbird Disturbance Mitigation Toolkit http://www.tide-toolbox.eu/tidetools/waterbird_disturbance_mitigation_toolkit/. Cutts et al. (2013) suggest a generic and precautionary no response level of 55 dB(A) at the location of the bird and a more evidence based threshold for observed responses of 70 dB(A) at the location of the bird (describing up to 70 dB(A) as an “acceptable ‘dose’ level” where between 55 and 70 dB(A) such a noise level “might occasionally induce a low level behavioural response such as a heads up”). Sound pressure levels of 85 dB(A) and above at the location of the bird are described in Cutts et al. (2013) as “a flight response is almost certain to occur”. Further refinement is given with a ranking of noise level effects as set out in Table 14 and the identification of brent goose, curlew and redshank as particularly sensitive and shelduck and bar-tailed godwit as more sensitive than average.

5 This is the maximum sound level recorded of the A-Weighted Sound Pressure Level recorded over a period of time. Details on how the noise levels have been derived or what noise index they represent were not provided.


http://www.tide-toolbox.eu/tidetools/waterbird_disturbance_mitigation_toolkit/

http://www.tide-toolbox.eu/tidetools/waterbird_disturbance_mitigation_toolkit/

Table 14 Guidance on effect levels for non-breeding waterbirds in estuaries (Cutts et al. 2013).

Level of effect Sudden noise event (dB(A) at receptor)

Continuous or regular noise event

(dB(A) at receptor) High 60 72 Moderate 55 60-72 Low <55 55-72

The text of Cutts et al. (2013) and the threshold levels identified with cut-offs at 55 dB(A) and 85 dB(A) strongly suggest that Natural England and the Environment Agency have based their advice to Atkins on the evidence based, technical content of Cutts et al. (2013).

Studies by Reijnen et al. (1995, 1996) as listed in Atkins (2016) on continuous road noise impacts on a variety of grassland and marshland bird species found “increasing severity of effect with increasing traffic and noise levels… measured in density of breeding birds. Data shown for the extremes in sensitivity for woodland and grassland/marshland species and mean community levels of 43 dB(A) and 47 dB(A) respectively.” Such continuous road noise effect levels are unlikely to be applicable to non-breeding waterbirds of estuarine habitats and are not applied here.

3.5.6 Measurements of noise from mussel dredging activities

Atkins conducted two sound surveys:

1. An operational noise survey on board an existing Wash Mussels’ boat operating out of Ramsgate approximately 0.6 nautical miles offshore (1.1 km) with a second logger placed on the shore line at Royal Harbour Approach. Noise levels were monitored in octave bands and full audio recordings were made.

2. A background noise survey was performed around the Stour Estuary covering a time period representative of the proposed operations. A logger was placed on Wrabness Point close to Wrabness Sluice for a period of 8 hours. This location is considered representative of dredging areas 4 and 5. Additional sample measurements were taken at Shotley Gate, approximately 100 m west of Bristol Hill, taken to be representative of areas 1 and 2, and on the shore at the end of Shore Lane, close to areas 3 and 4. This survey was used to establish the lowest background sound levels used in the assessment.

The noise levels measured on board the dredger were used to calculate the noise levels at varying distance from the boat (see Table 5-1 in Atkins 2016). Based on the thresholds of Cutts et al. (2013) and the advice of Natural England and the Environment Agency the important predictions are for the distance when the noise level falls below 85 dB(A), 70 dB(A) and 55 dB(A). The first three rows of Table 5-1 are presented below in Table 15 as that provides the relevant information needed for the assessment, where the units of LAeq dB are relevant to the ‘continuous or regular noise event’ of Cutts et al. (2013) and the units of LAmax, fast dB relevant to the ‘sudden noise event’.


Table 15 Dredging noise levels at varying distances (from Table 5-1 Atkins 2016).

Dredge distance, m LAeq dB LAmax, fast dB 100 55.9 63.0 200 49.9 57.0 300 46.3 53.5

This indicates that noise levels (both regular and sudden) fall within 100 m to below levels at which birds would be both predicted to take flight and be subject to a level greater than an “acceptable ‘dose’ level” (Cutts et al., (2013). The no effect level identified by Natural England and the Environment Agency occurs at just over 100 m for regular sounds and around 250 m for sudden sounds.

3.5.7 Scale of numbers of birds potentially disturbed by visual presence of the dredging activities

Boat based operations related to the proposed mussel fishery will only occur in WeBS count sectors 2 to 6 inclusive. Accordingly potential disturbance will be restricted as a maximum to the number of birds occurring in these WeBS count sectors. As there will be only two boats operating, working separate mussel beds, the potential disturbance will be restricted as a maximum to the number of birds occurring in two WeBS count sectors. The worse case on a species by species basis will be potential disturbance within the sectors that support the largest and second largest number of any individual species. This worse case is set out in Table 16, derived from the data in Table 9 above. It is clear that the two most sensitive locations are in the areas of Deep Fleet (WeBS count sector 3 / proposed mussel bed site 2) and Stutton to Holbrook Bay West / North Shoal (WeBS count sector 6 / proposed mussel bed site 5). This is due to the combined presence of peak numbers of dark-bellied brent goose, shelduck, wigeon and cormorant; and goldeneye, red-breasted merganser, great crested grebe and Slavonian grebe respectively.

Table 16 Species by species location for the worse case potential disturbance effect (largest and second largest peak counts).

Species Size of peak count

WeBS sector

Mussel bed

Five year mean peak count

Brent Goose Largest Sector 3 Site 2 827 2nd largest Sector 2 Site 1 629

Shelduck Largest Sector 3 Site 2 273 2nd largest Sector 6 Site 5 164

Wigeon Largest Sector 3 Site 2 627 2nd largest Sector 2 Site 1 608

Goldeneye Largest Sector 6 Site 5 61 2nd largest Sector 5 Site 4 17

Red-breasted Merganser Largest Sector 6 Site 5 58 2nd largest Sector 5 Site 4 42

Great Crested Grebe Largest Sector 6 Site 5 77 2nd largest Sector 4 Site 3 36

Slavonian Grebe Largest Sector 6 Site 5 7 2nd largest Sector 5 Site 4 1

Cormorant Largest Sector 3 Site 2 13 2nd largest Sector 2 Site 1 12


Not all of the birds in the identified in each of the WeBS sectors listed above are likely to be disturbed as the spatial extent of the potential disturbance zone is not the whole WeBS sector but is related to the ‘disturbance effect distance’ that is species specific. This factor is considered below.

3.5.8 Predicted spatial extent of disturbance within a WeBS sector / around a mussel bed: Visual presence

The ‘disturbance effect distance’ defined in Table 12 above can be used to identify the predicted spatial extent of disturbance that might potentially occur within each WeBS sector and/or around each mussel bed as it is worked. For a slow moving boat, as will be carrying out the proposed operations, this area will move with the boat and potentially will create a zone from which birds will depart and not re-enter (an exclusion zone within which habituation is not assumed to occur). Its extent will depend upon the species specific ‘disturbance effect distance’ that has been identified.

The ‘open water’ and particularly the fish-eating birds that are being assessed can be expected at high water to concentrate along the line of the deep water channel where the feeding would be better in contrast to within the flooded saltmarsh and tidal flats. The non-diving wildfowl (dark-bellied brent goose, shelduck and wigeon) will have very limited feeding opportunities at high tide within the intertidal area (other than close to the shoreline and hence outside the area which might be disturbed by the boat operations) as the water will be too deep to access their food. Accordingly the dark-bellied brent goose, shelduck and wigeon can only be using the open water of the central channel to roost (away from terrestrial predators such as foxes).

It is considered that prediction of the proportion of birds that are using the WeBS sector that might be disturbed is most appropriately carried out by considering the linear spatial extent – the length of the WeBS sector along the direction of the deep water channel in relation to the length (diameter) of the zone of potential disturbance6, that is two times the species specific disturbance effect distance. This calculation of the linear proportion is carried out using the information in Table 17 and the linear proportion, expressed as a ratio, is provided in Table 16 for those species and locations for which a worse case of potential disturbance is identified in Table 16 above. The ratio for each species for each WeBS sector is provided in Appendix 5. The length of the WeBS sector along the direction of the deep water channel has been estimated from the maps published by the BTO as specific figures of the dimensions of WeBS sectors are not published.

Table 17 Estimated lengths of WeBS count sectors along the deep water channel.

WeBS count sector name Sector number Sector length (m) Erwarton Bay 2 4,800 Deep Fleet 3 2,200 Holbrook Bay East 4 2,900 Copperas Bay 5 2,900 Stutton to Holbrook Bay West

6 4,800

6 An alternative method of considering the proportion of estuarine habitat that is made temporarily unavailable through disturbance to the species being assessed is given in Appendix 6.


Table 18 Length of the potential disturbance zone in relation to the estimated lengths of WeBS count sectors along the deep water channel for each species.

Species WeBS sector

Estimated sector length (m)

Species specific

disturbance effect distance

x2 (m)

Ratio of sector length to disturbance effect distance

Dark-bellied brent Goose 3 2,200 800 2.8:1 2 4,800 6:1

Shelduck 3 2,200 500 4.4:1 6 4,800 9.6:1

Wigeon 3 2,200 600 3.7:1 2 4,800 8:1

Goldeneye 6 4,800 600 8:1 5 2,900 4.8:1

Red-breasted merganser 6 4,800 130 37:1 5 2,900 22:1

Great crested grebe 6 4,800 300 16:1 4 2,900 9.7:1

Slavonian grebe 6 4,800 100 48:1 5 2,900 29:1

Cormorant 3 2,200 400 5.5:1 2 4,800 12:1

The species specific ratios presented in Table 18 can then be applied to the worse case potential disturbance effect numbers in Table 16 to calculate an assessment, based on worse case parameters, of the potential disturbance caused by the proposed boat activity. This is presented in Table 17. The numbers potentially affected in relation to the populations of the whole Stour Estuary using the most recent count data and to the populations of the Stour and Orwell Estuaries SPA as stated on the relevant designation document (English Nature, 2005) are presented in Table 18.

Table 19 Assessment, based on worse case parameters, of the potential disturbance caused by the proposed boat activity.

Species WeBS sector / mussel bed

Five year mean peak count for sector

Proportion affected (ratio in Table J)

Number of birds potentially affected

Brent Goose

Sector 3 / Site

2

827 2.8:1 295

Sector 2 / Site

1

629 6:1 105

Combined number: 400






Shelduck

Sector 3 / Site

2

273 4.4:1 62

Sector 6 / Site

5

164 9.6:1 17

Combined number: 79

Wigeon

Sector 3 / Site

2

627 3.7:1 169

Sector 2 / Site

1

608 8:1 76

Combined number: 245

Goldeneye

Sector 6 / Site

5

61 8:1 8

Sector 5 / Site

4

17 4.8:1 4

Combined number: 11

Red-breasted Merganser

Sector 6 / Site

5

58 37:1 2

Sector 5 / Site

4

42 22:1 2

Combined number: 3

Great Crested Grebe

Sector 6 / Site

5

77 16:1 5

Sector 4 / Site

3

36 9.7:1 4

Combined number: 9

Slavonian Grebe

Sector 6 / Site

5

7 48:1 <1

Sector 5 / Site

4

1 29:1 <1

Combined number: <1






Cormorant

Sector 3 / Site

2

13 5.5:1 2

Sector 2 / Site

1

12 12:1 1

Combined number: 3

Note: In producing the combined number, rounding effects from the constituent numbers become apparent, calculations were carried out in a spreadsheet on exact figures that are then rounded.

Table 20 Bird numbers potentially affected in relation to the populations of the whole Stour Estuary and to the populations of the Stour and Orwell Estuaries SPA.

Species Combined number of birds potentially affected

As a percentage of the Stour Estuary population

As a percentage of the Stour & Orwell Estuaries SPA population

Brent Goose 400 17.6% 15.2% Shelduck 79 4.4% 2.7% Wigeon 245 7.5% 6.2% Goldeneye 11 5.8% 5.2% Red-breasted Merganser

3 2.7% n/a

Great Crested Grebe 9 2.4% 3.5% Slavonian Grebe <1 2.3% n/a Cormorant 3 2.8% 1.4%

3.5.9 Predicted spatial extent of disturbance within a WeBS sector / around an operating mussel dredger: Noise

The predicted extent of disturbance due to noise from the mussel dredging activity has been identified as a result of the measurements and predictions of Atkins (2016), when applied to the thresholds identified by Cutts et al. (2013), Natural England and the Environment Agency, as being:

• <100 m to fall below levels at which birds would be predicted to take flight; • <100 m to fall below levels at which birds would be subject to a level greater than an

“acceptable ‘dose’ level”; • ~100 m to fall to the no effect level for regular sounds; and • ~250 m to fall to the no effect level for sudden sounds.


With respect to potential impacts on ‘open water’ and fish-eating bird species, the assessment carried out above of the effects of visual stimuli applied distances for the calculation of numbers of birds that might be affected that are greater than the identified effect distances due to noise identified here (other than for red-breasted merganser and Slavonian grebe). As a result it can be concluded that the assessment already carried out for visual stimuli provides a precautionary assessment for potential impacts on ‘open water’ and fish-eating bird species. This also follows the reasoning of Cutts et al. (2013) that concluded that noise stimuli rarely cause waterbird disturbance before associated visual stimuli have an effect.

The dredging operations occur at high tide and as a result consideration has to be given to the potential effects of dredging noise on roosting waders. The locations of the high tide wader roosts in the Stour Estuary are illustrated on Figure 2.2 of Musgrove et al. (2001). It was identified in Otto (2013) that all high tide roosts were more than 500 m away from the areas within which mussel dredging was to take place. Accordingly, applying the thresholds and noise level predictions identified above, all high tide wader roosts will receive sound levels from the dredging activity of below the no effect level.

3.5.10 Conclusion

An assessment has been carried out, based on worse case parameters, of the potential for disturbance caused by the proposed boat activity planned to relay, maintain and harvest the five proposed mussel beds in the Stour estuary. This has identified that short term potential disturbance of ‘open water’ and fish-eating birds that are interest features (either alone or as part of the waterbird assemblage) will be to no more than 15.2% of the dark bellied brent goose population of the Stour & Orwell Estuaries SPA and significantly less than this for shelduck, wigeon, goldeneye, red-breasted merganser, great crested grebe, Slavonian grebe and cormorant. The mussel bed sites where there is the potential for the disturbance of the largest numbers of birds that are ‘open water’ and fish-eating birds are Site 2 ‘Deep Fleet’ and Site 5 ‘North Shoal’.

An assessment has also been carried out of the potential for disturbance caused by noise generated by the proposed dredging to affect ‘open water’ and fish-eating birds and this has found that the potential for effects is less than that produced by the visual stimuli of the boat activity. The assessment of potential noise effects has also found that all high tide wader roosts will not be affected.


4. References

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Austin, G. and Ross-Smith, V. (2014). Guidance to Interpretation of Wetland Bird Survey Within-Site Trends. BTO Research Report No. 661. The British Trust for Ornithology, Thetford.

Beadman, H.A., Kaiser, M.J., Galanidi, M., Shucksmith, R & Willows, R.I. (2004). Changes in species richness with stocking density of marine bivalves. Journal of Applied Ecology. 41: 464-475.

Bregnballe, T., Aaen, K. and Fox, A.D. (2009). Escape distances from human pedestrians by staging waterbirds in a Danish wetland. Wildfowl Sp Iss 2: 115-130.

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Elner, R. W. (1978). The mechanics of predation by the shore crab, Carcinus maenas (L.), on the edible mussel, Mytilus edulis L. Oecologia, 36(3), 333-344.

English Nature (2005). Stour and Orwell Estuaries SPA Citation UK9009121 [May 2005, Version: 1.0].English Nature, Peterborough.

Franklin, P. & Mowlam, C. (2015). Stour Estuary Several Order Application: Review of potential impact to commercial, charter and recreational fishing. Report prepared by Homarus Ltd on behalf of Parkinson Wright.

Gill, J.A., Norris, K. and Sutherland, W.J. (2001) Why behavioural responses may not reflect the population consequences of human disturbance. Biological Conservation 97: 265 - 268.

Harwich Haven Authority. 2012. Maintenance Dredging Protocol Baseline Document: Stour and Orwell Estuaries, Harwich/Felixstowe Harbour and Deep Water Channel. Version 6.0. Available from: http://apps.hha.co.uk/library/files/F03C8865-664E-409F-A6E8-4F4BEC761C66.PDF. Accessed: November 2015.

Holloway, S. (1997). Winter Distribution and Disturbance of Wildfowl and Waders on Findhorn Bay. BTO Research Report No. 179.

Hume, R.A. (1976). Reactions of goldeneyes to boating. British Birds 69: 178-179.

JNCC (2008). Ramsar Information Sheet UK11067: Stour and Orwell Estuaries [Version 3.0, 13/06/2008]. JNCC, Peterborough.


http://apps.hha.co.uk/library/files/F03C8865-664E-409F-A6E8-4F4BEC761C66.PDF

http://apps.hha.co.uk/library/files/F03C8865-664E-409F-A6E8-4F4BEC761C66.PDF

Kaiser, M. J., Collie, J. S., Hall, S. J., Jennings, S., & Poiner, I. R. (2003). 12 Impacts of Fishing Gear on Marine Benthic Habitats. Responsible fisheries in the marine ecosystem, 57(3), 197.

Kirby, J.S., Davidson, N., Giles, N., Owen, M. and Spray, C. (2004). Waterbirds and wetland recreation handbook: A review of issues and management practice. Wildfowl & Wetlands Trust, Slimbridge.

Liley, D. and Fearnley, H. (2011). Bird Disturbance Study, North Kent 2010/11. Footprint Ecology, Wareham.

Liley, D., Cruickshanks, K., Waldon, J. and Fearnley, H. (2011). Exe Estuary Disturbance Study. Footprint Ecology, Wareham.

MacDonald DS., Little M., Eno C., and Hiscock K. (1996) Disturbance of benthic species by fishing activities: sensitivity index. Aquatic Conservation: Marine and Freshwater Ecosystems 6: 257-268.

Madsen, J. (1998). Experimental refuges for migratory waterfowl in Danish wetlands. I. Baseline assessment of the disturbance effects of recreational activities. J. Applied Ecology 35: 386-397.

Musgrove, A.J., Clark, N.A., Gill, J. and Ravenscroft, N.O.M. (2001). A Review of Wildfowling on the Stour Estuary. BTO Research Report 248. BTO, Thetford.

Natural England, (2015). Trial Several Fishery Order in the Stour Estuary for Blue Mussel, Mytilus edulis. [Consultation Letter dated 2nd April 2015.] Ref: 148290.

Otto, S. (2012). Environmental Statement in support of an application for a trial several fishery order in the Stour Estuary for blue mussel Mytilus edulis.

Otto, S. (2013). Supplementary Environmental Statement in support of an application for a trial several fishery order in the Stour Estuary for blue mussel Mytilus edulis.

Platteeuw, M. and Henkens, J.H.G. (1997). Possible impacts of disturbance to waterbirds: Individuals, carrying capacity and populations. Wildfowl 48: 225-236.

Ravenscroft, N., Parker, B., Vonk, R. and Wright, M. (2007). Disturbance to waterbirds wintering in the Stour-Orwell estuaries SPA. Report to Suffolk Coast and Heaths Unit, Wildside Ecology, Earl Soham.

Reijnen, R., R. Foppen, C. ter Braak, and J. Thissen. (1995). The effects of car traffic on breeding bird populations in woodland. III. Reduction of density in relation to the proximity of main roads. Journal of Applied Ecology 32: 187-202.

Reijnen, R., R. Foppen, and H. Meeuwsen. (1996). The effects of car traffic on the density of breeding birds in Dutch agricultural grasslands. Biological Conservation 75: 255-260.

RSPB (2015a). Annex attached to e-mail from Mark Nowers of 2nd April 2015 ref PGS/LCD/69739 – The River Stour Mussel Fishery Order 2015. RSPB, Manningtree.


RSPB (2015b). Letter sent by e-mail from Mark Nowers of 20th May 2015 ref PGS/LCD/69739 – The River Stour Mussel Fishery Order 2015. RSPB, Manningtree.

RSPB (2015c). E-mail from Mark Nowers of 1st June 2015 RE: River Stour Mussel Fishery Order. RSPB, Manningtree.

Ruddock, M. and Whitfield, D.P. (2007). A Review of Disturbance Distances in Selected Bird Species. Natural Research Ltd report to SNH. Natural Research Ltd, Banchory.

Saier, B. (2001). Direct and indirect effects of seastars Asterias rubens on mussel beds (Mytilus edulis) in the Wadden Sea. Journal of Sea Research, 46(1), 29-42.

Smit, C.J. and Visser G.J.M. (1993). Effects of disturbance on shore birds: A summary of existing knowledge from the Dutch Wadden Sea and Delta area. Wader Study Group Bulletin 68: 6-19.

Suffolk Coast and Heaths AONB Unit (2010). The Stour and Orwell Estuaries Scheme of Management 2010. SCHU, Melton.

Tydeman, C.F. (1978). Gravel pits as conservation areas for breeding bird communities. PhD Thesis. Bedford College.

Underwood G.J.C and Kromkamp J. (1999). Primary production by phytoplankton and microphytobenthos in estuaries. Advances in Ecological Research 29: 93-149.

Worsfold, T.M. (2005). Stour, Orwell and Harwich approaches benthos: review of data commissioned by Harwich Haven Authority, biotope distribution update and partial review of current knowledge for the area. Unicomarine Report HHABiot05 to Harwich Haven Authority, November 2005.


Appendices


Appendix 1 Stour Estuary WeBS ‘core’ count (high tide) 5 year mean peak counts for the winters 2009/10 to 2013/14 for

the whole estuary and for each count sector


Sector 1

Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Sector 7

Sector 8

Sector 9

Sector 10

Proposed mussel bed - Site 1 Site 2 Site 3 Site 4 Site 5 - - - - Brent Goose 2,276 145 629 827 292 198 627 350 380 103 118 Shelduck 1,816 141 85 273 52 100 164 237 750 247 174 Wigeon 3,280 0 608 627 123 153 360 464 931 48 680 Goldeneye 191 0 0 1 0 17 61 20 34 25 1 Red-breasted Merganser

130 0 4 6 15 42 58 32 21 9 0

Great Crested Grebe 349 1 11 4 36 15 77 35 38 10 4 Slavonian Grebe 8 0 0 0 0 1 7 1 0 0 0 Cormorant 121 8 12 13 12 11 5 28 39 44 15


Appendix 2 Relative importance of the bird populations in WeBS count sectors compared to the whole Stour Estuary (for

the winters 2009/10 to 2013/14)


Sector 1

Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Sector 7

Sector 8

Sector 9

Sector 10

Proposed mussel bed - Site 1 Site 2 Site 3 Site 4 Site 5 - - - - Brent Goose 2,276 6% 28% 36% 13% 9% 28% 15% 17% 5% 5% Shelduck 1,816 8% 5% 15% 3% 6% 9% 13% 41% 14% 10% Wigeon 3,280 0% 19% 19% 4% 5% 11% 14% 28% 1% 21% Goldeneye 191 0% 0% 1% 0% 9% 32% 10% 18% 13% 1% Red-breasted Merganser

130 0% 3% 5% 12% 32% 45% 25% 16% 7% 0%

Great Crested Grebe 349 0% 3% 1% 10% 4% 22% 10% 11% 3% 1% Slavonian Grebe 8 0% 0% 0% 0% 13% 88% 13% 0% 0% 0% Cormorant 121 7% 10% 11% 10% 9% 4% 23% 32% 36% 12%

Appendix 3 Relative importance of the bird populations in WeBS count sectors compared to the whole Stour Estuary and

the Stour and Orwell Estuaries SPA (sector data from 2009/10 to 2013/14 compared to SPA populations at time of

designation)

Species S&OE SPA

Sector 1

Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Sector 7

Sector 8

Sector 9

Sector 10

Proposed mussel bed - Site 1 Site 2 Site 3 Site 4 Site 5 - - - - Brent Goose 2,627 6% 24% 31% 11% 8% 24% 13% 14% 4% 4% Shelduck 2,955 5% 3% 9% 2% 3% 6% 8% 25% 8% 6% Wigeon 3,979 0% 15% 16% 3% 4% 9% 12% 23% 1% 17% Goldeneye 213 0% 0% 0% 0% 8% 29% 9% 16% 12% 0%


Species S&OE SPA

Sector 1

Sector 2

Sector 3

Sector 4

Sector 5

Sector 6

Sector 7

Sector 8

Sector 9

Sector 10

Red-breasted Merganser

n/a

Great Crested Grebe 245 0% 4% 2% 15% 6% 31% 14% 16% 4% 2% Slavonian Grebe n/a Cormorant 232 3% 5% 6% 5% 5% 2% 12% 17% 19% 6%

Colour coding is based on the guidance of Austin and Ross-Smith (2014).


Appendix 4 Literature reviewed to identify an appropriate ‘disturbance effect

distance’.


distance (m)

Environmental setting

Source

Dark-bellied brent Goose

58-152 Estuarine, land based activity

Smit & Visser 1993

20-158 Estuarine, all activities combined

Liley & Fearnley 2011


Liley et al. 2011


Cutts et al. 2013

265-384 (95%CI)

Estuarine, land based activity

Laursen et al. 2005

400 Estuarine, land based activity

Cutts et al. 2013 distance at which to consider mitigation for ‘high level disturbance’

Shelduck 25-100 Estuarine, all activities combined

Liley et al. 2011




Cutts et al. 2009

220 Estuarine, boat based activity

Smit & Visser 1993

206-246 (95%CI)


Laursen et al. 2005


Smit & Visser 1993


Cutts et al. 2013 distance at which to consider mitigation for ‘high level disturbance’

Wigeon 50-100 Estuarine, all activities combined

Liley et al. 2011



205 Freshwater, land based activity

Bregnballe et al. 2009

155-270 Estuarine, boat activity

Madsen 1998

239-303 (95%CI)


Laursen et al. 2005

Goldeneye 300 Estuarine, boat activity

“movements triggered by boats regularly occur at 300 metres” Nowers pers. obs.



distance (m)

Environmental setting

Source

350 - 400 Freshwater, sailing boat activity

Hume 1976

350 - 750 Freshwater, powerboat activity

Hume 1976


50 Estuarine, all activities combined

Liley et al. 2011

55-65 Estuarine, boat activity

Holloway 1997

Great crested grebe 20 Freshwater, boat based activity

Tydeman 1978

142 Freshwater, land based activity

Cooke 1987

Slavonian grebe 50 Estuarine, all activities combined

Liley et al. 2011

10-150 Freshwater breeding site, land & boat based activity

Ruddock & Whitfield 2007

Cormorant 193 Freshwater, land based activity

Bregnballe et al. 2009


Appendix 5 The length of the WeBS sector along the direction of the deep

water channel in relation to the length (diameter) of the zone of potential

disturbance.

WeBS count sector name WeBS sector


Species specific disturbance effect distance x2 (m)


Dark-bellied brent Goose Erwarton Bay 2 4,800 800 6:1 Deep Fleet 3 2,200 800 2.8:1 Holbrook Bay East 4 2,900 800 3.6:1 Copperas Bay 5 2,900 800 3.6:1 Stutton to Holbrook Bay West

6 4,800 800 6:1

Shelduck Erwarton Bay 2 4,800 500 9.6:1 Deep Fleet 3 2,200 500 4.4:1 Holbrook Bay East 4 2,900 500 5.8:1 Copperas Bay 5 2,900 500 5.8:1 Stutton to Holbrook Bay West

6 4,800 500 9.6:1

Wigeon Erwarton Bay 2 4,800 600 8:1 Deep Fleet 3 2,200 600 3.7:1 Holbrook Bay East 4 2,900 600 4.8:1 Copperas Bay 5 2,900 600 4.8:1 Stutton to Holbrook Bay West

6 4,800 600 8:1

Goldeneye Erwarton Bay 2 4,800 600 8:1 Deep Fleet 3 2,200 600 3.7:1 Holbrook Bay East 4 2,900 600 4.8:1 Copperas Bay 5 2,900 600 4.8:1 Stutton to Holbrook Bay West

6 4,800 600 8:1

Red-breasted merganser Erwarton Bay 2 4,800 130 37:1 Deep Fleet 3 2,200 130 17:1 Holbrook Bay East 4 2,900 130 22:1 Copperas Bay 5 2,900 130 22:1 Stutton to Holbrook Bay West

6 4,800 130 37:1

Great crested grebe Erwarton Bay 2 4,800 300 16:1 Deep Fleet 3 2,200 300 7.3:1 Holbrook Bay East 4 2,900 300 9.7:1 Copperas Bay 5 2,900 300 9.7:1


WeBS count sector name WeBS sector


Species specific disturbance effect distance x2 (m)


Stutton to Holbrook Bay West

6 4,800 300 16:1

Slavonian grebe Erwarton Bay 2 4,800 100 48:1 Deep Fleet 3 2,200 100 22:1 Holbrook Bay East 4 2,900 100 29:1 Copperas Bay 5 2,900 100 29:1 Stutton to Holbrook Bay West

6 4,800 100 48:1

Cormorant Erwarton Bay 2 4,800 400 12:1 Deep Fleet 3 2,200 400 5.5:1 Holbrook Bay East 4 2,900 400 7.3:1 Copperas Bay 5 2,900 400 7.3:1 Stutton to Holbrook Bay West

6 4,800 400 12:1


Appendix 6 An alternative method of considering the proportion of estuarine

habitat that is made temporarily unavailable through disturbance.

This alternative method considers the area that would be made temporarily unavailable to the relevant bird species (based on the ‘disturbance effect distance’) as a result of two boats displacing birds (calculated as the area of a circle of radius equal to the species specific ‘disturbance effect distance’) compared to the area of estuarine habitat available in the Stour estuary.

Area of estuarine habitat available to waterbirds is taken as ~2,250 ha (based on the area of the Stour Estuary SSSI http://www.sssi.naturalengland.org.uk/citation/citation_photo/1004172.pdf ). This figure is an underestimate (and hence a precautionary figure from which to calculate proportions) as a significant area opposite Harwich Port is not included in the SSSI.

The results of this calculation are:

Species Disturbance effect distance (m)

Potentially disturbed area around single boat (ha)

Potentially disturbed area around two boats (ha)

Two boat area in relation to estuarine habitat in Stour Estuary (%)

Brent Goose 400 50.3 100.5 4.5% Shelduck 250 19.6 39.3 1.7% Wigeon 300 28.3 56.5 2.5% Goldeneye 300 28.3 56.5 2.5% Red-breasted Merganser

65 1.3 2.7 0.1%

Great Crested Grebe 150 7.1 14.1 0.6% Slavonian Grebe 50 0.8 1.6 0.1% Cormorant 200 12.6 25.1 1.1%

This indicates that potential temporary displacement as a result of proposed boat operations occurs across a small percentage of the estuarine habitat available in the Stour estuary. The largest proportions identified are for dark-bellied brent goose (4.5%), wigeon (2.5%) and goldeneye (2.5%). For dark-bellied brent goose and wigeon it should be noted that both these species are grazers and they also use fields outside the SSSI and as a result a larger area of habitat is potentially available to them than has been used in this precautionary calculation.


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