12. Fish - Southampton VTS Oct 2012/SACD - Chapter 12 Fish.pdf12. Fish Executive Summary: Chapter...

Environmental Statement for Port of Southampton: Southampton Approach Channel Dredge

Updated by Further Information

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12. Fish

Executive Summary: Chapter 12. Fish The following potential impacts to fish will occur either in the short-term, during capital dredging and disposal, or in the long-term, as a result of hydrodynamic and sedimentary changes brought about by the proposed channel: 1) Potential Impacts Due to Direct Removal of the Subtidal Habitat The temporary impact of the direct removal of poor quality feeding habitat at the proposed dredge areas is considered to be insignificant for the majority of fish that frequent the study area and are of lower nature conservation importance (e.g. gobies and sprat). For those fish of higher nature conservation importance (e.g. migratory salmon and juvenile bass), the impact will be insignificant to minor adverse significant. The temporary risk of direct uptake of fish and fish eggs during dredging is negligible to low. Consequently, the impact on fish and eggs is considered to be insignificant. 2) Potential Impact Due to the Predicted Effect on Hydrodynamic Processes The very small (millimetric) changes to water levels and flow dynamics that are predicted to occur following the proposed dredge are so marginal that they are unlikely to be discernable from natural variability. Therefore, there is unlikely to be a change in the usage of areas by fish and the impact will be insignificant. 3) Potential Impact to Habitat Due to the Predicted Effect on Sedimentary Processes The small increase in the net import of sediment into Southampton Water, following the proposed dredge, will result in an increased potential for sedimentation over shallow subtidal and intertidal areas, and a negligible reduction in suspended sediment concentrations. Given the resulting insignificant impact to mudflat habitat levels the impact, with regards to the distribution of fish and overall feeding resource, will be insignificant. 4) Potential Impact Due to Deposition of Sediment The temporary impact of deposition of material on benthic invertebrates during dredging is assessed as being insignificant and, therefore, the consequent impact to fish feeding grounds is considered to be insignificant. The potential effect of shellfish smothering on the majority of fish that frequent Southampton Water and eat such prey will be insignificant. For those fish features of higher nature conservation importance the impact is considered to be minor adverse significant, albeit transient during the dredge. The exposure to changes in the quality of sediment is assessed as being negligible and, therefore, the temporary impact of any contamination being released and redistributed onto fish feeding grounds during dredging will be insignificant. The temporary impact to fish as a result of the falling plume of deposits at the Nab Deposit Ground is considered to be insignificant for pelagic fish and insignificant to minor adverse



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significant for bottom-dwelling demersal fish that are less likely to be able to move rapidly and avoid the impact. The initial disposal plume will impact on a very localised area of seabed and a relatively small number of fish may benefit from the potential food source, however, the scale is considered negligible and, therefore, the impact will be insignificant. The temporary disturbance to subtidal habitats that provide a feeding resource, as well as nursery and spawning grounds for fish during disposal is considered to be insignificant to minor adverse significant to fish populations. 5) Potential Impact Arising as a Result of the Effects to Water Quality Suspended Sediment Concentrations The temporary impact of elevated suspended sediment concentrations during dredging for the feeding ability of the majority of fish that frequent the study area will be insignificant to minor adverse significant, and minor to moderate adverse significant for fish of higher nature conservation importance. During dredging within Southampton Water and the Test Estuary, the temporary impact for migratory patterns of fish is considered to be moderate adverse significant for fish of higher nature conservation importance (e.g. salmon) and minor to moderate adverse significant for other migratory species (e.g. eels and flounder). Outside Southampton Water, the impact to migratory fish is considered to be insignificant to minor adverse significant. The temporary impact during dredging for spawning and juvenile fish is considered to be minor adverse significant for designated bass nursery areas within Southampton Water and insignificant to minor adverse significant for other spawning and juvenile fish activities in the estuary. Outside Southampton Water, the impact will be insignificant to minor adverse significant for nursery and spawning grounds. At the disposal ground, the temporary impact of enhanced suspended sediment concentrations to fish directly in the area of the deposit ground is considered to be insignificant to minor adverse significant. Away from the immediate vicinity of the deposit location, the impact on the feeding, nursery and spawning activity of fish in the surrounding area is considered to be insignificant to fish populations. Given the unconfined nature of the migratory passageway, the impact to migratory fish is considered to be insignificant to minor adverse significant. Organic Enrichment and Oxygen Depletion During dredging within Southampton Water and the Test Estuary, the temporary impact of a reduction in dissolved oxygen for migratory patterns of high nature conservation value fish is considered to be minor to moderate adverse significant and insignificant to minor adverse significant for other migratory species. Outside Southampton Water, the impact of oxygen depletion to migratory fish of conservation value is considered to be insignificant to minor adverse significant and insignificant for other migratory fish. Contaminants



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The temporary impact from the potential release of contaminants associated with the dredged materials and any uptake by fish in the water column is considered to be insignificant to minor adverse significant for the majority of fish that frequent the area, and minor adverse significant for fish of higher nature conservation value. 6) Potential Disturbance Due to the Generation of Underwater Noise and Vibration The temporary impact of backhoe dredging and trailing suction hopper dredging (TSHD) to migratory salmonids is considered to be insignificant and will not cause a barrier to migration. The impact to other fish species is also considered to be insignificant given that they will be able to avoid areas with temporary adverse conditions without any implications to the recruitment and sustainability of the population. 7) Potential Impact Due to Compounding Effects of Impacts The compounding effects of several impacts acting together i.e. temperature, dissolved oxygen, suspended sediments and noise on migratory salmonids are considered to be moderate adverse significant in Southampton Water and minor adverse significant outside Southampton Water and in the Test and Itchen Estuaries. 8) Potential Impact During Future Maintenance Dredging The change in future maintenance dredging work is expected to be small compared with existing annual variability and, therefore, the impact on fish will be insignificant. Mitigation For impacts that have been assessed as being moderate adverse significant to fish within Southampton Water, an adaptive management strategy is being discussed has been agreed with the Environment Agency and will be adopted as mitigation for the dredge. With the proposed mitigation measures in place, the residual effect on fish in Southampton Water is considered to be minor adverse significant. Conclusion The majority of impacts to fish will be temporary, occurring during the capital dredging and disposal of arisings, and insignificant to minor adverse significant. Any greater impacts that occur during the short-term increase in the level of suspended sediments, and subsequent depletion of dissolved oxygen during dredging in Southampton Water will be mitigated by the implementation of an adaptive management strategy. The impacts following the capital dredge are all considered to be insignificant to fish.

Baseline Information

12.1 The intertidal and shallow subtidal habitats in Southampton Water and the Solent provide food and shelter for resident fish populations and seasonal visitors, as well as juvenile fish. Migratory Atlantic salmon are of particular significance given that they are an Annex II species and a qualifying feature of the River Itchen Special Area of Conservation (SAC). Other species of conservation interest that have been recorded on an individual and occasional basis in



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Southampton Water and the Solent include twaite shad, allis shad, river lamprey and sea lamprey (Fisheries Joint Data Group, 2001), which are all protected under Annex II of the EC Habitats Directive (EEC/92/43) and Appendix III of the Bern Convention. The following sections review the baseline fish resource in the study area: General Estuarine and Inshore Fish Populations; Salmonids; Other Migratory Fish; and Spawning and Nursery Grounds.

General Estuarine and Inshore Fish Populations

12.2 There are extensive records of the species composition and numbers of fish retained on the intake screens of Fawley Power Station, which have been kept since 1973. During this time around 80 fish species have been recorded, with the most abundant fish frequenting the area comprising sand smelt, sprat, herring, sand goby, pouting, bass and flounder. The actual number of species using Southampton Water is likely to be greater than these records indicate as they are biased towards small fish that are unable to avoid being drawn in to the screens. Table 12.1 gives an indication of the relative (i.e. qualitative) abundance of fish recorded on the intake screens. Table 12.1 Relative abundance of fish species at the Fawley Power Station Intake

Relative Abundance Fish Species >100,000 Very abundant Sand smelt, sprat 50,000 - 100,000 Abundant Herring 10,000 - 50,000 Abundant Sand goby, pouting, bass, flounder 5,000-10,000 Common Golden mullet, greater pipefish, poor cod

1,000-5,000 Common Pollack, black goby, common dragonet, thin-lipped grey mullet, Nilsson’s pipefish, 5 bearded rockling, sole, 15 spined stickleback

>100 Uncommon 18 species <100 Rare 44 species

(Source: Reay & Culley, 1980; and updated by Fisheries Joint Data Group, 2001)

12.3 The potential geographical coverage of a survey grid is, however, considered a more representative means of recording fish than fixed-point sampling at power station intakes used in the past (Picket et al., 2002). For this reason, the data collected during the regular Cefas Solent Bass Surveys and Young Fish Surveys constitute a more representative assessment of the fish populations using Southampton Water and the Solent and have been reviewed below.

12.4 The Solent Bass Surveys have been undertaken by Cefas since 1981, twice a year at around

35 core station positions throughout the Solent, Southampton Water (up to Dock Head), and in the Langstone and Chichester Harbours. Although aimed at monitoring the abundance of pre-recruit (2-5 year old) bass, the survey catches a wide range of demersal and pelagic finfish species. A trawl of similar design has been used for the surveys since 1983, which are undertaken at the same part of the tidal cycle on each visit (falling springs). Since 1990, the surveys have been carried out in May and September so it is possible that certain migratory fish that are absent at that time would not have been recorded. The same amount of ground is fished at each station, at a consistent towing speed, depending on the strength of the tide. All



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biota caught are recorded and for some species the dataset constitutes a time series of their local distribution and relative abundance. Since 1987, a total of 86 species have been recorded in the Solent bass trawl surveys (Pickett et al., 2002). The variation in mean abundance between surveys is a function of various factors, such as size selectivity, mortality and seasonal distribution. An example of the typical species composition sorted in order of abundance, using data from the latest published September 2000 bass survey, is shown in Table 12.2. Table 12.2 Fish recorded in the Solent, September 2000

Species Numbers Caught Percentage of Total Numbers of Fish Caught

Sea bass 1990 53.62 Putting (bib) 495 13.34 Black sea-bream 230 6.2 Golden grey mullet 161 4.34 Sand-smelt 129 3.48 Plaice 103 2.78 Horse-Mackerel (Scad) 103 2.78 Cuckoo wrasse 70 1.89 Sole 70 1.89 Corkwing wrasse 63 1.7 Flounder 53 1.43 Pollack 39 1.05 Dragonet family 30 0.81 Whiting 30 0.81 Red mullet 28 0.75 Eel 18 0.49 Sand eel family 13 0.35 Greater pipefish 13 0.35 Dab 12 0.32 Thornback ray 12 0.32 Starry smooth hound 11 0.3 (European) Mackerel 9 0.24 Tub gurnard 6 0.16 Black goby 4 0.11 Rock cook 3 0.08 Pilchard 3 0.08 Solenette 2 0.05 Five-bearded rockling 2 0.05 Thin lipped mullet 2 0.05 Brill 2 0.05 Lesser spotted dogfish 2 0.05 Sea scorpion 2 0.05 Fifteen-spined stickleback 1 0.03 Herrings * * Crystal goby * * Sand goby family * * * Data excludes herring and gobies

(Source: Picket et al., 2002)



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12.5 The frequency of occurrence of each species recorded in the Cefas Solent bass surveys undertaken between 1983 and 1997 was compared to the data obtained during the Cefas Young Fish Survey that uses alternative sampling methods (push net and 2m beam trawl), from within a similar geographical area and over a similar sampling period (Picket et al., 2002). The Cefas Solent Bass Survey found that bass had the highest frequency of occurrence by station, occurring in over 60% of stations sampled in the study area. Other relatively regularly occurring species, recorded across 20-39% of the Solent Bass Survey stations, were sand eels, eels, sand smelt, garfish, dragonet, herring, dab, golden grey mullet and whiting. In contrast, dragonet most frequently occurred across the stations of the Young Fish Survey, with corkwing wrasse and black goby also being widely distributed, occurring in 40-59% of the stations. These differences reflect the importance of sampling design and methodology used to determine fish populations.

Salmonids

12.6 The study area forms part of the migratory route for salmonids, namely Atlantic salmon and sea

trout. Salmon and sea trout are anadromous fish species, migrating upstream from the sea to breed. They are thought to swim near the surface during migratory runs (Moore et al., 1998), and adults display a strong behaviour to return to natal rivers. The rivers in Southampton Water, principally the Test and Itchen, which are designated under the Freshwater Fish Directive (78/659/EEC) are important for migratory salmonids. The Environment Agency has also indicated the New Forest streams’ importance for these fish (Appendix A).

12.7 The Environment Agency and Cefas monitor salmon movements in the Test and Itchen Rivers.

Past data have shown that salmon stocks in the rivers of Hampshire have declined. Estimated numbers of salmon and smolt runs in the River Test and estimated salmon runs in the River Itchen between 1990 and 2007 are shown in Table 12.3. Based on this 17-year period, the returning stock is around 800 (361-1155) in the River Test and 400 (152-880) in the River Itchen. Although stocks remain stable, they are still below the Environment Agency’s conservation target. Poor survival of salmon eggs in the upper stretches of the rivers due to run-off from fields has been identified as a major reason for the decline in numbers (Environment Agency, 2004c).



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Table 12.3 Salmon statistics for the Rivers Test and Itchen

River Test River Itchen

Year Smolt Run Estimate

Returning Salmon Run Estimate

Returning Salmon Run Estimate

1990 - 790 367 1991 - 538 152 1992 11 967 614 357 1993 7131 1155 852 1994 3381 775 378 1995 6853 647 880 1996 4712 623 433 1997 7229 361 246 1998 14 672 898 453 1999 4138 867 213 2000 3516 595 208 2001 2625 410 217 2002 2190 1046 239 2003 7585 367 222 2004 5024 1129 410 2005 7580 1150 411 2006 6118 1058 419 2007 13461 646 302

No smolt counter data are available for the River Itchen

(Source: Environment Agency) 12.8 Salmon pass through Southampton Water; firstly as juveniles (smolts) migrating to sea, and

again as adult salmon returning to the river to spawn. The majority of adult salmon return to the Test and Itchen between June and September. Smolt move out of the rivers and migrate downstream to the sea in spring, with the main movements occurring between April and end of May (Adrian Fewings, Environment Agency, pers. comm., March 2008).

12.9 An acoustic tagging study found that the seaward migration of salmon smolts in the upper Test

Estuary between Redbridge and Eling takes place predominantly at night and during an ebb tide, close to the surface and within the fastest moving section of the water column (Moore and Ives, 1997; Moore et al., 1998). During the day the smolts would refuge close to the bed to avoid predation by birds and during flood tides, juveniles in the middle Test Estuary either remain in place or move into the deeper waters of the Bury Swinging Ground (Moore & Ives, 1997; Moore et al., 1998). The subsequent seaward movement of smolts down the lower Test Estuary, between Marchwood and Dock Head, and through Southampton Water occurs both day and night during the ebb and latter part of the flood tide, suggesting some active directed swimming. In this section of the estuary, the smolts continue to swim close to the surface of the water, either within the maintained channel or along the western shore, and were observed to avoid boat traffic by moving rapidly away from approaching vessels (Moore & Ives, 1997).

12.10 Sea trout exhibit similar migratory movements to salmon to and from Southampton Water and

the New Forest river systems. Sea trout enter the rivers between March and October, with a peak during the summer months, generally at the end of the fishing season. As with salmon,



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eggs are laid in gravel nests and spent sea trout (kelt) leave the rivers between December and April, while smolt leave during spring. Other Migratory Fish

12.11 Common seasonal visitors that frequent the estuaries of the Solent during their annual migrations are sprat, whiting, pouting, bass and herring. Eels and flounder are two further migratory fish species that have been recorded in Southampton Water. In contrast to salmonids, they are both catadromous species i.e. migrating to the sea to breed. European eels breed in the Atlantic Ocean southeast of Bermuda and the postlarvae cross the Atlantic changing into elvers in British coastal waters. Elvers migrate into estuaries and rivers during late winter and early spring and adult eels leave the rivers and coastal area in late summer and autumn. Eels are understood to follow the saltwater/freshwater interface and move close to the riverbank during strong river currents. It is believed that seaward movements take place at varying depths. Flounder is a common and widespread flatfish species found in estuaries and sandy rivers throughout the UK. Adult flounder migrate from the rivers through the estuarine brackish and marine waters out to sea to spawn. Juvenile fish are known to be common close inshore, entering estuaries at an early age and using incoming tides to help them move upstream (Davidson et al., 1991).

12.12 There is very little information on the movements of the rarer migratory species in Southampton

Water, such as twaite shad, allis shad, river lamprey and sea lamprey. The lampreys are known to migrate to the rivers that feed the Solent to spawn, but there does not appear to be any evidence that either of the shads have spawning ground runs in Southampton Water (Tubbs, 1999). In addition to the movements of true migratory fish, seaward migration of sea bass occurs in the autumn as they move to offshore prespawning and spawning areas to the south and west of the UK (Aprahamian & Barr, 1985). In the summer, bass tend to move back into shallow inshore warmer waters (Pawson and Pickett, 1996). Spawning and Nursery Grounds

12.13 Southampton Water and the Solent provide a nursery and spawning ground for a number of species of fish. The coastal waters of the Solent provide shelter, breeding substrates and feeding resources required for many species of resident estuarine fish and for the larvae and juveniles of seasonal visitors that spawn further offshore. Many small inshore fish species spawn in Southampton Water and the Solent, including sand smelt and gobies, and are generally associated with the shallow sandflats and mudflats. The nursery and spawning areas of the Solent and Southampton Water also support a number of commercial fish species, including bass, Dover sole, plaice and cod (Figure 12.1). Adult bass are known to aggregate in the East Solent and English Channel prior to spawning from March to May, and juveniles are found in estuaries from June onwards.



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Impact Assessment Key Impact Pathways

12.14 The potential impact of the dredge on fish (migratory fish, feeding, spawning and nursery grounds) arises mainly from the direct and indirect effects on subtidal and intertidal habitat during the construction and operational phases of the development. Following the Environment Agency’s response in the Scoping Opinion (Appendix A), the compounding effects of several impacts to fish interests acting together (e.g. temperature, dissolved oxygen, suspended sediments and noise) have also been considered.

12.15 The following sections review the key impact pathways relating to fish:

Potential Impact Due to Direct Removal of Subtidal Habitat; Potential Impact to Habitat Due to the Predicted Effect on Hydrodynamic Processes; Potential Impact to Habitat Due to the Predicted Effect on Sedimentary Processes; Potential Impact Due to Deposition of Sediment from the Dredging and Disposal

Activity; Potential Impact Arising as a Result of the Effects to Water Quality; Potential Disturbance Due to the Generation of Underwater Noise and Vibration; Potential Impact Due to Compounding Effects of Impacts; and Potential Impact During Future Maintenance Dredging.

12.16 For the purposes of this assessment it is assumed that all intertidal and subtidal habitats provide feeding, spawning and nursery habitat, although some areas may be more utilised than others. Estuarine fish are sensitive to the loss of these habitats, which may occur by the removal or smothering of habitat during dredging and disposal, or as a consequence of altered coastal processes resulting from the proposed channel design. The potential for an impact on fish can also occur as a result of increased levels of suspended sediment during the dredging and disposal activities, which in turn can increase turbidity and deplete dissolved oxygen.

12.17 The fish impact assessment has been undertaken based on the results of the hydrodynamic

and sediment modelling (Appendix C), the physical processes impact assessment (Chapter 8), the water and sediment quality impact assessment (Chapters 9 and 10), and the marine and coastal ecology impact assessment (Chapter 11), in the context of the previous baseline review. Potential Impact Due to Direct Removal of Subtidal Habitat Food Resource

12.18 Fish occupy many trophic levels of the estuarine food chain, feeding on phytoplankton, zooplankton, algae, invertebrates or other fish. In order to forage for these food items, their feeding habits comprise grazers, plankton filter feeders (e.g. herring, sprats and smelts), suckers and parasites (e.g. sea lamprey) and predators (e.g. bass, gobies, dab and plaice).



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12.19 The capital dredging would have an indirect effect on fish by removing a potential food source within the footprint of the dredge. The temporary reduction would be localised to the dredge, covering 130ha in the Test Estuary, 230ha in Southampton Water, 240ha in the Thorn Channel and 140ha in the Nab Channel (Figure 1.1). Furthermore, the reduction would only be temporary given that the sediments of the proposed dredge area are characterised by opportunistic species that are regularly disturbed by vessel disturbance and/or maintenance dredging, and capable of rapidly recolonising habitats, (Chapter 11). Therefore, although the probability of the change occurring is high, the temporary reduction in habitat would be a small part of the overall potential feeding area for fish i.e. the magnitude of effect would be small and the exposure to change considered low. The sensitivity of fish to this small and temporary change is considered to be low given that the species poor invertebrate assemblages present within the existing channel and in the proposed widening areas (Chapter 11) are likely to be of poorer quality for feeding fish than the more species diverse intertidal sediments, although this will vary for different fish species. The temporary impact of the direct removal of this feeding habitat on the majority of fish common to the study area, which are of lower nature conservation importance, such as gobies and sprat, would be insignificant. For those fish of higher nature conservation importance, including migratory salmon and juvenile bass, the temporary impact is considered to be insignificant to minor adverse significant. Direct Uptake (Entrainment)

12.20 During the capital dredging there is the potential for fish and fish eggs to be directly taken up by the dredger. Any eggs that are associated with the dredge material may be damaged, killed, or relocated to the deposit ground, where the habitat at that location will also be modified. Fish at greatest risk are demersal species, such as flatfish that burrow in sediments. Although there is a temporary risk of direct uptake of fish during dredging, fish tend to avoid sources of disturbance and are likely to exhibit an avoidance reaction to the dredger, thus greatly limiting their direct uptake. Furthermore, the proposed dredge areas are highly unlikely to form an important habitat for fish as the main navigational channel and its margins are regularly disturbed by vessel movements and/or maintenance dredging. In any case, any fish or fish eggs that are found in the channel are widespread elsewhere in the study area, i.e. the channel does not support exclusive populations of fish species. Overall, although there is a temporary risk of direct uptake of fish and fish eggs during dredging, the probability and magnitude of the loss would be negligible/low and, therefore, the exposure is considered to be negligible. Consequently, the temporary impact on fish and eggs is considered to be insignificant.

Potential Impact to Habitat Due to the Predicted Effect on Hydrodynamic Processes

12.21 The tidal propagation within Southampton Water is predicted to be marginally affected by the

proposed wider and deeper channel, resulting in very small (millimetric) changes to water levels in Southampton Water and the Solent. These negligible changes will result in a marginal reduction in tidal range in Southampton Water, upstream of Cadland, effectively converting a narrow strip of lower shore intertidal into shallow subtidal mudflat (2.6ha), and a slight increase in intertidal, mainly mudflat and sandflat habitat, in the Solent (3.8ha).

12.22 Many estuarine fish will be dependent upon the invertebrate feeding grounds provided by the

mudflats of Southampton Water, particularly the intertidal flats available to foraging fish when submerged at high tides. The most important predators on the intertidal flats include the flatfish



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(sole, dab, flounder and plaice), gobies and juvenile bass, which feed on a range of prey food items, including polychaete worms, bivalves, and tidally active crustaceans. Many demersal fish are opportunistic predators and their prey choice reflects the species that are available in the area (Elliott et al., 1998).

12.23 In addition to the small water level changes, which change the area of intertidal and subtidal

habitat, small changes to the flow regime could cause a change to fish usage of certain areas. The modelling, however, has shown that most flow changes are very small, well within the natural spring/neap variability that already occurs. Also, the most significant changes occur in the same areas that the dredging disturbance will take place.

12.24 The predicted changes to intertidal and subtidal habitats and flow dynamics are so marginal

that they would not be discernable from natural variability. Furthermore, the fact that fish feed over intertidal areas when they are covered by the tide and in subtidal areas at low water (LW) indicates that the slight modification of habitats as a result of these changes in water levels will be a negligible exposure to the fish communities in Southampton Water and the Solent. Small changes to the distribution of flow speeds could affect the distribution of fish, however, these changes are shown to occur, predominantly in areas where a greater effect will be caused by the dredging process. Overall, the impact on fish arising from the changes to hydrodynamic processes alone is considered to be insignificant, when the sensitivity and relative importance of fish are taken into account.

Potential Impact to Habitat Due to the Predicted Effect on Sedimentary Processes

12.25 The predicted additional net import of 1.5% of sediment into Southampton Water will result in

an increase in the rate of potential sedimentation in certain areas of the estuary, including a general increase of about 0.01m/year over the shallow subtidal and intertidal areas to the east and west of the main navigation channel between Dock Head and Fawley. Despite the increase in net fine sediment import, suspended sediment concentrations within the estuary will not be increased and in fact a minor reduction (maximum of around 1-2mg/l) will result following the dredge. The changes will affect the transient nature of accretion and erosion throughout the tides that occurs in most areas, therefore, accumulations on the mudflats due to the redistribution of material by natural and anthropogenic wave disturbance are unlikely to occur. The predicted changes to the mudflat habitat levels have been assessed as being negligible (Chapter 11), and, therefore, the subsequent impact to the distribution of fish and overall feeding resource is considered insignificant.

Potential Impact Due to Deposition of Sediment

During Capital Dredging

12.26 During dredging, the intertidal and subtidal sediments will be subject to short-term periods of

small depths (mostly millimetric) of increased depositions. These will vary in time and space depending on tidal state, range of tide, location of dredging and material type, lasting for a few minutes up to several days. Most of this material will be re-eroded over the period of the dredging, therefore, these periods and locations of deposition will be transient and any material remaining will be re-worked by long-term processes in most areas of the estuary, unless they move to net accretional areas that already exist, such as berth pockets. The scale of these



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short-term effects has been discussed in detail in Appendix C. The magnitude of the deposition that is predicted is generally within the natural variability that currently exists between spring and neap tides, not accounting for storm activity. Any deposition from the capital dredge will be temporary, generally occurring at slack water and particularly on neap tides, and will often be re-eroded, generally on ebb tides and particularly on spring tides, and redistributed elsewhere in the estuary. The only permanent accumulation will occur where the sediment is trapped (e.g. berth pockets). Therefore, although the probability of deposition is high, the overall exposure, taking into account the transient nature and relatively small magnitude of the change, is considered to be low. These processes already occur within the estuary and can be enhanced by the existing maintenance dredging. Therefore, taking account of the temporary nature of this change, the sensitivity of the fish to the increased deposition is low. The overall impact of these physical changes to specific aspects relating to fish is assessed in the following sections.

Food Resource

12.27 The impact of deposition of material on benthic invertebrates during dredging was assessed as being insignificant (Chapter 11), and therefore, the consequent temporary impact to fish feeding grounds is also considered to be insignificant. In the case of shellfish, the impact to the beds in the Central Solent and approaches to Southampton Water was assessed as being insignificant, and for those within Southampton Water, the impact considered to be moderate adverse significant (Chapter 11). Fish generally feed on a range of food items and, therefore, their sensitivity to the temporary change in a particular food resource is considered to be low. Furthermore, the high mobility of fish enables them to move freely to avoid areas of adverse conditions and to use other food sources in the estuary or the unaffected shellfish beds in the approaches to Southampton Water and the Central Solent. Therefore, the potential effect of shellfish smothering on the majority of fish that frequent Southampton Water and eat such prey, such as gobies and plaice, which are of lower nature conservation importance, is considered to be insignificant and only temporary. For those fish features of higher nature conservation importance, namely juvenile bass for which Southampton Water is a designated nursery, the temporary impact is considered to be minor adverse significant, albeit transient during the dredge.

Sediment Quality

12.28 Any chemical or microbiological contaminants associated with the sediments being removed during dredging may be dispersed, redistributed and deposited elsewhere in the estuary. There is a risk that some of these sediment-bound contaminants may be temporarily bioaccumulated in the tissues of certain fish prey, such as polychaete worms and marine bivalves, and made available for uptake by feeding fish. Demersal fish species, such as dab and flounder, which remain close to the seabed and feed mainly on benthic organisms, would experience a higher exposure to contaminated sediments than pelagic fish. The temporary exposure to changes in chemical or microbiological quality of sediment as a result of deposition during dredging was assessed as being insignificant (Paras 9.30 to 9.36). The temporary impact of any contamination of fish feeding grounds is as a consequence, also considered to be negligible in magnitude and, therefore, insignificant.



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During Disposal of Dredge Arisings 12.29 It is estimated that around 11.6 million m3 in situ of capital material (plus around an additional

1.3 million m3 of material in situ allowing for a maximum potential overdredge of 0.15m) will need to be disposed of as a result of the proposed dredge at the existing deposit ground at the Nab, subject to a beneficial use being identified. The deposition of capital arisings has the potential to affect fish that utilise the seabed and pelagic species, swimming through the deposit ground and the areas of enhanced suspended sediment concentration as the sediment is dispersed from the site. On release from the dredger, all the material will fall through the water column as a dynamic plume and impact on the seabed below the disposal vessel. Such disposals will have a maximum frequency of the order of 5 minutes every hour whilst dredging the Nab Channel, otherwise the deposit frequency will range between 5-10 minutes every 4-6 hours when dredging in Southampton Water. Immediately beneath the dredger, suspended sediment concentrations would be several thousand mg/l, however, decaying rapidly to a few hundred 100mg/l.

12.30 Any demersal fish under the footprint of the disposals will be smothered but it is likely that the

majority of pelagic fish, particularly adults, will be able to swim away from the area, thus avoiding the area of direct impact and temporary reductions in dissolved oxygen levels. It is important to note that young juvenile fish are likely to be less adept at escaping than adults. The majority of fish in the Solent area are pelagic and, therefore, free swimming and likely to be able to avoid the impact. Given the low probability of occurrence and the low scale of impact to pelagic fish species, the temporary exposure of pelagic fish being directly impacted by the falling plume is considered to be negligible and, therefore, impact insignificant. For bottom-dwelling demersal fish, the probability of occurrence and the magnitude of the change are both considered to be medium given their lower importance to the area in terms of numbers and their reduced ability to avoid the impact. The most common bottom-dwelling demersal fish frequenting the area, according to the Cefas surveys, and, therefore, the most likely to be impacted are gobies, dab and corkwing wrasse. The exposure of the demersal fish to being directly impacted by the disposals is considered to be medium/low and the temporary impact, therefore, insignificant to minor adverse significant, given their high sensitivity to the impact and relatively low importance to the area.

12.31 The intended spread of the material over the complete disposal ground is likely to cause humps

on the sea bed up to 3-4m in height at different locations within the ground, which will reduce in height over time, leaving minimal trace on the seabed within a few weeks following cessation of the deposits. Simulation of the dispersal and deposition of sediments from the deposit ground indicates that the magnitude of the impact on shallow coastal waters of the East Solent, and around the east and south coasts of the Isle of Wight will be small to negligible (Chapter 8). Although there will be disturbance to the feeding, nursery and spawning activity of fish in the area, namely bass, sole, plaice and cod (Figure 12.1), the disturbance will be temporary and similar to that experienced during existing disposal of maintenance dredge arisings. The deposition of sediment on particular communities of conservation interest in the area surrounding the Nab Deposit Ground was assessed as being insignificant to minor adverse significant (Chapter 11). Despite the fact that, such communities (e.g. Sabellaria spinulosa reefs) are important habitats for fish and the fact that the subtidal resource represents a feeding resource, as well as nursery and spawning grounds for fish, the impact on fish populations is expected to be insignificant to minor adverse significant.



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Potential Impact Arising as a Result of the Effects to Water Quality

Suspended Sediments

During Capital Dredging 12.32 During dredging and disposal, there will be temporary increases in suspended sediments in the

water column. The scale of these effects will vary in time and space depending on tidal state, range of tide, material type, as well as location, rates and methods of dredging and disposal. Suspended sediment levels are expected to return to background levels within one spring/neap cycle following the cessation of the dredging. The results of the dispersion modelling are presented in Chapter 8.

12.33 The direct effects of increased suspended sediments on adult fish are the physiological effects

resulting from the abrasion of sediment particles on gill tissues resulting in reduced gill function and possible death. These detrimental effects occur at suspended sediment levels of around 10,000mg/l (Britwell, 2000). These are only likely to occur for a few minutes at the exact position of the disposal at the Nab (Chapter 8). At lower concentrations, suspended sediments can indirectly impact fish by increasing turbidity, and thus reduce foraging and feeding ability due to a reduced visual range, or cause a sediment barrier to migration. Given the temporary and transient nature of the changes to suspended sediments, fish are likely to return to the same areas after the dredging and disposal activities have ceased. The significance of each of the potential impact pathways brought about by the proposed works is assessed in the following sections.

Feeding Ability

12.34 Reduced visual range and consequent effects to fish feeding ability are likely to occur for short periods during the dredging within Southampton Water. The high mobility of fish enables them to escape and avoid areas of adverse conditions, although the avoidance reaction will vary between species, with adult fish generally being able to avoid adverse conditions more rapidly and effectively than larval and juvenile fish. Fish are likely to find some difficulty in avoiding the disturbance caused by dredging over LW, when a greater proportion of the channel is affected by elevated suspended sediments. Although the magnitude of the change to suspended sediment concentrations in Southampton Water is considered to range from medium to high, given its short duration and temporary nature, the fish would have a low to medium exposure to elevated levels of suspended sediments, depending on the species. On cessation of dredging activities, enhanced suspended sediment concentrations are expected to return to background conditions within a spring/neap cycle. Given the moderate sensitivity of feeding fish to the short-term and transient increase in suspended sediments predicted to occur during dredging, the temporary impact to their feeding ability will be insignificant to minor adverse significant for the majority of fish that frequent the study area, and minor to moderate adverse significant to fish of higher nature conservation importance, such as salmon and juvenile bass.

12.35 As a moderate adverse significant impact to fish of high nature conservation value has been

assessed, an adaptive management strategy is being discussed with the Environment Agency and will be adopted to mitigate for the water quality effects of the dredge in Southampton Water, based around the monitoring of suspended sediment concentrations. An outline of the proposed mitigation management plan is included in Chapter 21. With the proposed mitigation measures in place, the residual effect will reduce to minor adverse significant.



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Migratory Patterns 12.36 Migratory fish are sensitive to any changes that act as a barrier to migration, which delay,

reduce or prevent spawning. The sensitivity of salmonids in Southampton Water is, therefore, highest in the inner estuary upstream of Dock Head, due to the highly confined nature of the migratory passageway in which the ability of fish to freely move to avoid adverse conditions is more restricted. Sensitivity is lower in the relatively unconfined passageway of the outer estuary and throughout the Solent.

12.37 Because of their migratory behaviour and dependence on marine, brackish and fresh water

conditions at different stages of their lifecycle, migratory fish are able to tolerate variations in a number of environmental parameters, including the full range of salinities and variable dissolved oxygen and turbidity. However, there are levels and durations of change that migratory fish are unable to tolerate. Lethal and sub-lethal effects may therefore, occur, such as avoidance, physiological stress and damage.

12.38 Salmon and sea trout will be vulnerable to elevated suspended sediment concentrations while

they migrate through Southampton Water on their way to and from the internationally important salmonid rivers of the Test and Itchen. Direct mortalities of salmonids from suspended sediments, however, have seldom been observed (EIFAC, 1965; cited in FARL, 1995). Increased suspended sediments and turbidity in the water column are more likely to induce sub-lethal effects in salmonids, such as swimming to surface waters to escape higher suspended sediments at depth, abrasion and damage to the gill filaments and impairment to osmoregulatory capacity. Fish are considered able to tolerate low levels of enhanced suspended sediment concentrations, and should recover from sub-lethal effects induced by moderate to high concentrations once conditions return to background levels. The sensitivity of different salmon populations also varies with the background suspended sediments that occur in the specific estuaries and rivers through which each population migrates. Salmon runs are known to occur against a wide range of background suspended sediment concentrations in UK estuaries, with salmon successfully passing through highly turbid estuaries, such as the Severn, Wye, Usk and Parrett, which contain concentrations of suspended sediments up to several thousand mg/l of (EIFAC, 1965; cited in FARL, 1995). Concentrations as high as 9,000mg/l, for example, have been recorded in the path of salmon runs in the Usk Estuary (Alabaster, 1993).

12.39 During the dredging, the highest suspended sediment concentrations would be predominantly

in the vicinity of the dredger, particularly during periods of overflow, and along the streamline of the dredging activity (i.e. generally along the navigation channel). Maximum suspended sediment levels will occur in the area of widening between Dock Head and Fawley during the dredging of alluvium, reaching around 600mg/l and occurring for no more than around 2 hours in every 6 hours over spring tides. The highest levels away from the dredger occur at the edges near LW in very shallow water, where the fish are unlikely to be swimming. Concentrations will vary throughout the dredge, both temporally and spatially throughout the estuary (Chapter 8). Migratory fish will be able to avoid the disturbance plume for at least 70% of the time during the period of dredging in the widening area between Dock Head and Fawley and move to less exposed areas.

12.40 In Southampton Water and the Test Estuary, the migratory passageway is more restricted, and

therefore, any changes will be more confined and any impacts of more concern. Although the



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passage of migratory fish and smolt will continue, there is likely to be intermittent disruption to the paths normally taken during the main smolt and adult salmonid runs, particularly in the middle to outer estuary, between Dock Head to Fawley, where the highest suspended sediment concentrations are predicted to occur (Chapter 8). The probability of a sediment barrier to migration is considered to be restricted to occurring for a maximum of around 2 hours in every 6 hours based on the findings of the modelling, and the magnitude would be small, given the scale, as well as the short-term and intermittent frequency of the change. Given that migratory fish inside the estuary will be particularly sensitive to any barriers to their migration, the impact is considered moderate adverse significant for migratory fish of high nature conservation importance, namely salmon, and minor adverse significant for other migratory species, such as eels and flounder.

12.41 Outside Southampton Water, the exposure to changes in suspended sediment levels will be

negligible/low and the sensitivity of migratory fish will be low given the unconfined nature of the migratory passageway in which fish will be able to freely move and avoid localised adverse conditions. Therefore, the impact to migratory fish outside Southampton Water is considered to be insignificant to minor adverse significant.


assessed, an adaptive management strategy is being discussed has been agreed with the Environment Agency and will be adopted to mitigate for the water quality effects of the dredge in Southampton Water, based around the monitoring of suspended sediment concentrations. An outline of the proposed mitigation management plan is included in Chapter 21. With the proposed mitigation measures in place, the residual effect will reduce to minor adverse significant.

Spawning and Nursery Grounds

12.43 High levels of suspended sediments can damage eggs and planktonic larvae, as well as cause abrasion or clogging of the fragile gills of larval and juvenile fish, resulting in mortality or reduced growth rates. Although spawning adults and juvenile fish may move to avoid adverse conditions, eggs and larvae are unable to avoid areas of elevated suspended sediments. Any reduction in spawning success and any sub-lethal or lethal effects on juveniles can have implications for fish populations.

12.44 During dredging, elevated suspended sediments are predicted to occur in Southampton Water,

particularly between Dock Head and Fawley, with maximum concentrations, for most of the time, being below about 300mg/l above background with maxima reaching around 600mg/l for short periods of around 1-2 hours. These changes will be very transient in both space and time (Chapter 8). The increase in suspended sediment levels outside Southampton Water is predicted to be far lower, generally below 50mg/l, and on average around 30mg/l, lasting for very short periods of time (Chapter 8).

12.45 Based on the predictions of the modelling, the exposure to changes in suspended sediment

levels outside Southampton Water will be low/negligible. Spawning and juvenile fish will have a moderate sensitivity to the change and given that the Solent is considered to be of moderate importance as a nursery and spawning ground, the impact will be insignificant to minor adverse significant.



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12.46 The exposure to changes in suspended sediments in Southampton Water will be low to medium, given the high probability of occurrence and the relatively small magnitude, due to the short-term and transient nature of the change (Chapter 8). The sensitivity of spawning and juvenile fish in the estuary is moderate, although it is important to consider that existing spawning and nursery activity in the estuary takes place against regular maintenance dredging. The impact is, therefore, considered to be minor adverse significant for designated bass nursery areas in Southampton Water, upstream of Fawley, and insignificant to minor adverse significant for other spawning and juvenile fish activities in the estuary.

During Disposal of Dredge Arisings

12.47 Modelling has indicated that maximum concentrations near to the point of disposal will exceed 4000mg/l, but rapidly reduce to below 1000mg/l within about 30 minutes following each disposal and within about 2.6km down stream on the ebb and less than 1km on flood flows (Chapter 8). Levels exceeding the 10,000mg/l level, whereby detrimental effects occur to fish (Britwell, 2000), are only likely to occur for a few minutes through the water column in close proximity to the exact deposit location at the Nab Deposit Ground (Chapter 8).

Distribution and Feeding Ability

12.48 Many adult fish are likely to be able to avoid the disposal plume, although it is possible that some fish may be impacted, particularly young fish that are less able to escape. Overall, the high suspended sediment concentrations of the initial disposal plume would impact on a very localised area of seabed, and be rapidly dispersed with the currents, thus affecting a relatively small number of individuals. Given the high probability of occurrence and the high magnitude of effect, the exposure of fish being directly impacted by the falling plume is considered to be high, both through the water column and on the bed, the latter due to smothering of demersal species. Within the water column, due to the unconfined deep-water site, the sensitivity of pelagic fish is considered to be low, as they will be able to swim away from the locations of high suspended sediment concentrations. The overall significance of enhanced suspended sediment concentrations to fish directly in the area of the deposit ground is considered to be insignificant to minor adverse significant.

12.49 It should be noted that similar deposits have occurred at the location in the past. A large

proportion of the sediment will be alluvial in origin and could contain a food source for some fish. A proportion of this is likely to be released into the water column during disposal. It is, therefore, possible that larger populations of fish could be attracted to the area than would be the norm. Thus, the direct effects on fish arising from the disposal could result in an increase in the abundance of fish above background. Overall, the initial disposal plume would impact on a very localised area of seabed and a relatively small number of fish would benefit from this food source. Given the low probability of occurrence and the low scale of impact, the temporary exposure of fish to a potential food source is considered to be negligible and, therefore, the impact is considered to be insignificant.

12.50 Overall, the elevated levels of suspended sediment that are predicted to occur away from the

immediate vicinity of the deposit location, along the line of the initial disposal plume, are considered to be tolerable to fish and likely to result in a minor redistribution of fish in the area, with some fish being attracted to the food source released from the dredged material. Elevations in suspended sediments elsewhere within the Solent and English Channel are unlikely to result in a change in the distribution of fish.



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Spawning and Nursery Grounds

12.51 As the disturbance will be temporary and similar to that experienced during existing disposal of maintenance dredge arisings, the impact on the feeding, nursery and spawning activity of fish in the area is considered to be insignificant to fish populations.

Migratory Patterns

12.52 Given the unconfined nature of the migratory passageway at the Nab Deposit Ground and surrounding area, the overall sensitivity is considered to be low for migratory fish and the impact will be insignificant to minor adverse significant.

Organic Enrichment and Oxygen Depletion

12.53 The resuspension of sediments containing organic material can cause oxygen depletion of the

water column. The subsequent settling of this organic rich sediment can deplete the sediments of oxygen and affect benthic prey used by fish. The response of fish to low concentrations of dissolved oxygen is determined by a range of factors, including the duration of exposure, water temperature and the presence of other pollutants. The duration of any low dissolved oxygen event is a key factor in determining its effect. Most fish would survive an extremely low concentration of dissolved oxygen, such as 2mg/l, for a few minutes, but a longer exposure would start to have sub-lethal and eventually lethal effects (ABP Research, 2000b).

12.54 The general water quality assessment (GQA) review that was undertaken by Nixon et al.

(1994) on the effects of dissolved oxygen to estuarine environments concluded the following:

The early life states of fish are the most sensitive to oxygen concentrations; The tolerance of various estuarine fish to low dissolved oxygen varies, with flounder

appearing the least sensitive and data indicating salmonids to be the most sensitive, particularly during upstream adult migration; and

Data indicated that fish, including salmonids are able actively to avoid low dissolved oxygen concentrations.

12.55 On the basis of their extensive review, Nixon et al. (1994) proposed the following classification

scheme:

Class A Oxygen > 8mg/l No effects; Class B Oxygen 4-8mg/l Low risk of effects; Class C Oxygen 2-4mg/l Possible effects; and Class D Oxygen <2mg/l Probable effects.

12.56 With regards to sensitive salmonids, Hugman et al. (1984) suggested a standard of 5mg/l for a

good quality migratory salmonid fishery. In general, estuarine waters require at least 4.5-5mg/l of dissolved oxygen to allow the free passage of migratory salmonids in UK waters. A threshold of 4mg/l, an annual mean of 5mg/l and a 95%-ile of 3mg/l was also considered to support the migratory passage of fish (Stiff et al., 1990).

12.57 There is currently no legally defined Environmental Quality Standard (EQS) for dissolved

oxygen in UK estuaries, although standards are in place under the Shellfish Waters Directive,



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whereby an individual measurement must not fall below 60% saturation and the average must not fall below 70% saturation. There are also standards that have been proposed by UK regulators to support ecological status classification under the Water Framework Directive (WFD). By ensuring that the statutory standards within the Shellfish Water Directive are met during dredging, the broader marine ecology of the estuary as well as commercial fisheries will be protected.

12.58 Mean background dissolved oxygen saturation at all Environment Agency sample sites within

Southampton Water and the Solent were high (Chapter 10). Based on previous monitoring and research, although it is probable that there will be a reduction in dissolved oxygen concentrations as a result of the proposed dredge, it is considered that the magnitude of change to dissolved oxygen levels will be small in effect, on the basis of the classification proposed by Nixon et al. (1994) and it is unlikely that levels would fall below the Hugman et al. (1984) and Stiff et al. (1990) migratory fish limits, or standards under the Shellfish Waters Directive or the good-moderate ecological status class boundary proposed by UK regulators to support the WFD (Chapter 10). Any change in dissolved oxygen will be variable in space and time, with the greatest reduction occurring at the bed in the local vicinity of the dredged area. The combination of a high to low probability of occurrence and small magnitude will lead to a low exposure of oxygen depletion during dredging. Depending on the scale of change, the sensitivity of fish to oxygen depletion events is considered to be moderate to high in the more confined area of Southampton Water and the Test Estuary, and low outside Southampton Water.

12.59 For fish of high nature conservation importance, such as migratory salmonids and juvenile

bass, the impact of a reduction in dissolved oxygen as a result of the dredge is considered to be variable between minor to moderate adverse significant in the estuary depending on time relative to individual dredging events and time in the tide, and insignificant to minor adverse significant outside Southampton Water.

12.60 For fish of lower conservation importance, such as resident fish populations that frequent the

study area, the impact is considered to be insignificant to minor adverse significant in the estuary, and insignificant outside the estuary.


assessed, an adaptive management strategy is being discussed has been agreed with the Environment Agency and will be adopted to mitigate for the water quality effects of the dredge in Southampton Water, based around the monitoring of dissolved oxygen levels. An outline of the proposed mitigation management plan is included in Chapter 21. With the proposed mitigation measures in place, the residual effect will reduce to minor adverse significant.

Contaminants

12.62 The potential release of contaminants associated with dredged materials may be available for

uptake by any fish in the water column. The chemical sediment sampling has indicated that for the most part the levels of contaminants are negligible to very low within the areas proposed for dredging, with levels largely falling below Cefas Action Level 1 (Chapter 9). The accumulation of moderate or high levels of contaminants in fish can cause or contribute to a range of lethal and sublethal effects, including genetic, reproductive and growth changes. There is less



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information available on the effects of low levels of contaminants, such as those likely to be experienced during dredge operations. A survey of the quality of UK coastal waters demonstrated that metal and organochlorine (including the pesticides dieldrin, PCB and DDT) contamination in fish is confined to a few well-known geographical locations (MPMMG, 1998). This study revealed that the accumulation of toxic contaminants in fish taken from a site in the East Solent was negligible. Given the moderate to high sensitivity of fish to the uptake of contaminants along with the negligible/low exposure to contaminants in the water column, the impact on most fish species frequenting the study area during dredging and disposal is considered to be insignificant to minor adverse significant. The impact on fish species of higher nature conservation importance, such as salmon and juvenile bass, is considered to be minor adverse significant.

Potential Disturbance Due to the Generation of Underwater Noise and Vibration

12.63 Exposure to high noise limits has the potential to cause a range of reactions in fish from lethal

injury at high levels to avoidance behaviour at lower levels. Although Atlantic salmon are considered to be relatively insensitive to underwater sound (pers comm. Adrian Fewings, 5 March 2008) it is important to consider whether the dredging activities will impact their migratory passage, i.e. affect more than 50% of the available channel width.

12.64 A report commissioned by Department of Business Enterprise and Regulatory Reform (BERR)

has issued guidance in connection with the auditory and behavioural effects of underwater noise (Nedwell et al., 2007). Based on this guidance, the following criteria were suggested for the effects of noise: 0-50 dBht (species) - low likelihood of disturbance; and 50-90 dBht (species) - stronger avoidance reaction by most individuals.

12.65 dBht is defined as the number of decibels (dB) above a given species’ hearing threshold. 12.66 The Southampton Approach Channel Dredge is likely to be undertaken by a combination of

backhoe and TSHD dredging. Further information on the proposed methodology is provided in Chapter 3.

12.67 Measurements undertaken by Subacoustech Ltd of backhoe dredging activities indicate that

the noise threshold levels for salmon are not likely to exceed 50dBht (Salmo salar) within several metres of the backhoe dredger (Appendix H). Given their low sensitivity to noise, the impact on migratory salmonids from backhoe dredging is considered to be insignificant throughout the study area i.e. in the Solent, throughout Southampton Water and the Test Estuary. Subacoustech Ltd has also undertaken underwater noise recordings from a suction dredger. Analysis of the data would indicate that a 50dBht (Salmo salar) loudness zone would not exceed a distance of 50m. A distance of 50m is considerably less than 50% of the available channel width at any given time and location of the dredging works. The impact to migratory salmonids from trailing suction hopper dredging (TSHD) is, therefore, considered to be insignificant throughout the study area. Potential Impact Due to Compounding Effects of Impacts



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12.68 Following the Environment Agency’s response in the Scoping Opinion (Appendix A), the compounding effects of several impacts to fish interests acting together i.e. temperature, dissolved oxygen, suspended sediments and noise have also been considered.

12.69 On the basis of the above assessment of effects on fish, migratory fish are particularly sensitive

to changes in suspended sediment concentrations, oxygen depletion and noise, which in isolation, or in combination, could potentially prevent their migration to or from the rivers and streams where they spawn, or even cause physical damage. This has been shown to be minor to moderate adverse significant for short periods on any tidal cycle and an adaptive management process will, therefore, be developed and is discussed in Chapter 21.

12.70 There can also be cumulative effects as a result of a number of different changes occurring at

the same time. Low dissolved oxygen concentrations, for example, can make salmon more susceptible to other stress, such as handling, as well as reduce their ability to swim away from polluted water and increase the toxicity of other pollutants such as ammonia (Priede et al., 1989; Alabaster et al., 1979; Thurston et al., 1981). Also, the rapid avoidance reaction of salmon to "unacceptable" conditions, such as noise, could result in highly energetic sprint behaviour which causes an oxygen deficit, making them more susceptible to the effects of low oxygen levels or regions of high suspended sediment (ABP Research, 2000b). In previous observations using acoustic tagging, salmon were reported to pass through a zone of working dredgers generating underwater noise and increased suspended sediments, with little or no delay (Westerberg, 1982).

12.71 The degree of dissolved oxygen reduction is closely linked to suspended sediment

concentrations and temperatures. On a horizontal spatial dimension, oxygen depletion events are more likely to be a problem during the period of dredging when higher elevations in suspended sediments of material with high organic content (alluviums) occur, i.e. in the mid estuary. On a temporal dimension, the maximum changes in suspended sediments are predicted to occur predominantly on spring tides, albeit for very short period of times, and, therefore, the greatest compounding effects with dissolved oxygen will be during the summer months when higher water temperatures occur, predominantly on spring tides at times of maximum flows. On a vertical dimension, the greatest changes to suspended sediments and dissolved oxygen will occur low down close to the bed during the initial release of the overflow, diffusing upwards to the surface of the water. This coincides with the greatest underwater noise levels of the dredger, attenuating and reducing with distance from the source. Therefore, although only occurring for very intermittent and short periods of time, the greatest exposure of compounding effects on the passage of migratory salmonids will occur between Dock Head to Fawley, at depth, close to the dredger, and on a summer spring tide. Outside Southampton Water and in the Test, Itchen and Hamble Estuaries, the exposure will be far lower.

12.72 Adult salmon are thought to swim near the surface during migratory runs and an acoustic

tagging study found that smolts actively swam seaward close to the surface of water in the middle and outer sections of Southampton Water. Therefore, the sensitivity of salmon to the greatest exposure of compounding effects is considered to be low. The impact of compounding effects to migratory salmonids is, therefore, considered to be moderate adverse significant in Southampton Water.



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12.73 Outside Southampton Water, the exposure to compounding effects will be low and the sensitivity to fish low/negligible in the unconfined passageway, and, therefore, the impact is considered to be minor adverse significant in the Solent.

12.74 In the Test and Itchen Estuaries, although the exposure of compounding effects is still low, the

more restricted passage and areas, such as the Upper Swinging Ground, at Bury where smolts are known to aggregrate for periods of time during the smolt run (Moore & Ives, 1997), suggests the sensitivity of migratory salmonids will be moderate. Therefore, the impact of compounding effects to migratory salmonids is considered to be minor adverse significant in the estuaries.

12.75 As an impact that is moderate adverse significant has been predicted, an adaptive

management strategy will be implemented as mitigation for the dredging in the middle part the estuary, based around monitoring of suspended sediment concentrations and dissolved oxygen concentrations. This is identified in more detail in Chapter 21. Taking into account the proposed mitigation measures, the exposure of salmon and sea trout to the disruption of migratory movements will be low and the residual effect would be minor adverse significant.

Potential Impact During Future Maintenance Dredging

12.76 The same potential impact to fish applies during maintenance dredging as for capital dredging,

although the scale of impact will be substantially less. Furthermore, the effects of future maintenance dredging work are expected to be small in the context of existing annual variability (Chapter 8). When taking into account the measures undertaken for existing maintenance dredging and acknowledging the pro-active involvement between the Agency’s fisheries staff and ABP, future maintenance dredging will have an insignificant impact on the fish of Southampton Water and the Solent. For these reasons, the above detailed assessment of the pathways of change concentrates on the effects of the capital dredging works.

Conclusions

12.77 The potential impacts to fish have been assessed according to the Marine Works (EIA)

Regulations 2007. As a qualifying feature for the River Itchen SAC, the assessment of the potential impacts to migratory Atlantic salmon have also informed the Appropriate Assessment signposting document (Appendix D) for the purposes of the Habitats Regulations. The main impacts to fish will be temporary, occurring during the capital dredging and disposal of arisings. The scale of the majority of the potential impacts to fish will be temporary and insignificant to minor adverse significant. The short-term increase in the level of suspended sediments, and subsequent depletion of dissolved oxygen during dredging in Southampton Water are considered to result in a potential temporary impact up to moderate adverse significant for the feeding ability and migratory patterns of fish of higher nature conservation importance, namely Atlantic salmon and juvenile bass. This will be mitigated by the implementation of an adaptive management strategy during the capital dredge. Following the capital dredge, the impacts of the changes to the hydrodynamic and sediment regime brought about by the proposed channel, including the impacts during future maintenance dredging, are all considered to be insignificant with respect to fish.

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