Shellfish
184. Of the key shellfish species of the fish and shellfish ecology study area, crustaceans such as European lobster and crab tend to be physiologically resilient to noise due to the lack of gas within their bodies (Popper et al., 2001). To date, no lethal effects of underwater noise have been described for edible crab, European lobster or Nephrops. A report by Christian et al. (2003) found no significant difference between acute effects of seismic airgun exposure (a similar impulsive high amplitude noise source to piling; >189 dB re 1 μPa (peak–peak) @ 1 m) upon adult snow crabs Chionoecetes opilio in comparison with control crabs, and Parry and Gason (2006) investigated whether there was a link between seismic surveys and changes in commercial rock lobster Panulirus cygnus based on rates associated with acute to mid-term mortality over a 26-year period. No statistically significant correlation was found (Parry and Gason, 2006).
185. Sub-lethal physiological effects have been identified from impulsive noise sources including bruised hepatopancreas and ovaries in snow crab exposed to seismic survey noise emissions (at unspecified SPLs) (DFO, 2004), changes in serum biochemistry and hepatopancreatic cells (Payne et al., 2007), increase in respiration in brown shrimp (Solan et al., 2016), and metabolic rate changes in green shore crab Carcinus maenas.
186. There is no evidence to suggest shellfish eggs and larvae are at risk of direct harm from underwater noise such as piling (Edmonds et al., 2016). Rather, of the few studies that have focussed on the eggs and larvae of shellfish species, evidence of impaired embryonic development and mortality has been found to arise from playback of seismic survey noise among gastropods and bivalves (De Soto et al., 2013, Nedelec et al., 2014). Limited information exists on the impact of impulsive sound upon crustacean eggs, and no research has been conducted on commercially exploited decapods around the UK. Of the evidence that is available all studies focus on the impact of seismic noise, which delays hatching of snow crab eggs, causing resultant larvae to be smaller than controls (DFO, 2004). Pearson et al. (1994) found no statistically significant difference between the mortality and development rates of stage II (their free-swimming, planktonic larval stage) Dungeness crab Metacarcinus magister larvae exposed to single field-based discharges (231 dB re 1 μPa (zero-peak) @ 1 m) from a seismic airgun.
187. Roach et al. (2018) examined the effects on catch rates of European lobster of a temporary closure of lobster fishing grounds during offshore wind farm construction (including piling). Monitoring data at the Westermost Rough Offshore Wind Farm (north-east coast of England) found that the size and abundance of European lobster increased following temporary closure of the area during its construction. While not looking specifically at the effects of underwater noise on shellfish species, this study implies that the activities associated with construction of the wind farm (which included piling of foundations for 80 wind turbines) did not impact on the resident European lobster populations and instead allowed some respite from fishing activities for a short period time before reopening following construction (Roach et al., 2018). The results therefore suggest that population level injury impacts on shellfish species are unlikely to occur due to piling operations.
Behaviour
189. Popper et al. (2014) provides qualitative behavioural criteria for fish from a range of noise sources. These categorise the risks of effects as “high”, “moderate” or “low” at three distances from the source: “near” (i.e. tens of metres), “intermediate” (i.e. hundreds of metres) or “far” (i.e. thousands of metres). The behavioural criteria for piling operations are summarised in Table 9.27 Open ▸ for the four fish groupings.
Table 9.27: Potential Risk for the Onset of Behavioural Effects in Fish from Piling (Popper et al., 2014)c
c Note: Relative risk (high, moderate, low) is given for animals at three distances from the source defined in relative terms as near field (N; i.e. tens of metres), intermediate (I; i.e. hundreds of metres), and far field (F; i.e. thousands of metres); Popper et al. (2014).
190. Highly sensitive hearing specialist species such as herring have an otic bulla; a gas filled sphere, connected to the swim bladder, which enhances hearing ability. The gas filled swim bladder in species groups such as cod and salmon may be involved in their hearing capabilities, so although there is no direct link to the inner ear, these species are able to detect lower noise frequencies and as such are considered to be of medium sensitivity to noise. Flat fish and elasmobranchs have no swim bladders and as such are considered to be relatively less sensitive to sound pressure.
191. Several studies have examined the behavioural effects of the sound pressure component of impulsive noise (including piling operations and seismic airgun surveys) on fish. For example, Mueller-Blenkle et al. (2010) recorded behavioural responses of cod and sole to sounds similar to those produced during marine piling, with variation noticed across specimens (i.e. depending on the age, sex, condition etc. of the fish, as well as the possible effects of confinement in cages on the overall stress levels in the fish). Mueller-Blenkle et al. (2010) concluded that it was not possible to find a clear relationship between the level of exposure and the extent of the behavioural response, although an observable behavioural response was reported at 140 dB to 161 dB re 1 μPa SPLpk for cod and 144 dB to 156 dB re 1 μPa SPLpk for sole. Regardless, these thresholds should not be interpreted as the level at which an avoidance reaction will be elicited, as the study was not able to show this.
192. Further, a study by Pearson et al. (1992) examined the effects of geophysical survey noise on caged rockfish Sebastes spp. and observed a startle or “C-turn response” at peak pressure levels beginning around 200 dB re 1 μPa. This response was less common with the larger fish. Studies by McCauley et al. (2000) exposed various fish species in large cages to seismic airgun noise and assessed behaviour, physiological and pathological changes. The study observed that:
- a general fish behavioural response was to move to the bottom of the cage during periods of high level exposure (greater than RMS levels of around 156 dB to 161 dB re 1 μPa; approximately equivalent to SPLpk levels of around 168 dB to 173 dB re 1 μPa);
- a greater startle response was seen in small fish to the above levels;
- a return to normal behavioural patterns was noticed some 14 to 30 minutes after airgun operations ceased;
- no significant physiological stress increases attributed to air gun exposure; and
- some preliminary evidence of damage to the hair cells was noticed when exposed to the highest levels, although it was determined that such damage would only likely occur at short range from the source.
193. Post construction monitoring at the Beatrice Offshore Wind Farm (BOWL, 2021) concluded that, for sandeel, there was no evidence of adverse impacts on sandeel populations between pre and post construction levels over a six year period. Similarly for cod, there was no change in the presence of spawning between pre and post construction (although spawning intensity was found to be low across both surveys). Based on these studies, it can therefore be assumed that noise impacts associated with installation of an offshore wind development are temporary and that fish communities (specifically cod and sandeel in this case) show a high degree of recoverability following construction.
194. Impacts of underwater noise on marine invertebrates is limited, and no attempt has been made to set exposure criteria (Hawkins et al., 2014). Aquatic decapod crustaceans are equipped with receptor types potentially capable of responding to the particle motion component of underwater noise (e.g. the vibration of the water molecules which results in the pressure wave) and ground borne vibration (Popper et al., 2001). It is generally their cilia that provide the sensitivity, although these animals also have other sensor systems which could be capable of detecting vibration. It has also been reported that sound wave signature of piling noise can travel considerable distances through sediments (Hawkins and Popper, 2016), with implications for demersal and sediment dwelling fish (e.g. sandeel) and shellfish (e.g. Nephrops) in close to piling operations.
195. At Westermost Rough Offshore Wind Farm, monitoring of European lobster revealed no population level effects on shellfish species. (Roach et al., 2018). While there may be some residual uncertainty regarding behavioural effects while piling operations are ongoing, the evidence suggests that long term effects will not occur, and any effects will be reversible.
Summary – marine fish and shellfish species
197. Behavioural effects are expected over much larger ranges than injury ranges. For example, Figure 9.7 Open ▸ shows the modelled underwater noise levels for SPLpk based on the results from volume 3 appendix 10.1, relative to key fish spawning habitats in the vicinity of the fish and shellfish ecology study area. Figure 9.8 Open ▸ shows noise contours for the maximum (4,400 kJ) hammer energy in relation to cumulative herring spawning larval densities (i.e. the core herring spawning habitat in the fish and shellfish study area). The northern piling location was chosen as the point closest to the most sensitive habitats/areas.
198. Noting that there are no published or agreed thresholds for behavioural effects on fish from piling operations, the noise contours presented below suggest that behavioural responses will extend over ranges of 33 km to 49 km; for example, assuming avoidance occurs at levels in excess of 160 dB re 1 μPa SPLpk, which is a lower threshold than the levels at which behavioural effects in fish were detected in a number of studies (including McCauley et al., 2000). These results broadly align with qualitative thresholds for behavioural effects on fish as set out in Table 9.27 Open ▸ , with moderate risk of behavioural effects in the range of hundreds of metres to thousands of metres from the piling activity, depending on the species. As previously discussed, these behavioural response thresholds are likely to be highly precautionary for the less sensitive group 1 and group 2 fish species. For some of the more sensitive groups 3 and 4 fish species in the fish and shellfish study area (e.g. cod and herring), further detail is given below.
199. For cod (group 3 fish; Figure 9.7 Open ▸ ), low intensity spawning grounds are ubiquitious in the north sea and overlap with the site boundary. Based on modelling at the south location, underwater noise levels with the potential to cause behaviour effects (approximately 160 dB re 1 μPa SPLpk) area predicted to coincide with a very small proportion of this spawning habitat, which is vast and surrounds the Array from inshore waters, out to the North Sea’s offshore waters. The same is true for sandeel (group 2 fish), plaice (group 1 fish) and whiting (group 3 fish; Figure 9.7 Open ▸ ); whilst the Array exists over low intensity spawning grounds exists for these species, underwater noise levels from piling using a 4,400 kJ hammer energy is expected to travel across a very small proportion of their spawning habitats, which, like for cod, is vast around the Array.
200. Herring (group 4) spawning grounds exist to the north of the Array, with Figure 9.8 Open ▸ showing the core spawning habitats (as mapped using cumulative herring larval abundance data) and noise contours associated with piling at the closest possible location within the Array. Based on modelling at the north location, underwater noise levels with the potential to cause behavioural effects (i.e. approximately 160 dB re 1 μPa SPLpk) is predicted to coincide with a small proportion of this spawning habitat. Further, the core, regular spawning ground for herring is well outside the 160 dB contour ( Figure 9.8 Open ▸ ). It is acknowledged that spawning grounds are not fixed boundaries, and spawning does not occur at an equal density across the mapped grounds, with variation inside and outside mapped grounds annually and throughout the spawning season.
201. A concurrent piling scenario was also modelled in addition to the single piling scenarios; see volume 3, appendix 10.1 for full details. This is presented in Figure 9.9 Open ▸ and it should be noted the contours presented are for single strike cumulative SEL metric (as opposed to SPLpk for the previous figures). Underwater noise modelling for concurrent piling assumed piling at the northern location concurrently with the central location, which is representative of the largest separation of the piling vessels, as detailed within volume 1, chapter 4, and a maximum separation of 30 km, to represent the scenario would result in disturbance over the greatest area. Although there is a possibility of a separation between vessels of up to 41 km, variation in seabed bathymetries and water depths make the separation modelled the scenario resulting in maximum disturbance. Figure 9.9 Open ▸ shows the noise contours associated with this concurrent piling scenario in the cumulative SELSS metric, alongside a single piling scenario in the same metric. These demonstrate that while the area of disturbance is expected to be greater, the range of effects from the site boundary is not greater than that of a single piling scenario and therefore cumulative piling would not result in a greater risk to the core herring spawning grounds within the fish and shellfish study area.
202. Most marine fish are deemed to be of low vulnerability, high recoverability and local to national importance. The sensitivity of the receptor is therefore, considered to be low.
203. Herring are deemed to be of medium vulnerability, high recoverability and regional importance. The sensitivity of the receptor is therefore, considered to be medium.
204. Shellfish are deemed to be of low vulnerability, high recoverability and regional importance. The sensitivity of the receptor is therefore, considered to be low.
Summary – diadromous species
205. As with marine fish, diadromous fish species close to piling operations may experience injury or mortality. However, diadromous fish species tend to be highly mobile and may only utilise the environment within the fish and shellfish ecology study area to pass through during migration. As such, piling is unlikely to result in significant mortality of diadromous species. The use of soft start piling procedures (see Table 9.18 Open ▸ ), may allow individuals in close proximity to piling to flee the ensonified area before the greatest hammer energies are reached, therefore reducing the likelihood of injury and mortality on diadromous species (depending on the species and their responses to elevated noise levels).
206. The studies discussed in paragraphs 188 to 196 are also relevant to diadromous fish species which, like marine species, may experience behavioural effects in response to piling noise, including a startle response, disruption of feeding, or avoidance of an area. As discussed in paragraph 198, behavioural effects (including avoidance) would be expected to occur at ranges of up to 33 km to 49 km, depending on the species and their relative sensitivities to underwater noise (i.e. in order of lowest to highest sensitivities: lamprey species, Atlantic salmon and sea trout, European eel and shad species). Harding et al. (2016) examined behavioural and physiological responses in Atlantic salmon when subjected to noise similar to piling. No responses were produced, though the noise levels tested were estimated at <160 dB re 1 µPa RMS, which is considerably below the level at which injury or behavioural disturbance would be expected for Atlantic salmon. Due to the distance between the Array and the coast, these behavioural impacts are unlikely to cause barrier effects between the fish and shellfish ecology study area and the migration routes of diadromous species along the east coast of Scotland, due to the relatively small area around piling events where noise levels are high enough to cause behavioural responses (as demonstrated in Figure 9.7 Open ▸ to Figure 9.9 Open ▸ ).
207. Diadromous fish species are deemed to be of low vulnerability, high recoverability and national to international importance. The sensitivity of the receptor is therefore considered to be low.
Figure 9.7: Cod, Plaice, Sandeel, and Whiting Spawning Grounds with Subsea 10 dB Sound SPLpk Contours for Piling at 4,400 kJ Hammer Energy at the South Modelled Location
Figure 9.8: Herring Larval Densities (combined 2007 to 2016 data) with Subsea 10 dB Sound SPLpk Contours for Piling at 4,400 kJ Hammer Energy at the North Modelled Location
Figure 9.9: Concurrent and Single Piling Scenarios Based Upon Using 3,000 kJ and 4,400 kJ Hammer Energies. Note, Contours are Shown in Cumulate SELSS Metric for Illustrative Purposes Only
Significance of the effect
209. For herring, the magnitude of the impact is deemed to be low, and the sensitivity of herring is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms. This is due to the hearing sensitivity of herring, coupled with the presence of a small proportion of undetermined intensity spawning grounds within range of underwater sound levels which may give rise to behavioural effects.
210. For diadromous fish, the magnitude of the impact is deemed to be low, and the sensitivity of most marine fish IEFs is considered low. The effect will, therefore, be of minor significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
211. No secondary fish and shellfish ecology mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 9.10) is not significant in EIA terms.
Underwater noise from the operation of floating wind turbines and anchor mooring lines impacting fish and shellfish receptors
212. Underwater noise has the potential to arise from wind turbine operation and movement of anchor mooring lines. This impact is relevant to the operation and maintenance phase and has the potential to cause direct and indirect impacts to fish and shellfish receptors.
213. The assessment presented below is informed by volume 3, appendix 10.1, which presents evidence for the conclusions for the impact.
Operation and maintenance phase
Magnitude of impact
214. The presence of operational floating wind turbines may result in the generation of underwater noise, occurring at a very low frequency and low sound pressure level (Andersson et al., 2011). As shown in Table 9.13 Open ▸ , the MDS assumes the maximum scale of the Array (based upon the maximum number of turbines), which accounts for up to 265 semi-submersible floating wind turbine foundations with up to 224 m hub height, placed 25 m deep in the water column with up to 100 m excursion limit. The MDS also accounts for noise generated from up to 1,590 catenary mooring lines and movement of these during the operation and maintenance phase. This impact has the potential to affect fish and shellfish receptors for the 35 year operation and maintenance phase.
215. Studies have demonstrated that underwater noise from operational fixed wind turbines is only high enough to possibly cause a behavioural reaction in fish and shellfish species within metres from a wind turbine. In addition, noise generated by operational fixed wind turbines is of a low frequency and low sound pressure level (Andersson et al., 2011). Therefore, noise levels from operational wind turbines at a level where there is a potential effect on fish and shellfish receptors are considered highly unlikely to occur (Sigray and Andersson, 2011). These observations from earlier fixed offshore wind farms (with smaller wind turbines) are supported by modelling of the noise emissions from larger fixed offshore wind turbines, which demonstrate that the risk of injury or behavioural effects on fish and shellfish populations is negligible (SSER, 2022a).
216. Putland (2022) presented a study into operational noise of floating offshore wind turbines; their findings indicate that operational noise is comparable to that of fixed bottom wind turbines, generating low level noise which is unlikely to cause significance disturbance effects to fish. Risch et al., (2023) have also reported consistent results. In this study, acoustic data was collected from two floating offshore wind farms, currently deployed off the Scottish east coast: Kincardine and Hywind Scotland. At Kincardine five wind turbines rated at 9.5 MW were deployed on semi-submersible foundations, while at Hywind Scotland five 6 MW rated wind turbines were deployed on spar-buoys. As described in volume 3 appendix 10.1, it was found that the predicted noise fields for unweighted sound pressure levels were above the median ambient noise levels in the North Sea for a maximum of 3.5 km to 4.0 km from the centre of the Kincardine site and 3.0 km to 3.7 km from the centre of Hywind Scotland (Risch et al., 2023). As noted above, while fish and shellfish receptors may be able to perceive noise, the noise levels are too low to result in injury or behavioural effects. The study also concluded that noise emissions from floating offshore wind turbines were predicted to be similar to the operational noise of fixed offshore wind turbines and found that the biggest difference between fixed and floating offshore wind turbines in relation to underwater noise generation is related to moorings, rather the operational wind turbine noise.
217. It is acknowledged in volume 3, appendix 10.1 that underwater noise may occur due to mooring line slackening and tensioning which has the potential to produce transient ‘pinging’ or ‘snapping’ noises during the operation and maintenance phase of the Array (Liu, 1973). Presence of snapping transient noise was identified during acoustic underwater noise measurements at the floating Hywind Demonstrator Project in Norway in 2011 (Martin et al., 2011). The data were subsequently analysed and Stephenson (2015) extrapolated results from a single wind turbine to a theoretical array and it was found that with up to 115 snapping events per day, the resultant potential cumulative SEL over a 24 hour period was 156 dB re 1 µPa2s at 150 m from the wind turbines. This value is below the ranges for recoverable injury and Temporary Threshold Shift (TTS) for Groups 3 and 4 fish.
218. With specific reference to operational turbines, the distances and exposures of fish reported by various studies (as set out in volume 3, appendix 10.1) conclude that while sound levels would likely be audible, these would not be at a level sufficient to cause injury or behavioural changes to fish. This is due to the slight increase in SPL compared to the ambient noise measured before the construction of the wind farms and even when the highest increases in SPL was assumed (i.e. 20 to 25 dB re 1 μ Pa), these are unlikely to result in a measurable impact on fish and shellfish receptors.
219. Therefore, it is concluded that the risk of effects on fish (either injury or behavioural responses) from underwater noise from this impact is very low, whether that is from the structure-borne noise expected from any offshore wind turbine, regardless of foundation type, and the additional noise generated by movements in the mooring lines.
220. Therefore, this impact is predicted to be highly localised in extent, long term duration and continuous and low reversibility during the operation and maintenance phase (impact is reversible upon decommissioning). The magnitude is therefore considered to be negligible.
Sensitivity of the receptor
221. The sensitivity of fish and shellfish IEFs to underwater noise for both marine fish and shellfish and diadromous fish species can be found in the assessment of ‘underwater noise from piling and UXO clearance impacting fish and shellfish receptors’ in the construction phase assessment (see paragraph 159 et seq.) with a summary of these sensitivities presented in in paragraph 197 et seq.).
Marine fish and shellfish species
222. Most marine fish are deemed to be of low vulnerability, high recoverability and local to national importance. The sensitivity of the receptor is therefore, considered to be low.
223. Herring are deemed to be of medium vulnerability, high recoverability and regional importance. The sensitivity of the receptor is therefore, considered to be medium.
224. Shellfish are deemed to be of low vulnerability, high recoverability and regional importance. The sensitivity of the receptor is therefore, considered to be low.
Diadromous species
225. Diadromous fish species are deemed to be of low vulnerability, high recoverability and national to international importance. The sensitivity of the receptor is therefore considered to be low.
Significance of the effect
Marine fish and shellfish species
226. Overall, for all marine fish and shellfish considered as IEFs, the magnitude of the impact is deemed to be negligible and the sensitivity of the receptor is considered to be low to medium.
227. The effect for all marine fish and shellfish IEFs will, therefore, be of negligible adverse significance, which is not significant in EIA terms.
Diadromous species
228. Overall, the magnitude of the impact is deemed to be negligible and the sensitivity of the receptor is considered to be low. The effect will, therefore, be of negligible adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
229. No secondary fish and shellfish ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.
Increased SSCs and associated deposition
230. Increased SSCs and associated deposition may arise due to the movement of hanging mooring lines along the seabed during the operation and maintenance phase of the Array.
231. Sediment modelling was undertaken related to the MDS as outlined in Table 9.13 Open ▸ with the detail of the assessment provided in volume 3, appendix 10.1.
Site preparation and construction phases
Magnitude of impact
- seabed preparation activities: boulder and sand wave clearance;
- DEA installation; and
- inter-array and interconnector cable installation and burial ( Table 9.13 Open ▸ ).
233. Boulder and sandwave clearance may be required for along inter-array and interconnector cables within a corridor of up to 25 m width, as set out in Table 9.13 Open ▸ . For perspective, modelling conducted for Berwick Bank Offshore Wind Farm considered a clearance width of 25 m for site preparation activities such as sand wave clearance (SSER, 2022b). This modelling showed that the resulting sediment plume would be very small, with SSCs of <100 mg/l. SSCs were predicted to peak during the deposition of cleared material, with concentrations reaching 2,500 mg/l at the release site, but the plume was predicted to be at its most extensive during the redistribution of the deposited material on successive tides (SSER, 2022b). Under these circumstances, concentrations of 100 mg/l to 250 mg/l were predicted with average values <100 mg/l extending out to one tidal excursion (SSER, 2022b). Sedimentation of deposited material was focussed within 100 m of the site of release with a maximum depth 0.5 m to 0.75 m, whilst the finer sediment fractions were distributed in the vicinity at much smaller depths (circa 5 mm to 10 mm) over a maximum distance of one tidal excursion (SSER, 2022b). As the seabed sediments at Berwick Bank Offshore Wind Farm are more coarse than those of the Array fish and shellfish ecology study area (which comprises largely deep circalittoral sand; Figure 9.2 Open ▸ ), the smaller sedimentation depths associated with finer sediment fractions (5 mm to 10 mm; (SSER, 2022b)) are more likely to be associated with site preparation activities for the Array.
234. Up to 1,590 DEAs may be pulled up to 60 m along the seabed during the construction phase; this will be undertaken in a controlled manner to ensure that DEAs are installed at the correct position and to appropriate depth. DEAs were not assessed in any publicly available EIAs for projects within the regional fish and shellfish ecology study area, though are discussed in a study on the environmental effects of wind turbine foundations (Horwath et al., 2020). This study concluded that floating foundations that use embedded anchors may have similar seabed-disturbing activities during installation when compared to monopiles, depending on the size of the anchors and method of installation (Horwath et al., 2020). The study noted that the extent that anchors drag along the seabed due to the forces on floating foundations is unknown but is likely to produce some additional SSCs (Horwath et al., 2020). Therefore, the low magnitude of impact associated with foundation installation at Berwick Bank Offshore Wind Farm, could be applied to the use of DEAs at the Array. Modelling of SSCs associated with foundation installation at Berwick Bank Offshore Wind Farm predicted plumes to have peak concentrations of <5 mg/l, with average values typically less than one fifth of this, and dropping to 1 mg/l to 2 mg/l within a very short distance, typically less than 500 m of the installation activity (SSER, 2022b). The sediment plumes were expected to be temporary, returning to background levels within a few tidal cycles (SSER, 2022b). The average sedimentation depth was predicted to be typically 0.05 mm to 0.1 mm during pile installation, with that maximum dropping to <0.003 mm one day following cessation of operations (SSER, 2022b). This suggests that associated deposition would be imperceptible from the background sediment transport activity, with plotted sediment depths less than typical grain diameters (SSER, 2022b). As per the Array, drill arisings will result from foundation installation at Berwick Bank Offshore Wind Farm. The assessment for these however, is considered under long term habitat loss and disturbance (paragraphs 106 et seq.) as this material will be deposited on the seabed in the same area which will be occupied by scour protection and is unlikely to be redistributed as a result of hydrodynamic processes.
235. Finally, cable installation and burial have the potential to result in increased SSCs and associated deposition. The MDS considers up to 1,261 km of inter-array cables and 236 km of interconnector cables (noting that up to 116 km of the total inter-array cables will be dynamic, and not buried at the seabed) ( Table 9.13 Open ▸ ). As described in the Project Description (volume 1, chapter 3), the final cable installation methods have not yet been confirmed, and will be identified at the final design stage (post-consent), however cable plough, jet trencher, mass flow excavator, and mechanical cutter are potential options. At the Berwick Bank Offshore Wind Farm, jet trenching was assumed for the modelling, which predicted peak increases in SSCs of 100 mg/l in the immediate vicinity of the cable installation, with the sediment subsequently re-suspended and dispersed on subsequent tides, giving rise to concentrations of up to 500 mg/l (SSER, 2022b). The material was predicted to settle during slack water and then be resuspended to form an amalgamated plume. Sedimentation was predicted to be greatest at the location of the trenching and up to 30 mm in depth one day following cessation of inter-array cable installation (SSER, 2022b). Levels of sedimentation were predicted to reduce significantly, down to single figures, within close proximity (i.e. 100 m) of the trench (SSER, 2022b).
236. The impact is predicted to be of local spatial extent, long term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
Sensitivity of the receptor
Marine fish and shellfish species
238. A study by Appleby and Scarratt (1989) found development of eggs and larvae have the potential to be affected by suspended sediments at concentrations of thousands of mg/l. Modelling undertaken of SSC associated with the fish and shellfish ecology study area operation and maintenance phase identified increases in SSC due to movement of mooring lines and cabling. These concentrations of SSC may affect the development of eggs and larvae; however, these concentrations are only expected to be present in the immediate vicinity of the release site with dispersion of the released material continuing on successive tides. These levels are unlikely to affect the development of eggs and larvae.
239. Many shellfish species, such as edible crab, have a high tolerance to SSC and are reported to be insensitive to increases in turbidity; however, they are likely to avoid areas of increased SSC as they rely on visual acuity during predation (Neal and Wilson, 2008). Berried crustaceans (e.g. European lobster and Nephrops) are likely to be more vulnerable to increased SSC as the eggs carried by these species require regular aeration. Increased SSC within the fish and shellfish ecology study area will only affect a small area at any one time and will be temporary in nature, with sediments settling to the seabed quickly following disturbance. Nephrops are not considered to be sensitive to increases in SSC or subsequent sediment deposition, since this is a burrowing species with the ability to excavate any sediment deposited within their burrows (Sabatini and Hill, 2008).
240. The species which are likely to be affected by sediment deposition are those which either feed or spawn on or near the seabed. Demersal spawners within the vicinity of the Array include sandeel, which have low intensity spawning and nursery grounds within the fish and shellfish ecology study area (Ellis et al., 2012), however sandeel eggs are likely to be tolerant to some level of sediment deposition due to the nature of re-suspension and deposition within their natural high energy environment. Therefore, effects on sandeel spawning populations are predicted to be limited. Sandeel populations are also sensitive to sediment type within their habitat, preferring coarse to medium sands and showing reduced selection or avoidance of gravel and fine sediments (Holland et al., 2005). This is as identified by the FeAST tool as the pressure ‘siltation changes’ (low) which has identified that sandeel have medium sensitivity to this impact (Wright et al., 2000). Therefore, any increase in the fine sediment fraction of their habitat may cause avoidance behaviour until such time that currents remove fine sediments from the seabed, although modelled sediment deposition levels are expected to be highly localised and at very low levels.
241. Herring occur mostly in pelagic habitats, but utilise benthic environments for spawning, and are known to prefer gravelly and coarse sand environments for this purpose, with low intensity nursery grounds present within the site boundary and low intensity spawning grounds nearby (Coull et al., 1998). With respect to the effects of sediment deposition on herring spawning activity, it has been shown that herring eggs may be tolerant of very high levels of SSC (Messieh et al., 1981; Kiorbe et al., 1981). Detrimental effects may be seen if smothering occurs and the deposited sediment is not removed by the currents (Birklund and Wijsmam, 2005), however this natural removal by the currents and tidal physical processes would be expected to occur quickly in this case (i.e. within a couple of tidal cycles), given the low levels of deposition expected close to the installed foundations and the mooring lines during the operation and maintenance phase.
242. All fish and shellfish ecology IEFs in the fish and shellfish ecology study area, including sandeel, herring, Nephrops, and elasmobranch species, are deemed to be of low to medium vulnerability, high recoverability and local to national importance. The sensitivity of these IEFs is therefore considered to be low.
Diadromous species
243. Diadromous fish species known to occur in the area are also expected to have some tolerance to naturally high SSC, given their migration routes typically pass through estuarine habitats which have background SSC which are considerably higher than those expected to occur because of the operation and maintenance phase of the Array. As it is predicted that operation and maintenance activities associated with the Array will produce only temporary and rapidly dissipating increases in SSC, with levels well below those experienced in estuarine environments, it would be expected that any diadromous species should only be temporarily affected (if they are affected at all, based on the migration routes). Any adverse impacts on these species are likely to be short term behavioural effects, such as avoidance (Boubee et al., 1996), or temporary slightly erratic alarmed swimming behaviour (Chiasson, 2011), and are not expected to create a barrier to migration between feed grounds in the North and Atlantic and natal rivers or estuaries used by these species. However, these studies were laboratory based, and do not cover the species found within the fish and shellfish ecology study area, so the potential for other responses does exist, but these are unlikely, given the naturally highly turbid nature of estuarine environments that these species are adapted to traverse. Investigations into the impacts of offshore increased suspended sediments on diadromous species such as Atlantic salmon are limited (Kjelland et al., 2015), although there is the potential for increased turbidity to improve salmon survival rates during migrations due to a lowering of predation rates from reduced visibility (Gregory and Levings, 1998).
244. Diadromous fish species IEFs in the fish and shellfish ecology study area are deemed to be of low vulnerability, high recoverability and national to international importance. The sensitivity of the receptors is therefore considered to be low.
Significance of the effect
Marine fish and shellfish species
245. Overall, for all marine fish and shellfish species considered as IEFs, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be low. The effect will, therefore, be of negligible to minor adverse significance, which is not significant in EIA terms.
Diadromous species
246. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be low. The effect will, therefore, be of negligible to minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
247. No secondary fish and shellfish ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.