13.3.2  Changes in Prey Availability

1001         Potential changes to the fish assemblages during the construction and decommissioning phases of the Proposed Development, as identified in volume 2, chapter 9 of the Offshore EIA Report, may have indirect effects on marine mammals, as assessed in volume 2, chapter 10 of the Offshore EIA Report.

1002         This section addresses the effects due to changes in prey availability associated with the construction and decommissioning phases of the proposed project. For each type of underwater noise effect, the nature of the effect is described, the source activities generating the effect and the potential changes to marine mammal receptors are outlined. Effects are categorised as permanent or temporary.

1003         The subsequent sub-sections provide more information on each of these underwater noise effects and the sensitivity of the Annex II marine mammal features to these effects:

• a summary of the relevant components of the Proposed Development, outlined in the maximum design scenario ( Table 13.38   Open ▸ ) and designed in measures;
• an overview of the methodology/modelling/assessment undertaken to quantify and assess effects due to changes in prey availability on marine mammals;
• an overview of relevant marine mammal information gathered to aid the assessment (paragraph 1021 et seq.).

1004         Potential effects on fish assemblages during the construction, operation and maintenance and decommissioning phases of the Proposed Development, as identified in volume 2, chapter 9 of the Offshore EIA Report, may have indirect effects on marine mammals. The assessment includes temporary and long-term habitat loss/disturbance, increased SSC and associated sediment deposition, injury and/or disturbance from underwater noise and vibration, EMF, as well as colonisation of foundations, scour protection and cable protection.

1005         The key prey species for marine mammals include sandeel, gadoids (e.g. cod Gadus morhua, haddock Melanogrammus aeglefinus, whiting Merlangius merlangus), clupeids (herring), plaice Pleuronectes platessa, flatfish and mackerel. These prey species have been identified as being of regional importance within the Proposed Development fish and shellfish ecology study area (see volume 2, chapter 9 of the Offshore EIA Report). For example, there are important spawning grounds for cod, herring, plaice, sandeel, whiting and sprat within the Proposed Development array area and Proposed Development export cable corridor. Consequently, adverse effects on fish receptors may have indirect adverse effects on marine mammal receptors.

Construction phase

1006         Potential impacts on marine mammal prey species during the construction phase have been assessed in volume 2, chapter 9 of the Offshore EIA Report using the appropriate maximum design scenarios for these receptors. Construction impacts include temporary subtidal habitat loss/disturbance, long term subtidal habitat loss, injury and/or disturbance to fish and shellfish from underwater noise and vibration and increased SSCs and associated sediment deposition.

Decommissioning phase

1007         Potential impacts on marine mammal prey species during the decommissioning phase have been assessed in volume 2, chapter 9 of the Offshore EIA Report using the appropriate maximum design scenarios for these receptors. These impacts include temporary subtidal habitat loss/disturbance, long term subtidal habitat loss and increased SSCs and associated sediment deposition.

Maximum design scenario relevant to the assessment of adverse effects on integrity

1008         The Maximum Design Scenario considered for the assessment of potential impacts on Annex II marine mammal features from changes in prey availability during construction and decommissioning are set out in Table 13.38   Open ▸ below.

Table 13.38: Maximum Design Scenarios Considered for the Assessment of Potential Impacts on Annex II Marine Mammal Features from Changes in Prey Availability during Construction and Decommissioning

Designed-in measures relevant to the assessment of adverse effects on integrity

1009         There are no designed in measures of relevance to the assessments of potential effects on Annex II marine mammal features from changes to prey availability during construction and decommissioning.

Information to inform appropriate assessments

1010         The installation of infrastructure within the Proposed Development may lead to temporary subtidal habitat loss/disturbance as a result of a range of activities including use of jack-up vessels during foundation installation, installation of inter-array, interconnector and offshore export cables and associated seabed preparation, and anchor placements associated with these activities. There is the potential for temporary habitat loss/disturbance to affect up to 113,974,700 m2 of seabed during the construction phase, which equates to 9.7% of the Proposed Development area, representing a relatively small proportion of the Proposed Development fish and shellfish ecology study area. Habitat loss/disturbance could potentially affect spawning, nursery or feeding grounds of fish and shellfish receptors. Due to the highly localised nature of the effects (i.e. spatially restricted to within the Proposed Development array area and Proposed Development export cable corridor) and the small proportion of habitats affected as a proportion of the northern North Sea fish and shellfish ecology study area and medium term duration, temporary habitat loss/disturbance during the construction phase was assessed as being of low risk to fish and shellfish communities.

1011         As suggested in volume 2, chapter 9 of the Offshore EIA Report, only a small proportion of the maximum footprint of habitat loss/disturbance may be affected at any one time during the construction phase with areas starting to recover immediately after cessation of construction activities in the vicinity. Additionally, habitat disturbance during the construction phase will also expose benthic infaunal species from the sediment (see volume 2, chapter 8 of the Offshore EIA Report), potentially offering foraging opportunities to some fish and shellfish species (e.g. opportunistic scavenging species) immediately after completion of works. Most fish and shellfish receptors found within the Proposed Development fish and shellfish ecology study area are deemed to be of low vulnerability, high recoverability and local to international importance and therefore sensitivity of these receptors was considered to be low. However, sensitivity of some species has been assessed as medium, including larger crustacea (e.g. Nephrops, European lobster Homarus gammarus) and sandeel.

1012         The presence of infrastructure within the Proposed Development, including foundations and associated scour protection, cable protection, OSP/Offshore convertor station platform interconnector and offshore export cables will result in long term habitat loss of up to 7,798,856 m2. Many species of fish and shellfish are reliant upon the presence of suitable sediment/habitat for their survival and therefore seabed habitats removed by installation of the infrastructure will reduce the area available for foraging, spawning and nursing. However, the area that will be impacted represents a very low proportion of the available habitat (0.7% of the Proposed Development fish and shellfish ecology study area). Moreover, as presented in more detail in volume 2, chapter 9 of the Offshore EIA Report, there is scientific evidence that presence of offshore wind farms is associated with an increase in density of soft sediment-associated fish species and of species associated with hard substrate. The sensitivity of fish and shellfish receptors ranged from low to medium with the majority of fish receptors deemed to be of low vulnerability, high recoverability and local to international importance.

1013         An increase in SSC and associated sediment deposition as a result of the installation of all wind turbines and offshore substation foundations and the installation of inter-array, interconnector and offshore export cables may result in short-term avoidance of affected areas by fish and shellfish. The maximum design scenario assessed in volume 2, chapter 9 of the Offshore EIA Report assumed all wind turbine and offshore substation foundations will be installed by drilling 5.5 m diameter piles and installation of inter-array cables through jet-trenching. Modelling of SSCs associated with the foundation installation showed the plume related directly to the sediment releases was < 5 mg/l and this drops to lower levels within a very short distance, typically < 500 m. Modelling of SSC for installation of inter-array and offshore export cables indicated concentrations of up to 500 mg/l and between 50 mg/l and 500 mg/l, respectively. Adult fish have high mobility and may show avoidance behaviour in areas of high sedimentation, however, there may be impacts on the hatching success of fish and shellfish larvae and consequential effects on the viability of spawning stocks due to limited mobility. Spawning grounds for sandeel overlap with the Proposed Development fish and shellfish ecology study area; eggs of these species are attached to the seabed for couple of weeks before hatching. Sandeel eggs are known to be tolerant to sediment deposition due to the nature of re-suspension and deposition within their natural high energy environment, therefore it is very likely that the effect on sandeel spawning populations will be limited. Herring spawning grounds are also found within the Proposed Development fish and shellfish ecology study area, however, herring eggs are tolerant of very high levels of SSC. Additionally, elevations in SSC during the construction phase will be of short duration, returning to background levels relatively quickly. SSC will not reach the concentrations required for an extended period for there to be any effect on survival. Additionally, deposited sediments are expected to be removed quickly by the currents resulting in small amount of sediment being deposited. It has been assessed that the impact of SSC and associated sediment deposition is likely to be localised, short term and intermittent.

1014         There is the potential for underwater noise and vibration during construction pile-driving to result in injury and/or disturbance to fish and shellfish communities (see volume 2, chapter 9 of the Offshore EIA Report). For SPLpk and the maximum design scenario assessed (installation of one 5.5 m diameter pile with absolute maximum hammer energy of 4,000 kJ) in volume 2, chapter 9 of the Offshore EIA Report, the maximum recoverable injury range is estimated at 138 m to 228 m from the piling location. The potential for mortality or mortal injury to fish eggs would also occur at distances of up to 228 m. However, this is considered to be highly conservative due to the implementation of soft starts during piling operations which will allow fish to move away from the areas of highest noise levels, before the received noise reaches a level that would cause an injury. As such, the maximum injury ranges predicted for soft start initiation (i.e. of the order of tens of meters) are likely to be more realistic. For SELcum, subsea noise modelling showed that TTS, from which animals will recover, was predicted to occur out to a maximum distance of 4,161 m for single piling scenario at 4,000 kJ. The potential onset of behavioural effects (such as elicitation of a startle response, disruption of feeding, or avoidance of an area) may occur to ranges of approximately 17 km to 23 km. A qualitative assessment of behavioural effects in fish to underwater noise suggested, however, that responses will differ depending on the sensitivity of the species and the presence/absence of a swim bladder. For the least sensitive species (e.g. flatfish), the risk of behavioural effects is moderate to high in the nearfield (tens of metres) and intermediate field (i.e. hundreds of metres). For more sensitive species (e.g. herring, gadoids, sprat etc.) behavioural effects may occur further away from the source (i.e. over several kilometres or more from the source).

1015         With respect to indirect effects on marine mammals, no additional indirect effects other than those assessed for injury and disturbance to marine mammals as a result of elevated underwater noise during piling (see paragraph 881 et seq.) have been predicted. This is because if prey were to be disturbed from an area as a result of underwater noise, it is assumed that marine mammals would be disturbed from the same or greater area, and so any changes to the distribution of prey resources would not affect marine mammals as they would already be disturbed from the same (or larger) area.

1016         On the basis of the assessments presented in volume 2, chapter 9 of the Offshore EIA Report, no significant adverse effects have been predicted to occur to fish and shellfish species (marine mammal prey) as a result of the construction of the Proposed Development.

1017         Decommissioning activities such as use of jack-up vessels during foundation removal, removal of inter-array, interconnector and offshore export cables, and associated anchor placements may result in temporary habitat loss/disturbance of up to 34,571,200 m2. The impact is predicted to be of localised extent and affect only a small proportion of this total area at any one time during the decommissioning phase.

1018         Decommissioning of infrastructure will lead to increases in SSC and associated sediment deposition. The maximum design scenario is represented by the cutting and removal of piled jacket foundations at seabed level and removal of inter-array, OSP/Offshore convertor station platform interconnector and offshore export cables by jet dredging mobilising material from a 0.5 m deep and 2 m wide trench.

1019         Leaving infrastructure, such as the scour protection associated with wind turbine and OSP/Offshore convertor station platform foundations and cable protection associated with array, OSP/Offshore convertor station platform interconnector and offshore export cables, in situ after decommissioning will result in permanent habitat loss with a maximum design scenario of up to 7,562,609 m2. An overview of potential impacts to fish and shellfish receptors and sensitivity conclusions were previously presented in paragraph 1012 et seq.

1020         The impact on marine mammals is therefore predicted to be of local spatial extent, medium term duration, intermittent and of high reversibility. The assessment of the effect of changes in fish and shellfish communities affecting prey availability on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.

Relevant information of Annex II marine mammals

1021         Marine mammals exploit a range of different prey items and can forage widely, sometimes covering extensive distances. Given that the impacts of construction to prey resources will be highly localised and largely restricted to the boundaries of the Proposed Development, only a small area will be affected when compared to available foraging habitat in the northern North Sea. Marine mammals occurring within this small impact area also have the potential to be directly affected as a result of impacts such as injury and disturbance from elevated underwater noise during piling and it is likely that the effects to prey resources (e.g. behavioural displacement) will occur over a similar, or lesser, extent and duration as those for marine mammals. There would, therefore, be no additional displacement of marine mammals as a result of any changes in prey resources during construction, as they would already be potentially disturbed as a result of underwater noise during piling. In addition, as prey resources are displaced from the areas of potential impact, marine mammals are likely to follow in order to exploit these resources.

1022         Following placement on the seabed, submerged parts of the wind turbines provide hard substrate for the colonisation by high diversity and biomass in the flora and fauna. Faecal deposits of dominant communities of suspension feeders are likely to alter the surrounding seafloor communities by locally increasing food availability (Degraer et al., 2020). Higher trophic levels, such as fish and marine mammals, are likely to profit from locally increased food availability and/or shelter and therefore have the potential to be attracted to forage within offshore wind farm array area. However, still relatively little is known about the distribution and diversity of marine mammals around offshore anthropogenic structures. Species such as harbour porpoise, minke whale, white-beaked dolphin, harbour seal and grey seal were frequently recorded around offshore oil and gas structures (Todd et al., 2016; Delefosse et al., 2018; Lindeboom et al., 2011). Acoustic results from a T-POD measurement within a Dutch wind farm found that relatively more harbour porpoises are found in the wind farm area compared to the two reference areas (Scheidat et al., 2011; Lindeboom et al., 2011). Authors of this study concluded that this effect is directly linked to the presence of the wind farm due to increased food availability as well as the exclusion of fisheries and reduced vessel traffic in the wind farm (shelter effect). Russell et al. (2014) monitored the movements of tagged harbour seals within two active wind farms in the North Sea and demonstrated that animals commonly showed grid-like movement patterns which strongly suggested that the structures were used for foraging. During research on a Danish wind farm, no statistical differences were detected in the presence of harbour porpoises between inside and outside the wind farm (Diederichs et al., 2008). Diederichs et al. (2008) suggested, however, that a small increase in detections during the night at hydrophones deployed in close proximity to single wind turbines may indicate increased foraging behaviour near the monopiles. Whilst there is some mounting evidence of potential benefits of man-made structures in marine environment (Birchenough and Degrae, 2020), the statistical significance in EIA terms of such benefits and details about trophic interactions in the vicinity of artificial structures and their influence on ecological connectivity remain largely unknown (Petersen and Malm, 2007; Inger et al., 2009; Rouse et al., 2020, McLean et al., 2022; Elliott and Birchenough, 2022).

Grey seal

1023         Along the Scottish coast, grey seals exhibit an offshore foraging behaviour (Damseaux et al., 2021). Grey seal has a selective diet, mostly comprising flatfish and sandeel. A study on the diet of grey seals in Scottish waters found that 50% of prey items were plaice and sole Solea solea and 46% of prey items were sandeel (Damseaux et al., 2021). Hammond et al. (2001) corroborated this finding and highlighted sandeel as an important prey item for grey seals in Scottish waters where they account for approximately 50% of the diet. Gosch (2017) reported that there are significant regional and temporal differences in the diet of grey seal. Seals in shallow waters show a preference for demersal and groundfish species such as cephalopods and flatfish, whilst seals foraging in deeper waters, over sandy substrates, will target pelagic and bentho pelagic species such as blue whiting Micromesistius poutassou and sandeel (Gosch, 2017).

1024         Grey seals tend to forage in the open sea, returning to land regularly to haul out. Foraging trips can be wide-ranging, however, tracking studies have shown that most foraging is likely to occur within 100km of a haul out site (SCOS, 2018). Grey seal are deemed to be of low vulnerability, high recoverability and overall low sensitivity.

Harbour seal

1025         Harbour seals are central place foragers, requiring haul-out sites on land for resting, moulting and breeding, and dispersing from these sites to forage at sea. In order to reduce time and energy searching for prey, animals are likely to travel directly to areas of previously or predictably high foraging success (Bailey et al., 2014). Harbour seals tend to stay within 50 km of the coast, although most foraging trips are over shorter ranges (Russell and McConnell, 2014; supported by tagging studies in SCOS, 2018). Since females need to regularly return to their pups at the haul-out site, they may be more limited in foraging distance. Because of the constraint on their foraging range, particularly during the breeding season, harbour seals may be particularly vulnerable to changes in prey abundance (Bailey et al., 2014).

1026         Harbour seals are generalist feeders and their diet varies both seasonally and from region to region (Hammond et al., 2001). The analysis of stable isotopic composition and concentration of Hg and Se ions in blood of harbour seals from the North Sea demonstrated that harbour seal diet comprised 30% juvenile cod, 29% plaice and 23% monkfish as well as European hake and haddock (Damseaux et al., 2021).

1027         There may be an energetic cost associated with increased travelling and harbour seal may be particularly vulnerable to this effect as individuals typically forage close to haul out sites (i.e. within nearest 50 km). Despite this, if animals do have to travel further to alternative foraging grounds, the impacts are expected to be short-term in nature and reversible. It is expected that all harbour seals would be able to tolerate the effect without any impact on reproduction and survival rates and would be able to return to previous activities once the impact had ceased. Harbour seal are deemed to be of low vulnerability, high recoverability and overall low sensitivity.

Harbour porpoise

1028         Harbour porpoise has a higher metabolic rate than dolphins and therefore need to feed more frequently and consume more prey per unit body weight, in order to maintain their body temperature and other energy needs (Rojano-Doñate et al., 2018). For this reason, porpoise may be highly susceptible to changes in the abundance of prey species or disturbance from foraging areas. Harbour porpoise feed on a wide range of fish species, but mainly small shoaling species from demersal or pelagic habitats (Santos and Pierce, 2003; Aarfjord, 1995). There are regional and seasonal differences in diet; interannual variation depending on the availability of prey species and ontogenetic variation (adults and juveniles), with juveniles targeting smaller species such as gobies (Gobiidae) or smaller individuals of the same prey species targeted by adults (Santos and Pierce, 2003). A harbour porpoise’s field metabolic rate remains stable over seasonally changing water temperatures. Heat loss is deemed to be managed via cyclical fluctuations in energy intake to build up a blubber layer that offsets the extra cost of thermoregulation during winter (Rojano-Doñate et al., 2018). Ransijn et al. (2019) produced energy maps for various harbour porpoise prey species and found that the energy available in the North Sea is highest during the summer and the main energic contributions were from sandeel and whiting Merlangius merlangus. During the winter season European sprat Sprattus sprattus and Atlantic herring Clupea harengus also contributed to the overall energy density (Ransijn et al., 2019). This study corroborated findings of previous harbour porpoise off the east coast of Scotland which reported that sandeel is the dominant prey item during summer (Santos et al., 2004).

1029         Harbour porpoise regularly forage around tidal races, overfalls, and upwelling zones during the ebb phase of the tide (Pierpoint, 2008). Embling et al. (2010) analysed results of dedicated surveys conducted in the southern Inner Hebrides and found that maximum tidal current is the best environmental explanation of persistent harbour porpoise abundance, although in contrast to other studies, they found that densities were higher in areas of low current. Although harbour porpoise generally hunts alone or in small groups, this species is often seen in larger aggregations of 50 or more individuals, either associated with food concentrations or seasonal migrations. Within these loose aggregations, segregation may occur, with females travelling with their calves and yearlings, and immature animals of each sex being segregated into groups.

1030         The fish and shellfish communities found within the fish and shellfish ecology study area (see chapter 9) are characteristic of the fish and shellfish assemblages in the northern North Sea. It is therefore reasonable to assume that, due to the highly mobile nature of marine mammals, there will be similar prey resources available in the wider area. There may be an energetic cost associated with increased travelling and harbour porpoise may be particularly vulnerable to this effect. Harbour porpoise has a high metabolic rate and only a limited energy storage capacity, which limits their ability to buffer against diminished food. Despite this, if animals do have to travel further to alternative foraging grounds, the impacts are expected to be short term in nature and reversible. It is expected that all marine mammal receptors would be able to tolerate the effect without any impact on reproduction and survival rates and would be able to return to previous activities once the impact had ceased. Harbour porpoise are deemed to be of low vulnerability, high recoverability and overall low sensitivity.

Bottlenose dolphin

1031         There is variation in the patterns of habitat use of bottlenose dolphin, even within a population, and generally the distribution of this species is influenced by factors such as tidal state, weather conditions, resource availability, life cycle stage, or season (Hastie et al., 2004). Typical prey items in Scottish waters include cod Gadus morhua, saithe Pollachius virens, whiting, salmon Salmo salar and haddock Melanogrammus aeglefinus (Santos et al., 2001).

1032         There is a seasonal pattern of a higher intensity of bottlenose dolphin movement from the Tay estuary and adjacent waters to the Moray Firth SAC in early summer months, and from the Moray Firth SAC to the Tay estuary and adjacent waters in late summer. These are anticipated to be driven by environmental and biological factors (Arso Civil et al., 2021). Wilson et al. (1997) and Hastie et al. (2004) reported that these two areas share topographically distinct characteristics with increased observations of dolphins foraging. Seasonal changes in prey presence over variable temporal scales throughout the year may therefore enable dolphins to exploit these areas within their range at different times.

1033         Bottlenose dolphin are deemed to be of low vulnerability, high recoverability and overall low sensitivity.

13.4 Operation and Maintenance

13.4.1  Underwater Noise

1034         Increases in underwater noise associated with the operation and maintenance of the Proposed Development have the potential to cause injury and disturbance to marine mammals. Underwater noise assessment of effects of has been informed by subsea noise modelling, the scope of which was agreed through the Road Map process (volume 3, appendix 10.2 of the Offshore EIA Report).

1035         This section addresses the underwater noise effects associated with the operation and maintenance phase of the proposed project. For each type of underwater noise effect, the nature of the effect is described, the source activities generating the effect and the potential changes to marine mammal receptors are outlined. Effects are categorised as permanent or temporary.

1036         The subsequent sub-sections provide more information on each of these underwater noise effects and the sensitivity of the Annex II marine mammal features to these effects:

• a summary of the relevant components of the Proposed Development, outlined in the maximum design scenario ( Table 13.39   Open ▸ ) and designed in measures ( Table 13.40   Open ▸ );
• an overview of the methodology/modelling/assessment undertaken to quantify and assess underwater noise effects on marine mammals;
• an overview of relevant marine mammal information gathered to aid the assessment.

Maximum design scenario relevant to the assessment of adverse effects on integrity

1037         The maximum design scenarios considered for the assessment of potential impacts on Annex II marine mammal features from underwater noise during the operation and maintenance phases are set out in Table 13.39   Open ▸ .

Table 13.39: Maximum Design Scenarios Considered for the Assessment of Potential Impacts on Annex II Marine Mammal Features from Underwater Noise during Operation and Maintenance

Designed-in measures relevant to the assessment of adverse effects on integrity

1038         Designed in measures (and the associated commitments) of relevance to the assessments of potential impacts on Annex II marine mammal features from underwater noise during operation and maintenance are set out in Table 13.40   Open ▸ .

Table 13.40:  Designed in Measures Relevant to the Assessment of Adverse Effects on Integrity on European Sites Designated for Annex II Marine Mammal Features from Underwater Noise During Operation and Maintenance

Information to inform Appropriate Assessments

Site investigation surveys

1039         Site investigation surveys during the operation and maintenance phase have the potential to cause direct or indirect effects (including injury or disturbance) on Annex II marine mammals. A detailed underwater noise modelling assessment has been carried out to investigate the potential for injurious and behavioural effects on marine mammals as a result of geophysical and geotechnical surveys, using the latest criteria (volume 3, appendix 10.1 of the Offshore EIA Report), which is drawn upon in the appropriate assessments presented below.

1040         The maximum design scenario comprises of routine geophysical surveys estimated to occur every six months for first two years and annually thereafter. This equates to up to 37 surveys over the 35-year life cycle of Proposed Development ( Table 13.39   Open ▸ ).

Auditory injury and behavioural disturbance

1041         Vessel use during the operation and maintenance phase of the Proposed Development may lead to injury and/or disturbance to marine mammals. The maximum design scenario is for up to 2,323 vessel round trips per year over the operational lifetime of the Project. Vessel types which will be required during the operation and maintenance phase include those used during routine inspections, repairs and replacement of equipment, major component replacement, painting or other coatings, removal of marine growth, replacement of access ladders, and geophysical surveys ( Table 13.39   Open ▸ ).

1042         An overview of potential impacts from auditory injury due to elevated underwater noise during geophysical site investigation surveys is described in paragraph 945 et seq. for the construction phase and has not been reiterated here for the operation and maintenance phase. Similarly, the magnitude of potential impacts for behavioural disturbance to marine mammals is described in paragraph 955 et seq. The magnitude of the impact of underwater noise from geophysical surveys during operation and maintenance phase could result in a negligible alteration to the distribution of marine mammals. Surveys are anticipated to be short-term in nature (weeks to a few months) and occur intermittently over the operation and maintenance phase.

1043         With designed-in measures implemented for the geophysical surveys, the impact is predicted to be of local to regional spatial extent, short-term duration, intermittent and low reversibility (PTS) or high reversibility (behaviour). It is predicted that the impact will affect the receptor directly. The assessment of the effect of underwater noise due to site investigation surveys on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.

Vessel use and other activities

1044         The uplift in vessel activity during the operation and maintenance is considered to be relatively small in the context of the baseline levels of vessel traffic in the Proposed Development marine mammal study area described in paragraphs 989 et seq. Presence of the operational wind farm may divert some of the shipping routes and therefore, current traffic within the Proposed Development array area, which is not associated with Proposed Development, is likely to be reduced. The extent of that change cannot be quantified at the time of writing, however, it is anticipated this reduction will be ultimately counterbalanced by presence of maintenance vessels. Vessel movements will be within the Proposed Development array area and Proposed Development export cable corridor and will follow existing shipping routes to/from the ports. In addition, Codes of Conduct will be issued to all project vessel operators.

1045         The size and noise outputs from vessels during the operation and maintenance phase will be similar to those used in the construction phase and therefore will result in a similar maximum design spatial scenario (paragraph 988 et seq.). However, the number of vessel round trips and their frequency is much lower for the operation and maintenance phase compared to the construction phase.

Auditory injury

1046         An overview of potential impacts for auditory injury to marine mammals from elevated underwater noise due to vessel use and other activities is described in paragraph 988 et seq. for the construction phase with effect ranges presented in Table 13.35   Open ▸ and Table 13.36   Open ▸ and have not been reiterated here for the operation and maintenance phase. The impact is predicted to be of local spatial extent, long term duration, intermittent and low reversibility. It is predicted that the impact will affect the receptor directly. The assessment of the effect of injury from underwater noise due to site investigation surveys on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.

Behavioural disturbance

1047         An overview of potential impacts for behavioural disturbance to marine mammals from elevated underwater noise due to vessel use and other activities is described in paragraph 995 et seq. for the construction phase with impact ranges presented in Table 13.37   Open ▸ and have not been reiterated here for the operation and maintenance phase. The impact is predicted to be of local spatial extent, long term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The assessment of the effect of behavioural disturbance from underwater noise due to site investigation surveys on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.

13.4.2  Changes in Prey Availability

1048         Potential impacts on marine mammal prey species during the operation and maintenance phase have been assessed in volume 2, chapter 9 of the Offshore EIA Report using the appropriate maximum design scenarios for these receptors. These impacts include temporary subtidal habitat loss/disturbance, long-term subtidal habitat loss, increased SSC and associated sediment deposition, EMF from subsea electrical cabling and colonisation of foundations, scour protection and cable protection and the findings are summarised here.

Maximum design scenario relevant to the assessment of adverse effects on integrity

1049         The maximum design scenarios considered for the assessment of potential impacts on Annex II marine mammal features from changes in prey availability during the operation and maintenance phase are set out in Table 13.41   Open ▸ below.

Table 13.41: Maximum Design Scenarios Considered for the Assessment of Potential Impacts on Annex II Marine Mammal Features from Changes in Prey Availability During Operation and Maintenance

Designed in measures relevant to the assessment of adverse effects on integrity

1050         There are no designed in measures of relevance to the assessments of potential effects on Annex II marine mammal features from changes to prey availability during operation and maintenance.

Information to inform Appropriate Assessments

1051         There is the potential for up to 989,000 m2 of temporary habitat loss/disturbance during the operation and maintenance phase as a result of the use of jack-up vessels during any component replacement activities and during any inter-array, OSP/Offshore convertor station platform interconnector and offshore export cable repair activities. These impacts will be similar to those identified for temporary habitat loss/disturbance the construction phase (as discussed in paragraph 1010) and will be highly restricted to the immediate vicinity of these operations.

1052         The presence of infrastructure within the Proposed Development, will result in long term habitat loss of up to 7,798,856 m2 during the operation and maintenance phase (0.7% of the Proposed Development fish and shellfish ecology study area). An overview of potential impacts to fish and shellfish receptors and sensitivity conclusions were previously presented in paragraph 1012 for construction phase and will not be reiterated here for operation and maintenance phase.

1053         Increased SSC could occur as a result of repair or remedial burial activities during the operation and maintenance phase. The maximum design scenario assessed in volume 2, chapter 9 of the Offshore EIA Report for increased SSC and associated deposition is for the repair of cables of up to 30,000 m in length and reburial of cables of up to 10,000 m in length for inter-array cables; and repair of cables of up to 4,000 m in length and reburial of cables of up to 4,000 m in length for offshore export cables, using similar methods as those for cable installation activities (e.g. jet-trenching), undertaken at intervals over the 35 years operation and maintenance phase. The assessment in volume 2, chapter 9 of the Offshore EIA Report considered that any suspended sediments and associated deposition will be of the same magnitude, or lower as for construction.

1054         The presence and operation of inter-array, interconnector and offshore export cables will result in emissions of localised electrical and magnetic fields, which could potentially affect the sensory mechanisms of some species of fish and shellfish. Species for which there is evidence of a response to electrical and/or magnetic fields include elasmobranchs (sharks, skates and rays), river lamprey Lampetra fluviatilis, sea lamprey Petromyzon marinus, European eel Anguilla ecommis, plaice and Atlantic salmon Salmo salar (Gill et al., 2005, CSA, 2019). A range of their life functions is supported by either electric or magnetic sense, including detection of prey, predator avoidance, social or reproductive behaviours, orientation, homing, and navigation (Gill et al., 2005; Normandeau et al., 2011). The range over which species can detect EMF will be very localised to within a few centimetres of the buried cable, with rapid decay of the EMF with increasing distance.

1055         Artificial structures introduced to the marine environment, such as wind turbine foundations and scour/cable protection, provide hard substrate for settlement of various organisms, including small crustaceans and polychaete worms. These communities can provide a valuable food source for fish species and therefore, hard substrate habitat is likely to be colonised within days after construction by demersal and semi-pelagic species. The maximum design scenario assessed in volume 2, chapter 9 of the Offshore EIA Report assumes up to 10,198,971 m2 of habitat created due to the installation of jacket foundations, associated scour protection and cable protection associated with inter-array cables, OSP/Offshore convertor station platform interconnector cables and offshore export cables. The dominant natural substrate character (e.g. soft sediment or hard rocky seabed) will determine the number of new species found on the introduced vertical hard surface and associated scour protection. When placed on a soft seabed, most of the colonising fish tend to be associated with hard bottom habitats, thus the overall diversity of the area is expected to increase. If infrastructure is introduced to the area of rocky substrates, few species will be added to the area, but the increase in total hard substrate could sustain higher abundance (Andersson et al., 2009).

1056         The impact on marine mammals is predicted to be of local spatial extent, long-term duration, continuous and the effect on marine mammals is of high reversibility. The assessment of the effect of changes in fish and shellfish communities affecting prey availability on grey seal, harbour seal, harbour porpoise and bottlenose dolphin as features of relevant designated sites is provided in section 13.6.