Construction Phase

Magnitude of Impact

Auditory injury

  1. During the construction phase of the Proposed Development, the increased levels of vessel activity will contribute to the total underwater noise levels. The maximum design scenario for construction activities associated with site preparation and inter-array and offshore export cables is up to 316 return trips of up to nine boulder clearance vessels and 104 return trips of up to three sandwave clearance vessels, throughout the construction period. Additionally, vessel movements associated with other activities such as foundation and OSPs/Offshore convertor station platform installation, will contribute to a maximum scenario of up to 11,484 vessel round trips over the construction phase. Vessel types will include main installation vessels, cargo barges, support vessels, tug/anchor handlers, guard vessels and others (see Table 10.16   Open ▸ for full list of construction vessels and volume 3, appendix 10.1 for SPLs associated with each vessel type). Whilst this will lead to an uplift in vessel activity, the movements will be limited to within the Proposed Development array area and Proposed Development export cable corridor and will follow existing shipping routes to/from the ports.
  2. The main drivers influencing the magnitude of the impact are vessel type, speed and ambient noise levels (Wilson et al., 2007). Based on information presented in volume 2, chapter 13, baseline levels of vessel traffic in the Proposed Development marine mammal study area are relatively high. An average of 14 vessels per day were recorded within a 10 nm buffer around the Proposed Development array area (hereinafter Proposed Development shipping and navigation study area) over a 14-day survey period in August 2022. The vessel traffic surveys also showed an average of three to four vessels intersecting the Proposed Development array area per day, over summer. Throughout the season, a maximum of 25 vessels were recorded within the Proposed Development array shipping and navigation study area over one day. For the winter survey period (January 2021), there was an average of 16 unique vessels per day recorded within the Proposed Development array area shipping and navigation study area.
  3. As described in the Navigational Risk Assessment (NRA) (volume 3, appendix 13.1), occasional vessel traffic movements associated with jack-ups, semi-submersibles and other platforms also occur in the region.
  4. Other noise generating activities for the Proposed Development will include drilled piling, with a maximum of 176 piles over the period of 135 days ( Table 10.16   Open ▸ ). Rotatory drilling is non-impulsive in character and the source sound levels associated with this activity have been based on pile drilling for the Oyster 800 project. The other noise sources potentially active during the construction phase are related to cable installation (i.e. trenching and cable laying activities), and their related operations such as the jack-up rigs. See volume 3, appendix 10.1 for more information about SELs associated with above construction activities.
  5. A detailed underwater noise modelling assessment has been carried out to investigate the potential for injurious and behavioural effects on marine mammals resulting from elevated underwater noise (non-impulsive sound), using the latest criteria (volume 3, appendix 10.1). A conservative assumption has been made that all individual marine mammals will respond aversively to increases in vessel noise (i.e. that there is no intra or inter-specific variation or context-dependent differences). The distance over which effects may occur will, however, vary according to the species, the ambient noise levels, hearing ability, vertical space use and behavioural response differences. SELs have been estimated for each vessel type based on 24 hours continuous operation, although it is important to note that it is highly unlikely that any marine mammal would stay at a stationary location or within a fixed radius of a vessel for 24 hours. Therefore, the acoustic modelling has been undertaken based on an animal swimming away from the source (or the source moving away from an animal). The noise modelling results indicate that ranges (within which there is a risk of PTS occurring to marine mammals as a result of elevated underwater noise due to vessel use) are either not exceeded or relatively low ( Table 10.51   Open ▸ ). The maximum range within which the PTS could occur across all species has been estimated for harbour porpoise at 525 m for a rock placement vessel ( Table 10.51   Open ▸ ).

 

Table 10.51:
Vessels Involved in the Construction of the Proposed Development and Estimated PTS Ranges for Marine Mammals

Table 10.51: Vessels Involved in the Construction of the Proposed Development and Estimated PTS Ranges for Marine Mammals

N/E = Not Exceeded

 

  1. Of the other noise-producing activities, cable laying is most likely to result in PTS compared to drilling, trenching and jack-up rigging ( Table 10.52   Open ▸ ). As before, the modelled effect ranges for cable laying suggest that harbour porpoise is the most sensitive species with PTS predicted up to 525 m from the source ( Table 10.52   Open ▸ ). The same activity is likely to result in a PTS to bottlenose dolphin and white-beaked dolphin within 15 m from the source and to seal species within only 5 m from the source. The jack-up rig has the potential to result in PTS to harbour porpoise within 5 m from the source. For all other activities and for all other species, the thresholds for PTS will not be exceeded as a result of underwater noise during construction activities.

 

Table 10.52:
Estimated PTS Ranges for Marine Mammals During Other Activities

Table 10.52: Estimated PTS Ranges for Marine Mammals During Other Activities

 

  1. The number of marine mammals potentially injured within the modelled ranges for PTS from vessels ( Table 10.51   Open ▸ ) and other activities ( Table 10.52   Open ▸ ), were calculated and found to be less than one individual for all species. Whist the numbers of animals likely to be affected at any one time are extremely low, the maximum duration of the piling phase is up to 52 months.
  2. The impact is predicted to be of local spatial extent, medium term duration, intermittent and the effect of PTS on sensitive receptors is of low reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.

 

Behavioural disturbance

  1. Disturbance from vessel noise is likely to occur only where vessel noise associated with the construction of the Proposed Development exceeds the background ambient noise level. As discussed in paragraph 354 et seq., the Proposed Development is located in a relatively busy shipping area and therefore background noise levels are likely to be relatively high.
  2. A detailed underwater noise modelling assessment has been carried out to investigate the potential for behavioural effects on marine mammals resulting from increased vessel noise and other activities. The estimated ranges within which there is a potential for disturbance to marine mammals are presented in Table 10.53   Open ▸ . Estimated impact ranges are presented for different vessel types in isolation. It is likely that during construction, operation and maintenance and decommissioning phases, there will be a number of different types of vessels present within the Proposed Development marine mammal study area at the same time. However, given that the exact type, numbers and distances between vessels are unknown at this stage, the cumulative areas of effect were not quantified. Therefore, the discussion presented in paragraph 363 et seq. are based on worst-case scenario for each type of vessel at any given time.
  3. Installation and construction vessels as well as rock placement vessels result in the greatest modelled disturbance out to 4,320 m for all marine mammal species. Similar ranges for behavioural effects are predicted to occur due to underwater noise from cable laying activities with disturbance ranges of 4,389 m. In comparison, vessels such as excavator, backhoe dredger, pipe laying, geophysical survey vessel and jack up vessel as well as jack-up rig were predicted to result in disturbance ranges out to 300 m.

 

Table 10.53:
Estimated Disturbance Ranges for Marine Mammals and Number of Animals Potentially Disturbed as a Result of Vessels and Other Activities

Table 10.53: Estimated Disturbance Ranges for Marine Mammals and Number of Animals Potentially Disturbed as a Result of Vessels and Other Activities

 

  1. As discussed previously in paragraph 357, there is likely to be a proportionate response of animals within the modelled contours (i.e. not all animals will be disturbed to the same extent). The life history of an individual and the context will also influence the likelihood of an individual to exhibit an aversive response to noise.
  2. Numbers of animals with the potential to be disturbed are presented in Table 10.53   Open ▸ , based on the most precautionary species-specific density estimates ( Table 10.13   Open ▸ ) with offshore density estimates applied for bottlenose dolphin (see paragraph 367). Grey seal is likely to be the most sensitive species to disturbance from vessel traffic with potentially the greatest numbers of individuals disturbed compared with other species. The second most sensitive marine mammal (based on numbers of animals potentially affected) is harbour porpoise.
  3. The numbers of animals with the potential to be disturbed (as presented in Table 10.53   Open ▸ ) are considered to be highly conservative, especially for harbour porpoise and grey seal, as these estimates were based on the peak seasonal densities from the Proposed Development aerial digital survey data during spring months and maximum density based on at-sea mean usage maps (Carter et al., 2020), respectively.
  4. Given that activities with the largest disturbance ranges, including installation, construction, rock placement and cable laying vessels, will be operating at distances from the outer Firth of Tay (the highest bottlenose dolphin densities) and are unlikely to affect coastal bottlenose dolphin population, bottlenose dolphins that could be exposed to potential disturbance ( Table 10.53   Open ▸ ) would belong to offshore populations.
  5. The impact is predicted to be of local spatial extent, medium term duration, intermittent and the effect of behavioural disturbance is of high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the Receptor
  1. Increased vessel movement during all phases of Proposed Development have the potential to result in a range of impacts on marine mammal including injury as a result of elevated underwater noise; avoidance behaviour or displacement; and masking of vocalisations or changes in vocalisation rate.

Auditory injury

  1. The sensitivity of marine mammal receptors to auditory injury has been assessed in paragraph 197 et seq., and is not reiterated here.
  2. All marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and low recoverability. The sensitivity of the receptor is therefore, considered to be high.

Behavioural disturbance

  1. Disturbance levels for marine mammal receptors will be dependent on individual hearing ranges and background noise levels within the vicinity. Sensitivity to vessel noise is most likely related to the marine mammal activity at the time of disturbance (IWC, 2006; Senior et al., 2008).
  2. Cetaceans can both be attracted to, and disturbed by, vessels. For example, resting dolphins are likely to avoid vessels, foraging dolphins will ignore them, and socialising dolphins may approach vessels (Richardson et al., 1995).
  3. Harbour porpoise is particularly sensitive to high frequency noise and likely to avoid vessels; Heinänen and Skov (2015) identified that the occurrence of harbour porpoise declines significantly when the number of vessels in a 5 km2 area exceeds 80 in one day. Wisniewska et al. (2018) studied the change in foraging rates of harbour porpoise in response to vessel noise in highly trafficked coastal waters. The results show that occasional high-noise levels coincided with vigorous fluking, bottom diving, interrupted foraging and even cessation of echolocation, leading to significantly fewer prey capture attempts at received levels greater than 96 dB re 1 µPa (16 kHz third-octave). Heinänen and Skov (2015) found that the occurrence of harbour porpoise declines significantly when the number of vessels in a 5 km2 area exceeds 20,000 ships per year (approximately 80 ships per day or 18 ships per km2).
  4. Other species of dolphin (e.g. common dolphin) are regularly sighted near vessels and may also approach vessels (e.g. bow-riding). However, dolphins are also known to show aversive behaviours to vessel presence, including increased swimming speed, avoidance, increased group cohesion and longer dive duration (Miller et al., 2008). Reactions of marine mammals to vessel noise are often linked to changes in the engine and propeller speed (Richardson et al., 1995). Watkins (1986) reported avoidance behaviour in baleen whales from loud or rapidly changing noise sources, particularly where a boat approached an animal. Disturbance in dolphins and porpoises is likely to be associated with the presence of small, fast-moving vessels as they are more sensitive to high frequency noise, whilst baleen whales, such as minke whale, are likely to be more sensitive to slower moving vessels emitting lower frequency noise. Pirotta et al. (2015) found that transit of vessels (moving motorised boats) in the Moray Firth resulted in a reduction (by almost half) of the likelihood of recording bottlenose dolphin prey capture buzzes. They also suggest that vessel presence, not just vessel noise, resulted in disturbance. Anderwald et al. (2013) suggested that in the study of displacement responses to construction-related vessel traffic, minke whale and grey seal were avoiding the area due to noise rather than vessel presence. In the same study, the presence of bottlenose dolphin was positively correlated with overall vessel numbers, as well as the number of construction vessels. It was, however, unclear whether the bottlenose dolphins were attracted to the vessels themselves or to particularly high prey concentrations within the study area at the time. Richardson (2012) investigated the effect of disturbance on bottlenose dolphin community structure in Cardigan Bay and found that group size was significantly smaller in areas of high vessel traffic.
  5. There is, however, evidence of habituation to boat traffic and therefore a slight increase from the existing levels of traffic in the vicinity of the Proposed Development may not result in high levels of disturbance. For example, Lusseau et al. (2011) (Scottish Natural Heritage commissioned report) undertook a modelling study which predicted that increased vessel movements associated with offshore wind development in the Moray Firth did not have a adverse effect on the local population of bottlenose dolphin, although it did note that foraging may be disrupted by disturbance from vessels.
  6. Seals are particularly sensitive to disturbances in regions where vessel traffic overlaps with productive coastal waters (Robards et al., 2016). Richardson et al. (2005) reported avoidance behaviour or alert reactions in harbour seal when vessels approach within 100 m of a haul-out (Richardson et al., 2005); when disturbed, seals that are hauled-out typically flush into the water which could be detrimental during pupping season (e.g. Terhune and Almon, 1983; Johnson and Acevedo-Gutiérrez, 2007). The presence of vessels in foraging grounds could result in reduced foraging success, particularly in harbour seals given reduced foraging ranges (c. 50 km from haul-outs) when compared to grey seals (c. 150 km from haul-outs) (SCOS, 2017). However, seals can be curious and have been recorded approaching tour boats that regularly visit an area and may habituate to sounds from tour vessels (Bonner, 1982). Mikkelsen et al. (2019) used long term sound and movement tagging data to study reaction to ship noise in grey seals in the North Sea and found that animals were exposed to audible vessel noise 2.2% – 20.5% of their time when in water and that high vessel noise coincided with interruption of functional behaviours such as resting.
  7. As mentioned previously, a study on grey seals by Hastie et al. (2021) demonstrated how foraging context is important when interpreting avoidance behaviour and should be considered when predicting the effects of anthropogenic activities, with avoidance rates depending on the perceived risk (e.g. silence, pile driving noise, operational noise from tidal turbines) versus the quality of the prey patch. It highlights that sound exposure in different prey patch qualities may result in markedly different avoidance behaviour, and should be considered when predicting impacts in EIAs. Given the existing levels of vessel activity in the Proposed Development shipping and navigation study area (see volume 2, chapter 13) it is expected that marine mammals could tolerate the effects of disturbance without any impact on reproduction and survival rates and would return to previous activities once the impact had ceased.
  8. All marine mammals, which are IEFs of international value, are deemed to be of low vulnerability and high recoverability. The sensitivity of the receptor is therefore considered to be medium.
Significance of the Effect

Auditory injury

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures (vessels following Code of Conduct) and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Behavioural disturbance

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect

Auditory injury and behavioural disturbance

  1. No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms. Therefore, the residual effect is considered to be of minor adverse significance, which is not significant in EIA terms.

Operation and Maintenance Phase

Magnitude of Impact
  1. Vessel use during operation and maintenance phase of the Proposed Development may lead to injury and/or disturbance to marine mammals. 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 10.16   Open ▸ ).
  2. 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 354 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 can not be quantified at the time of writing, however, it is anticipated this reduction will be ultimately counterbalanced by the presence of maintenance vessels. Vessel movements will be within the Proposed Development array area and the 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 to minimise the potential for collision risk as described in Table 10.21   Open ▸ .
  3. 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 357 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

  1. 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 353 et seq. for the construction phase with effect ranges presented in Table 10.51   Open ▸ and Table 10.52   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 magnitude is therefore considered to be low.

Behavioural disturbance

  1. 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 361 et seq. for the construction phase with impact ranges presented in Table 10.53   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 magnitude is therefore considered to be low.
Sensitivity of the Receptor

Auditory injury

  1. The sensitivity of marine mammal receptors to auditory injury has been assessed in paragraph 197 et seq. and is not reiterated here. PTS ranges that are a result of vessels involved in the construction phase (non-impulsive sound) are lower than PTS ranges for piling (impulsive sound) and the numbers of animals potentially injured are very low for all species.
  2. All marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and low recoverability. The sensitivity of the receptor is therefore considered to be high.

Behavioural disturbance

  1. The sensitivity of the receptors during the operation and maintenance is not expected to differ from the sensitivity of the receptors during the construction phase. The sensitivity of marine mammal receptors to elevated underwater noise due to vessel use and other activities is as described previously in paragraph 372 et seq. and is deemed to be medium.
Significance of the Effect

Auditory injury

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures (vessels following Code of Conduct) and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Behavioural disturbance

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect

Auditory injury and behavioural disturbance

  1. No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms. Therefore, the residual effect is considered to be of minor adverse significance, which is not significant in EIA terms.

Decommissioning Phase

Magnitude of Impact
  1. Vessel use during the decommissioning phase of Proposed Development may lead to injury and/or disturbance to marine mammals. Vessel types which will be required during the decommissioning phase include those used during removal of foundations, cables and cable protection ( Table 10.16   Open ▸ ).
  2. Since the numbers and types of vessel used to remove infrastructure (and hence their size and outputs) are expected to be similar to those used for installation, this impact is expected to result in a similar maximum design spatial scenario as the construction phase. The magnitude of the impact of the decommissioning phase for both auditory injury and disturbance as a result of elevated underwater noise due to vessel use, for all marine mammal receptors, is therefore not expected to differ or be greater than that assessed for the construction phase, where it has been assessed as low.

Auditory injury

  1. 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 353 et seq. for the construction phase and has not been reiterated here for the decommissioning phase. The impact is predicted to be of local spatial extent, medium term duration, intermittent and low reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.

Behavioural disturbance

  1. 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 361 et seq. for the construction phase and have not been reiterated here for the decommissioning phase. The impact is predicted to be of local spatial extent, medium term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the Receptor

Auditory injury

  1. The sensitivity of marine mammal receptors to auditory injury has been assessed in paragraph 197 et seq. and is not reiterated here. PTS ranges that are a result of vessels involved in the construction phase (non-impulsive sound) are in majority lower than PTS ranges for piling (impulsive sound), so auditory damage is likely to be less severe. PTS ranges that are a result of vessels involved in the decommissioning phase (non-impulsive sound) are in majority lower than PTS ranges for piling (impulsive sound) and the numbers of animals potentially injured are very low for all species.
  2. All marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and low recoverability. The sensitivity of the receptor is therefore considered to be high.

Behavioural disturbance

  1. The sensitivity of the receptors during the decommissioning phase is not expected to differ from the sensitivity of the receptors during the construction phase. The sensitivity of marine mammal receptors to elevated underwater noise due to vessel use and other activities is as described previously in paragraph 372 et seq. and is deemed to be medium.
Significance of the Effect

Auditory injury

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. The potential risk of injury will be reduced by appropriate designed-in measures (vessels following Code of Conduct) and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Behavioural disturbance

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect

Auditory injury and behavioural disturbance

  1. No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms. Therefore the residual effect is considered to be of minor adverse significance, which is not significant in EIA terms.

Increased Risk of Injury of Marine Mammals Due To COllision With Vessels

Construction Phase

Magnitude of Impact
  1. Vessel traffic associated with the Proposed Development has the potential to lead to an increase in vessel movements within the Proposed Development marine mammal study area. This increase in vessel movement could lead to an increase in interactions between marine mammals and vessels during offshore construction. Whilst a broad range of vessel types are involved in collisions with marine mammals (Laist et al., 2001), vessels travelling at higher speeds pose a higher risk because of the potential for a stronger impact (Schoeman et al., 2020).
  2. Collisions of vessels with marine mammals have the potential to result in both fatal and non-fatal injuries (Laist et al., 2001; Vanderlaan and Taggart, 2007; Cates et al., 2017). Evidence for fatal collisions has been gathered from carcasses washing up on beaches (Laist et al., 2001; Peltier et al., 2019), carcasses caught on vessel bows (Laist et al., 2001; Peltier et al., 2019) and floating carcasses; injuries including propeller cuts, significant bruising, oedema, internal bleeding radiating from a specific impact site, fractures and ship paint marks have strongly suggested ship strike as cause of death (Jensen and Silber, 2004; Jensen and Silber, 2003; Douglas et al., 2008). Fatalities from ship strikes, however, often go unreported (Authier et al., 2014). For non-fatal injuries there is evidence of animals which have survived ship strikes with no discernible injury; animals which survive with non-fatal injuries from propellers have been widely documented (Wells et al., 2008; Luksenburg, 2014).
  3. Guidance provided by National Oceanic and Atmospheric Administration (NOAA) has defined serious injury to marine mammals as ‘any injury that will likely result in mortality’ (NMFS, 2005). NMFS clarified its definition of ‘serious injury’ in 2012 and stated their interpretation of the regulatory definition of serious injury as any injury that is ‘more likely than not’ to result in mortality, or any injury that presents a greater than 50% chance of death to the marine mammal (NMFS, 2012; Helker et al., 2017). Non-serious injury is likely to result in short-term impacts and may also have long-term effects on health and lifespan.
  4. Vessel traffic associated with the construction activities will result in an increase in vessel movements within the Proposed Development marine mammal study area as up to 11,484 return trips by construction vessels may be made throughout the construction phase. This increase, described in more detail in paragraph 353 et seq., could lead to an increase in interactions between marine mammals and vessels. Vessels travelling at 7 m/s or faster are those most likely to cause death or serious injury to marine mammals (Laist et al., 2001; Wilson et al., 2007). With the exception of CTVs, vessels involved in the construction phase are likely to be travelling considerably slower than this, and all vessels will be required to follow a Project Code of Conduct, included as a part of the NSPVMP (volume 4, appendix 25), The Code of Conduct outlines instructions for vessel behaviour and vessel operators, including advice to operators to not deliberately approach marine mammals and to avoid sudden changes in course or speed. ( Table 10.21   Open ▸ ). Therefore, with Project designed in measures in place, the risk of collision is anticipated to be reduced and would only be present for transiting vessels (as opposed to stationary).
  5. A proportion of vessels involved in construction will be relatively small in size (e.g. tugs, vessels carrying ROVs, CTVs, dive boats, barges and RIBs) and due to good manoeuvrability able to move to avoid marine mammals, when detected (Schoeman et al., 2020). Larger vessels with lower manoeuvrability may need larger distances to avoid an animal, however they will also be travelling at slower speeds and have more time to react when marine mammal is detected. In addition, the noise emissions from vessels involved in the construction phase are likely to deter animals from the potential zone of impact. The vessel movements will be contained within the Proposed Development array area and Proposed Development export cable corridor and will follow existing shipping routes to/from the ports.
  6. The impact is predicted to be of local spatial extent, medium term duration, intermittent and, whilst the risk will only occur during vessel transits, the effect of collision on sensitive receptors is of medium to low reversibility (depending on the extent of injuries). It is predicted that the impact will affect the receptor directly. With designed-in measures in place the risk of collision will be reduced, however, given the potential for a collision to lead to injury the magnitude is, conservatively, considered to be low.
Sensitivity of the Receptor
  1. Marine mammals are generally able to detect and avoid vessels, however, it is unclear why some individuals do not always move out of the path of an approaching vessel (Schoeman et al., 2020). It has been suggested that behaviours such as resting, foraging, nursing, and socialising could distract animals from detecting the risk posed by vessels (Dukas, 2002). There can be consequences to a lack of response to disturbance for all marine mammals; behavioural habituation can result in decreased wariness of vessel traffic, which has the potential to result in an increased collision risk (Cates et al., 2017). Vessel strikes are known to be a cause of mortality in marine mammals (Carrillo and Ritter, 2010), and it is possible that mortality from vessel strikes is under-recorded (Van Waerebeek et al., 2007). Laist et al. (2001) reported that collisions between vessels and large whales tended to lead to death, but non-lethal collision has also been reported by Van Waerbeek et al. (2007). As described above in paragraph 404, collisions between cetaceans and vessels, are not necessarily lethal on all occasions.
  2. Harbour porpoise, as the most abundant cetacean species in the study area, are small and highly mobile and considering their potential avoidance responses to vessel noise (see paragraph 374), it can be assumed that they will largely avoid vessel collisions. UK Cetacean Stranding’s Investigation Programme (CSIP) (CSIP, 2015) reported results of post-mortem analysis conducted on 53 harbour porpoise strandings in 2015. A cause of death was established in 51 examined individuals (approximately 96% of examined cases) and, of these, only four (8%) had died from physical trauma of unknown cause, which could have been vessel strikes (CSIP, 2015).
  3. Collision risk for seals is less understood than for cetaceans. Trauma ascribed to collisions with vessels has been identified in < 2% of both live stranded (Goldstein et al., 1999) and dead stranded seals in the USA (Swails, 2005). The Onoufriou et al. (2016) study in the Moray Firth, Scotland showed that seals utilise the same areas as vessels during trips between haul-outs and foraging sites but that seals tended to remain beyond 20 m from vessels with only three instances over 2,241 days of seal activity resulted in passes at < 20 m.
  4. Although the potential to experience injury from construction traffic is relatively low, the consequences of collision risk could be fatal. All marine mammal receptors would have limited tolerance to a collision risk, and the effect of the impact could cause a change in both reproduction and survival of individuals, and receptors would have limited ability for the animal to recover from the effect.
  5. In summary, there is a high likelihood that marine mammals will avoid vessels and therefore, collision risk. On the basis that not all collisions that do occur are lethal, there is considered to be a medium potential for recovery.
  6. All marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and medium to low recoverability. The sensitivity of the receptor is therefore considered to be medium.
Significance of the Effect
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect
  1. No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms. Therefore the residual effect is considered to be of minor adverse significance, which is not significant in EIA terms.

Operation and Maintenance Phase

Magnitude of Impact
  1. Vessel use during operation and maintenance phase of Proposed Development may lead to injury to marine mammals due to collision with vessels. 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 10.16   Open ▸ ). The types of vessels are similar to those presented for the maximum design scenario for the construction phase. An overview of the potential impacts due to vessel collision are described in paragraph 404 for the construction phase and have not been reiterated here for the operation and maintenance phase.
  2. The impact is predicted to be of local spatial extent, long term duration, intermittent and the effect will be of medium to low reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the Receptor
  1. The sensitivity of the receptors during the operation and maintenance phase is not expected to differ from the sensitivity of the receptors during the construction phase. Therefore, the sensitivity of marine mammal receptors to collision risk is as described previously in paragraph 410 et seq., where it has been assessed as medium.
Significance of the Effect
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect
  1. No secondary mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms. Therefore the residual impact is considered to be of minor adverse significance, which is not significant in EIA terms.

Decommissioning Phase

Magnitude of Impact
  1. An overview of the potential impacts to marine mammals from vessel collision risk are described in paragraphs 404 et seq. for the construction phase and have not been reiterated here for the decommissioning phase.
  2. Vessel use during the decommissioning phase of Proposed Development may lead to injury to marine mammals due to collision with vessels. Vessel types which will be required during the decommissioning phase include those used during removal of foundations, cables and cable protection ( Table 10.16   Open ▸ ). The types of vessels used during the decommissioning will result in a similar maximum design scenario as the construction phase.
  3. The impact is predicted to be of local spatial extent, medium term duration, intermittent and the effect will be of medium to low reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the Receptor
  1. The sensitivity of the receptors during the decommissioning phase is not expected to differ from the sensitivity of the receptors during the construction phase. Therefore, the sensitivity of marine mammal receptors to collision risk is as described previously in paragraph 410 et seq., where it has been assessed as medium.
Significance of the Effect
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect
  1. No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms. Therefore the residual effect is considered to be of minor adverse significance, which is not significant in EIA terms.

Changes In Fish and Shellfish Communities Affecting Prey Availability

  1. 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, 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.
  2. The key prey species for marine mammals include sandeels, 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). For example, there are important spawning grounds for cod, herring, plaice, sandeel, whiting and sprat within the Proposed Development array area and export cable corridor. Consequently, adverse effects on fish receptors may have indirect adverse effects on marine mammal receptors.

Construction Phase

Magnitude of Impact
  1. Potential impacts on marine mammal prey species during the construction phase have been assessed in volume 2, chapter 9 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.
  2. 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 magnitude.
  3. As suggested in volume 2, chapter 9, 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), 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 sandeels. The magnitude of the impact was considered to be low. Consequently, the effect of temporary habitat loss/disturbance was assessed as being of minor adverse significance.
  4. Long-term habitat loss will occur directly under all wind turbine and OSP/Offshore convertor station platform foundation structures, associated scour protection and cable protection (including at cable crossings) where this is required. Long-term subtidal habitat loss within the Proposed Development fish and shellfish ecology study area will occur during construction (i.e. through placement of infrastructure) although effects will extend throughout the operation and maintenance phase (see paragraph 448). The presence of infrastructure within the Proposed Development 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, 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. The magnitude of the impact was considered to be low. Consequently, the effect of temporary long-term habitat loss was assessed as being of minor adverse significance.
  5. 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 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, the magnitude of impact was deemed to be low and the sensitivity of fish and shellfish receptors was considered to be low to medium. The effect was therefore assessed as being of negligible to minor adverse significance.
  6. 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). 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, 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). The magnitude of underwater noise effects was considered to be low and the sensitivity of the fish and shellfish receptors was assessed as low to medium. Therefore, the effect was of negligible to minor adverse significance.
  7. 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 116 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.
  8. On the basis of the assessments presented in volume 2, chapter 9, negligible or minor 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, which are not significant in EIA terms.
  9. The impact on marine mammals is predicted to be of local spatial extent, medium-term duration, intermittent and the effect on marine mammals is of high reversibility. The magnitude is therefore, considered to be low.
Sensitivity of the Receptor
  1. 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.
  2. The fish and shellfish communities found within the Proposed Development fish and shellfish ecology study area (see volume 2, 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 two species, harbour porpoise and harbour seal, 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 while harbour seal 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 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.
  3. Minke whale has the potential to be particularly vulnerable to potential effects on sandeels, particularly if there is a potential for reduced abundance. Studies analysing the stomach contents of minke whale found that in the North Sea this species is their key food resource, followed by clupeids Clupeidae and to a lesser extent mackerel (Robinson and Tetley, 2005; Tetley et al., 2008; see volume 3, appendix 10.2 for more details). Minke whale moves inshore during summer months to exploit key prey species. There was a spatial overlap between positions of minke whale sightings during Firth of Forth Round 3 boat-based surveys from May 2009 to November 2011 (see volume 3, appendix 10.2) and areas of high probability of sandeel presence (Langton et al., 2021). Various studies reported seasonal movement of minke whales to favoured feeding grounds, optimal for sandeel (from May to August; Robinson et al., 2009; Risch et al., 2019) and suggested some degree of generality regarding their habitat preferences that would favour sandeel, including association with the 50 m isobath, gravel/sand sediments and steep slopes (de Boer, 2010). Anderwald et al. (2012) studied flexibility of minke whales in their habitat use and found that although significantly higher sighting rates often occur in habitats associated with sandeel presence, an area of high occupancy by minke whale, coincided with high densities of sprat during spring. Hence, the low energetic cost of swimming in minke whales and their ability to switch between different prey according to their seasonal availability indicates that these species are able to readily respond to temporal changes in pelagic prey concentrations.
  4. All marine mammals, which are IEFs of international value, are deemed to be of low vulnerability and high recoverability. The sensitivity of the receptor is therefore considered to be low.