Construction phase

Magnitude of impact
Auditory injury

Vessel noise

  1. During the construction phase of the Array, the increased levels of vessel activity will contribute to background underwater noise levels. The MDS for construction activities associated with the Array assumes up to a total of 97 vessels to be present within the site boundary at any one time making up to 7,902 return trips over the duration of site preparation and construction phases (72 months). Detailed information about numbers of each type of vessel along with number of return trips for each is provided in  Table 10.17   Open ▸ .
  2. Whilst there will be an uplift in vessel activity during the site preparation and construction phases of the Array, the movements will be limited to within the site boundary and are likely to follow existing shipping routes to and from the ports. Based on long term vessel Automatic Identification System (AIS) traffic data from the entire 12 month period of 2023, on average, nine to ten unique vessels per day were recorded within the shipping and navigation study area (site boundary plus 10 nm buffer, see volume 3, appendix 13.1 for more details). Out of the vessels recorded within the shipping and navigation study area, 21% intersected the site boundary and the most common vessel types to intersect was cargo vessels (47%), oil and gas vessels (21%), and tankers (15%). Oil and gas vessels showed seasonal variation with only slight seasonality present in cargo vessels and tankers. Fishing vessels were recorded all year-round with higher volumes between March and September 2022. Additionally, visual observation surveys undertaken in winter 2022 (07 December 2022 to 21 December 2022) and summer 2023 (02 July 2023 to 18 July 2023) covering the shipping and navigation study area (see volume 3, appendix 13.1 for more details). During the winter vessel traffic survey period, there was an average of nine unique vessels per day recorded within the shipping and navigation study area, with two to three per day within the site boundary. The busiest full day during winter within the site boundary was 16 December 2022, when eight unique vessels were recorded. During the summer vessel traffic survey period, there was an average of 11 unique vessels per day recorded within the shipping and navigation study area, with three to four per day within the site boundary. The busiest full day during summer within the site boundary was the 05 July 2023, when seven unique vessels were recorded.
  3. The main drivers influencing the magnitude of the impact are vessel type, speed and ambient sound levels (Wilson et al., 2006b). As described in the navigational risk assessment for the Array, baseline levels of vessel traffic within the site boundary are at a relatively high-level largely due to movements of cargo, followed by oil and gas, tankers, tugs and fishing vessels (refer to volume 3, appendix 13.1 for more details).
  4. A detailed underwater sound modelling assessment has been carried out to investigate the potential for injurious and behavioural effects on marine mammals resulting from elevated underwater noise from vessels and non-piling activity, using the latest criteria from Southall et al. (2019) (vessel noise is classed as non-impulsive, see 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 interspecific variation or context-dependent differences). This is a precautionary approach as in reality, the distance over which effects may occur will vary according to the species, the ambient sound levels, hearing ability, vertical space use and behavioural response differences. Furthermore, vessel noise will be temporary and transitory, as opposed to permanent and fixed. Due to the mobile nature of marine mammals, it is highly unlikely that any marine mammal would stay at a stationary location or within a fixed radius of a vessel and therefore the underwater noise modelling has been undertaken based on an animal swimming away from the source (or the source moving away from an animal, see volume 3, appendix 10.1 for more details).
  5. The underwater noise modelling results indicate that the threshold for PTS was not exceeded for all species for all vessels, except harbour porpoise ( Table 10.48   Open ▸ ). There is a risk of injury (PTS) to harbour porpoise within 15 m from the noise source for sand wave clearance, main installation vessels, cable laying and rock placement vessels. However, it should be noted that the PTS ranges based on SEL threshold do not take into account any ambient noise levels and therefore are likely to be over-precautionary. With designed in measures in place, e.g. adherence to a Navigational Safety and Vessel Management Plan (NSVMP) (volume 4, appendix 24) where vessels will not deliberately approach animals and will remain at low speeds, the risk of auditory injury to marine mammals is considered to be negligible. The designed in measures to reduce the risk of injury to marine mammals (such as the NSVMP, see Table 10.22   Open ▸ ) will be followed at all times.

 

Table 10.48:
Estimated Potential PTS Ranges From Different Vessels For Marine Mammals (N/E = Threshold Not Exceeded)

Table 10.48: Estimated Potential PTS Ranges From Different Vessels For Marine Mammals (N/E = Threshold Not Exceeded)

 

Drilled piling

  1. Additionally, up to 10% of piles at wind turbine anchors (159 piles) and OSPs (216 piles) are anticipated to require drilling ( Table 10.17   Open ▸ ) and may be a source of underwater noise. The underwater noise modelling found that the PTS threshold will not be exceeded for all marine mammals exposed to drilled pile installation (see volume 3, appendix 10.1 for more details).
  2. With regard to injury, the impact (elevated underwater noise during vessel activity and other noise producing activities) is predicted to be of local spatial extent, medium-term duration, intermittent and, although the impact itself is reversible (i.e. the elevation in underwater noise only occurs during vessel activity and other noise producing activities), the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Given very small potential injury ranges for harbour porpoise only and considering the application of designed in measures (the NSVMP, volume 4, appendix 24), there is considered to be no residual risk of injury and therefore no population-level effects. The magnitude was therefore considered to be negligible.
Behavioural disturbance
  1. Behavioural disturbance from vessel noise is likely to occur only where vessel sound associated with the site-investigation and construction phases of the Array exceeds the background ambient sound level. As discussed in paragraph 381 above, the site boundary is located in waters with relatively high traffic associated with maritime transport, hence the presence of a high proportion of cargo vessels amongst all recorded vessels. Additionally, the site boundary is located in proximity to oil and gas structures in the North Sea and as such the traffic of oil and gas vessels is substantial. Considering the current levels of vessel traffic, it can be anticipated that marine mammals present in the vicinity of the Array marine mammal study area are exposed to some level of background noise.
  2. For non-impulsive (continuous) sound sources such as from vessels, there is a single available threshold (120 dB re 1 μPa (rms) based on NMFS (2005)), which is proposed as the basis for the onset of a strong behavioural reaction. However, it must be noted that thresholds that relate single exposure parameters (e.g. received sound level) to behavioural responses across species and sound types may lead to over-simplification in prediction of effects. Ideally differences between species, situational context, spatial scales and interacting effects of multiple stressors would be quantified to predict effects, but Southall (2021) highlights few studies report this critical data in a systematic structured way. Using a single threshold assumes that 100% of animals above this threshold are disturbed, whilst in reality, for those animals disturbed there is likely to be a proportional response (i.e. not all animals will be disturbed to the same extent). Joy et al. (2019) derived a dose-response for killer whales and underwater noise from vessels, indicating that marine mammals display a proportional response to non-impulsive noise. However, there is no dose-response curve available to apply in the context of non-impulsive sound sources for key species in the North Sea.
  3. JNCC et al. (2010) state that “it is most unlikely that a passing vessel would cause more than trivial disturbance. It is the repeated or chronic exposure to vessel noise that could cause disturbance”. Therefore, it is important to note that the 120 dB re 1 μPa (rms) criterion is very precautionary and that ambient sound levels in the North Sea could well exceed this value (NMFS, 2005, Xodus, 2014). This conservative assumption has been corroborated by Farcas et al. (2020), where the authors constructed a computational model of underwater noise levels in the North-east Atlantic using AIS data and environmental parameters and found that the annual median broadband noise level exceeded 120 dB re 1 μPa around offshore installations in the northern North Sea. Given the close proximity of the site boundary to the offshore oil and gas installations, it is anticipated that the background noise levels within the Array marine mammal area are close to or exceeding the 120 dB re 1 μPa (rms) criterion. Therefore, behavioural disturbance ranges and number of animals potentially disturbed presented in this section should be interpreted with caution.
  4. The estimated ranges within which there is a potential for disturbance to marine mammals are presented in Table 10.49   Open ▸ . Survey vessels, CTVs, SOVs, support vessels, CSVs, trenching support vessels, UXO clearance vessel, PLGR vessels and DSVs resulted in the greatest modelled disturbance out to 3,259 m for all marine mammal species ( Table 10.49   Open ▸ ). The greatest disturbance range for other non-vessel continuous sound behavioural effects was predicted to be 2,224 m due to underwater sound from sand wave clearance, cable laying and rock placement activities ( Table 10.49   Open ▸ ). In comparison, behavioural disturbance ranges for activities such as boulder clearance, offshore construction vessels, excavators and backhoe dredgers were predicted out to only 302 m ( Table 10.49   Open ▸ ).

Table 10.49:
Estimated Potential Disturbance Ranges From Different Vessels For All Marine Mammals (N/E = Threshold Not Exceeded)

Table 10.49: Estimated Potential Disturbance Ranges From Different Vessels For All Marine Mammals (N/E = Threshold Not Exceeded)

 

  1. Additionally, up to 10% of piles at wind turbine anchors (159 piles) and OSPs (216 piles) are anticipated to require drilling ( Table 10.17   Open ▸ ) and may be a source of underwater noise. The underwater noise modelling found that disturbance range for drilled piling was out to 309 m, comparable to disturbance ranges from boulder clearance, offshore construction vessel, excavator, backhoe dredger vessels.
  2. The number of animals predicted to experience behavioural disturbance due to vessel use and other noise producing activities is presented in Table 10.50   Open ▸ . Given the largest behavioural disturbance ranges and precautionary peak seasonal site-specific densities ( Table 10.13   Open ▸ ), the largest number of animals affected was found for harbour porpoise where up to 22 animals could experience strong disturbance as a result of activity of survey vessels, CTVs, SOVs, support vessels, CSVs, trenching support vessels, UXO clearance vessel, PLGR vessels and DSVs (0.01% of the North Sea MU population). The second and third largest number of animals disturbed was predicted for grey seal and white-beaked dolphin with up to six and four individuals potentially disturbed respectively (0.02% of the East Scotland MU plus North-east England seal MU for grey seal and 0.01% of the CGNS MU for white-beaked dolphin) due to the activity of the same type of vessels as listed for harbour porpoise ( Table 10.50   Open ▸ ). For bottlenose dolphin and minke whale the number of animals predicted to be disturbed was very small with no more than one animal within the predicted effect zones ( Table 10.49   Open ▸ , Table 10.50   Open ▸ ). It is important to highlight that multiplying numbers of animals presented in Table 10.50   Open ▸ by the numbers of vessels expected over the site preparation and construction phases ( Table 10.17   Open ▸ ) could lead to unrealistic estimates as it does not allow for any overlap between vessels (and therefore would double count), nor does it account for periods when vessels are stationary.
  3. The behavioural disturbance ranges will not overlap with any known important areas for any of the species, e.g. Southern North Sea SAC (harbour porpoise), CES2 MU (bottlenose dolphin), Southern Trench ncMPA (minke whale), Berwickshire and North Northumberland SAC (grey seal) ( Table 10.15   Open ▸ , Figure 10.3   Open ▸ ).

 

Table 10.50:
Maximum Number of Animals With the Potential to Experience Disturbance Due to Vessel use and Other Noise Producing Activities

Table 10.50: Maximum Number of Animals With the Potential to Experience Disturbance Due to Vessel use and Other Noise Producing Activities

 

  1. The impact (elevated underwater noise during vessel use and other noise producing activities) is predicted to be of local to regional spatial extent, medium-term duration, intermittent and the effect of behavioural disturbance is of high reversibility (with animals returning to baseline levels soon after they moved from the impact zone). It is predicted that the impact will affect the receptor directly. Whilst there may be effects at an individual level, these are not predicted to be at a scale that would lead to any population-level effects. The magnitude was 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 detail in paragraph 217 et seq., and therefore is not reiterated here. PTS ranges that are a result of vessels involved in the construction phase (non-impulsive sound) are far 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 are deemed to have limited resilience, low recoverability and high international value. 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 (International Whaling Commission (IWC), 2006, Senior et al., 2008).
  2. It is understood that 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). It varies by species, for example Anderwald et al. (2013) showed that bottlenose dolphin were beneficially correlated with total number of boats and number of utility vessels, but minke whale and grey seal were displaced by high levels of vessel traffic.
  3. Harbour porpoise, as a VHF cetacean, is particularly sensitive to high frequency sound and likely to avoid vessels. Wisniewska et al. (2018b) studied the temporary change in foraging rates of harbour porpoise in response to vessel sound in coastal waters with high traffic rates, and showed that occasional high sound levels coincided with vigorous fluking, bottom diving, interrupted foraging and even cessation of echolocation. This led to significantly fewer prey capture attempts at received levels greater than 96 dB re 1 µPa SPL rms (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). Benhemma-Le Gall et al. (2021) recently suggested increased vessel activity (and other construction activities) led to a decrease in harbour porpoise acoustic detections and activity at distances of up to 4 km, when comparing occurrence and foraging activity between two offshore wind farms in the Moray Firth.
  4. Other species of cetacean 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, greater time travelling, less time resting or socialising, avoidance, increased group cohesion and longer dive (Marley et al., 2017, Miller et al., 2008, Toro et al., 2021). In a recent study Meza et al. (2020) looked at behaviour of cetaceans when exposed to purse seine vessels in the Istanbul Strait, Turkey, which has high levels of human pressure with many vessels in a narrow space. The study found increased foraging in bottlenose and common dolphin (HF cetaceans) behavioural budgets, but a decrease in time spent foraging by harbour porpoise (a VHF cetacean).
  5. Fouda et al. (2018) studied concurrent ambient sound levels on social whistle calls produced by bottlenose dolphins in the western North Atlantic. The study demonstrated increases in ship sounds (both within and below the dolphin call bandwidth) resulted in simplified vocal calls, with higher dolphin whistle frequencies and a reduction in whistle contour complexity. Therefore, the sound-induced simplification of whistles may reduce the information content in these acoustic signals and decrease effective communication, parent–offspring proximity or group cohesion. This upward shift in whistle frequency has also been observed in bottlenose dolphin related to vessel presence in Walvis Bay, Namibia (Heiler et al., 2016).
  6. Reactions of marine mammals to vessel sound are often linked to changes in the engine and propeller speed (Richardson et al., 1995). Watkins (1986) reported avoidance behaviour in mysticetes from loud or rapidly changing sound 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 sound, whilst mysticetes, such as minke whale, are likely to be more sensitive to slower moving vessels emitting lower frequency sound. 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. The study also suggested that vessel presence, not just vessel sound, resulted in disturbance.
  7. 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 sound rather than vessel presence. 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 dolphin were attracted to the vessels themselves or to particularly high prey concentrations within the study area at the time. A study by Richardson (2012) on the effect of disturbance on bottlenose dolphin community structure in Cardigan Bay, Wales, found that group size was significantly smaller in areas of high vessel traffic.
  8. Observed reactions of pinnipeds to approaching vessels commonly includes increased alertness (Henry and Hammill, 2001), head raising (Niemi, 2013) and flushing off haul-out sites into the sea (Andersen et al., 2012, Blundell and Pendleton, 2015, Jansen et al., 2015, Johnson and Acevedo-Gutiérrez, 2007) but these studies focused on the presence of the vessel rather than vessel sound. Mikkelsen et al. (2019) recently found when studying the behaviour of grey and harbour seal to ship sound, a tagged grey seal changed its diving behaviour, switching rapidly from a dive ascent to descent. In a recent study which assessed the responses of grey seal to ecotourism during breeding and pupping seasons at White Strand Beach in south-west Ireland, Pérez Tadeo et al. (2021) found that vessels approaching within 500 m of the beach showed strong influence on the proportion of grey seal entering the water and increase in vigilance and decrease in resting behaviour. This is similar to a previous study on harbour seal which showed avoidance behaviour or alert reactions in harbour seal when vessels approach within 100 m of a haul-out (Paterson et al., 2015). Such disturbance to seal haul-outs could have adverse consequences during the pupping season, due to trade-offs between feeding and nursing. Harbour seal have been shown to be alerted and move away when a boat approaches (Andersen et al., 2012, Blundell and Pendleton, 2015), but this response varies by season. For example, they exhibit weaker and shorter lasting responses during the breeding season, appearing more reluctant to flee and return to the haul-out site after being disturbed (Andersen et al., 2012), likely attributed to a trade-off between moving away and nursing, rather than habituation. In a study of harbour seal in Alaska, haul-out probability was adversely affected by vessels, with cruise ships having the strongest effect (Blundell and Pendleton, 2015).
  9. The presence of vessels in foraging grounds could also result in reduced foraging success. The presence of whale-watching boats within an important feeding ground for minke whale led to a reduction in foraging activity Christiansen and Lusseau (2015). As a capital breeder, such a reduction could lead to reduced reproductive success since female body condition associated with foetal growth (Christiansen et al., 2014). However, it is worth noting that the study by Christiansen and Lusseau (2015) was conducted in Faxafloi Bay in Iceland where baseline sound levels (compared to the North Sea) are very low (McGarry et al., 2017). In addition, a subsequent study in the same area found no significant long term effects of disturbance from whale-watching on vital rates, as whales moved into disturbed areas when sandeel numbers were lower across their wider foraging area (Albert et al. (2022). Hastie et al. (2021) demonstrated how foraging context is important when interpreting avoidance behaviour in grey seals, and should be considered when predicting the effects of anthropogenic activities. Avoidance rates appeared to depend on the perceived risk (e.g. silence, pile driving sound, operational sound from tidal turbines) versus the quality of the prey patch Hastie et al. (2021). Therefore, it must be highlighted 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 Array shipping and navigation study area, 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.
  10. There is indication of tolerance to boat traffic (and anthropogenic sounds and activities in general) and so a slight increase from the existing levels of traffic in the vicinity of the Array may not necessarily result in high levels of disturbance (Vella et al., 2001). Whilst it cannot be assumed that tolerance to a stressor is evidence of absence of detrimental consequences for targeted animals (e.g. physiological responses are not easily detectable in free-ranging wild animals), there is evidence of animals (from multiple species) remaining in areas of high vessel traffic, described in paragraphs 407 below.
  11. For example, high co-occurrence between grey seal/harbour seal and shipping traffic within 50 km of the coastline near to haul-out sites were shown in a national scale assessment of seals and shipping in the UK (Jones et al., 2017). Thompson et al. (2011) (Scottish Natural Heritage (SNH) commissioned report) undertook a modelling study which predicted that increased vessel movements associated with offshore wind development in the Moray Firth would not have an adverse effect on the local population of bottlenose dolphin (although, similar to Benhemma-Le Gall et al. (2021), it did note that foraging may be disrupted by disturbance from vessels).
  12. Potlock et al. (2023) used cetacean porpoise detector (C-POD) detections of sonar activity as a proxy for vessel disturbance during construction of wind turbines foundations off Blyth, Northumberland. The vessel sonar variable was significant in both the dolphin (potentially bottlenose dolphin and/or white-beaked dolphin) and harbour porpoise models. The effect size was substantial in both species, with around eight minutes of sonar occurrence per hour leading to a 50% decline in harbour porpoise occurrence and around 13 minutes of sonar occurrence per hour leading to a 50% decline in dolphin occurrence. Despite this, dolphin occurrence during and after construction were not significantly different to the occurrence before the construction phase. Similarly, the increase in harbour porpoise occurrence across this study suggests that construction and after construction vessel activity did not result in any overall decline in area usage (Potlock et al., 2023).
  13. Bottlenose dolphins have been found to both increase and decrease whistle frequencies in noisy environments, avoiding acoustic masking and improving signal transmission (Heiler et al., 2016, La Manna et al., 2013, May-Collado and Wartzok, 2008, Peters, 2018, Rako Gospić and Picciulin, 2016). Therefore, it is suggested that if marine mammals depend on specific areas to maintain their activities, and the benefits exceed the cost of disturbance, animals may show increased tolerance instead of site avoidance (Antichi et al., 2022). Marine mammals therefore could continue to regularly visit the areas where they may be affected by the vessel presence (Antichi et al., 2022, Rako Gospić and Picciulin, 2016). Wisniewska et al. (2018a) found tagged porpoises did not appear to avoid highly trafficked areas, potentially because these overlapped with important foraging habitats (deep waters which may aggregate important prey items).
  14. Additionally, Joy et al. (2019) conducted a voluntary commercial vessel slowdown trial through 16 nm of shipping lanes which overlapped with critical habitat of at-risk southern resident killer whales. Disturbance metrics were simplified to a “lost foraging time” measure and demonstrated (when compared to baseline sound levels in the region) the slowdown trial achieved 22% reduction in ‘potential lost foraging time’ for killer whales (with 40% reductions when 100% of vessels were under the 11 knot speed limit). With the exception of CTVs, most vessels involved in the construction phase are likely to be travelling considerably slower than 11 knots (with all vessels travelling at at safe speeds at all times and reduce speed if appropriate when a marine mammal is in the vicinity, detailed in the NSVMP (volume 4, appendix 24, Table 10.22   Open ▸ ).
  15. All marine mammals are deemed to have some resilience to behavioural disturbance, high recoverability and high international value. The sensitivity of the receptor is therefore, considered to be medium.
Significance of the effect
Auditory injury
  1. Designed in measures adopted as part of the Array include the development of and adherence to a NSVMP, volume 4, appendix 24 (or equivalent) ( Table 10.22   Open ▸ ) which includes requirements to not deliberately approach marine mammals as a minimum, avoid abrupt changes in course or speed should marine mammals approach the vessel to bow-ride and to remain at safe speeds at all times and reduce speed when a marine mammal is in the vicinity. Therefore, these measures will further reduce the potential risk of injury and the scale of effect (injury radius and number of animals affected) was predicted to be very small.
  2. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. 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
  1. No further marine mammal mitigation is considered necessary (beyond design-in measures detailed in Table 10.22   Open ▸ ) because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Operation and maintenance phase

Magnitude of impact
  1. During the operation and maintenance phase of the Array, the increased levels of vessel activity will contribute to background underwater noise levels. The MDS for operation and maintenance activities associated with the Array assumes up to a total of 30 vessels to be present within the site boundary at any one time making up to 508 return trips over the duration of operation and maintenance phase (35 years). Detailed information about numbers of each type of vessel along with number of return trips for each is provided in  Table 10.17   Open ▸ .
  2. The uplift in vessel activity during the operation and maintenance phase is considered to be relatively small in the context of the baseline levels of vessel traffic in the vicinity and within the site boundary (see paragraph 381 above). Presence of the operational wind farm may divert some of the vessel routes and therefore, current traffic within the site boundary, which is not associated with the Array, is likely to be reduced. It is likely that this reduction will ultimately be counterbalanced by presence of maintenance vessels. Vessel movements will be limited to within the site boundary and are likely to follow existing shipping routes to and from the ports. The designed in measures to reduce the behavioural disturbance to marine mammals (such as the NSVMP, volume 4, appendix 24, see Table 10.22   Open ▸ ) will be followed at all times.
  3. The size and sound 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 ( Table 10.48   Open ▸ and Table 10.49   Open ▸ ). However, the number of vessels and round trips is much lower for the operation and maintenance phase compared to the construction phase.
Auditory injury
  1. An overview of potential impacts from elevated underwater noise due to vessel use and other (non-piling) noise producing activities as well as associated effects (auditory injury) are described in paragraph 380 et seq. for the construction phase and have not been reiterated here for the operation and maintenance phase of the Array.
  2. The impact (elevated underwater noise during vessel activity and other noise producing activities) is predicted to be of local spatial extent in the context of the geographic frame of reference, long term duration, intermittent and the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. The magnitude was therefore considered to be negligible.
Behavioural disturbance
  1. An overview of potential impacts from elevated underwater noise due to vessel use and other (non-piling) noise producing activities as well as associated effects (behavioural disturbance) are described in paragraph 387 et seq. for the construction phase and have not been reiterated here for the operation and maintenance phase of the Array.
  2. The impact (elevated underwater noise during vessel use and other noise producing activities) is predicted to be of local to regional spatial extent in the context of the geographic frame of reference, long term duration, intermittent and the effect of behavioural disturbance is reversible. It is predicted that the impact will affect the receptor directly. Whilst there may be effects at an individual level, these are not predicted to be at a scale that would lead to any population-level effects. The magnitude was therefore considered to be low.
Sensitivity of the receptor
Auditory injury
  1. The sensitivity of marine mammal receptors to auditory injury has been considered in detail in paragraph 217 et seq., and therefore is not reiterated here. PTS ranges that are a result of vessels involved in the operation and maintenance 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 are deemed to have limited resilience, low recoverability and adaptability and high international value. 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, which is described previously in paragraph 397 et seq. and is deemed to be medium.
Significance of the effect
Auditory injury
  1. Designed in measures adopted as part of the Array includes the development of and adherence to a NSVMP (volume 4, appendix 24) (or equivalent) ( Table 10.22   Open ▸ ) which includes requirements to not deliberately approach marine mammals as a minimum, avoid abrupt changes in course or speed should marine mammals approach the vessel to bow-ride and to remain at safe speeds at all times . Therefore, these measures will further reduce the potential risk of injury and the scale of effect (injury radius and number of animals affected) was predicted to be very small.
  2. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. 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
  1. No further marine mammal mitigation is considered necessary (beyond design-in measures detailed in Table 10.22   Open ▸ ) because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Decommissioning phase

Magnitude of impact

Vessel noise

  1. During the decommissioning phase of the Array, the increased levels of vessel activity will contribute to background underwater noise levels. Vessel types which will be required during the decommissioning phase include those used during removal of foundations, cables and cable protection, however the exact number of vessels and return trips is unknown at this stage.

Underwater cutting

  1. It is anticipated that maximum levels of underwater noise during the decommissioning phase would originate from underwater cutting required to remove structures (e.g. jacket foundations at OSPs). This is likely to be much less than pile driving and therefore impacts are likely to be less than as assessed during the construction phase. Given that the types of vessels used to remove infrastructure (and hence their size and outputs) are expected to be similar to those used for installation, the potential impacts from elevated underwater noise due to vessel use and other (non-piling) noise producing activities is expected to result in a similar maximum design spatial scenario as the construction phase. As such, the magnitude of the impact of the decommissioning phase for both auditory injury and behavioural disturbance for all marine mammal receptors, is not expected to differ or be greater than that assessed for the construction phase (paragraph 380 et seq.).
Auditory injury
  1. An overview of potential impacts from elevated underwater noise due to vessel use and other (non-piling) noise producing activities as well as associated effects (auditory injury) are described in paragraph 380 et seq. for the construction phase and have not been reiterated here for the decommissioning phase of the Array.
  2. The impact (elevated underwater noise during vessel activity and other noise producing activities) is predicted to be of local spatial extent, medium term duration, intermittent and, although the impact itself is reversible (i.e. the elevation in underwater noise only occurs during vessel activity and other noise producing activities), the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Given very small potential injury ranges for harbour porpoise only and considering the application of designed in measures (NSVMP, volume 4, appendix 24), there is considered to be no residual risk of injury and therefore no population-level effects. The magnitude was therefore considered to be negligible.
Behavioural disturbance
  1. An overview of potential impacts from elevated underwater noise due to vessel use and other (non-piling) noise producing activities as well as associated effects (behavioural disturbance) are described in paragraph 387 et seq. for the construction phase and have not been reiterated here for the decommissioning phase of the Array.
  2. The impact (elevated underwater noise during vessel use and other noise producing activities) is predicted to be of local to regional spatial extent, medium-term duration, intermittent and the effect of behavioural disturbance is of high reversibility (with animals returning to baseline levels soon after they moved from the impact zone). It is predicted that the impact will affect the receptor directly. Whilst there may be effects at an individual level, these are not predicted to be at a scale that would lead to any population-level effects. The magnitude was therefore considered to be low.
Sensitivity of the receptor
Auditory injury
  1. The sensitivity of marine mammal receptors to auditory injury has been considered in detail in paragraph 217 et seq., and therefore is not reiterated here. PTS ranges that are a result of vessels involved in the decommissioning 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 are deemed to have limited resilience, low recoverability and high international value. 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, which is described previously in paragraph 397 et seq. and is deemed to be medium.
Significance of the effect
Auditory injury
  1. Designed in measures adopted as part of the Array includes the development of and adherence to a NSVMP, volume 4, appendix 24 (or equivalent) ( Table 10.22   Open ▸ ) which includes requirements to not deliberately approach marine mammals as a minimum, avoid abrupt changes in course or speed should marine mammals approach the vessel to bow-ride and to remain at safe speeds at all times. Therefore, these measures will further reduce the potential risk of injury and the scale of effect (injury radius and number of animals affected) was predicted to be very small.
  2. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. 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
  1. No further marine mammal mitigation is considered necessary (beyond design-in measures detailed in Table 10.22   Open ▸ ) because the likely effect in the absence of mitigation is not significant in EIA terms.

Injury due to collision with vessels

                        Construction phase

Magnitude of impact
  1. Vessel traffic associated with the Array has the potential to lead to an increase in vessel movements within the Array 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). The severity of lesions seems also to be a function of speed e.g. Laist et al. (2001) reported among collisions with lethal or severe injuries, 89% of the 28 vessels investigated were moving at 14 knots or faster.
  2. Collisions of vessels with marine mammals have the potential to result in both fatal and non-fatal injuries (Cates and Acevedo-Gutiérrez, 2017, Laist et al., 2001, Vanderlaan and Taggart, 2007). 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 site, fractures and ship paint marks have strongly suggested ship strike as cause of death (Douglas et al., 2008, Jensen et al., 2003). However fatalities from ship strikes do often go unreported (Authier et al., 2014). There is evidence of animals which have survived ship strikes with no discernible injury (non-fatal injuries) and have been widely documented (Luksenburg and Parsons, 2014, Wells et al., 2008).
  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 (Helker et al., 2017, NMFS, 2023). In contrast, non-serious injury is likely to result in short term impacts which may have long term effects on health and lifespan.
  4. As discussed in paragraph 380, vessel traffic associated with the construction activities will result in an increase in vessel movements within the Array marine mammal study area, as up to 7,902 return trips by construction vessels may be made throughout the construction phase ( Table 10.17   Open ▸ ). This increase, described in more detail in paragraph 380 et seq., could lead to an increase in interactions between marine mammals and vessels. Vessels travelling at 7 m/s (~14 knots) or faster are those most likely to cause death or serious injury to marine mammals (Laist et al., 2001, Wilson et al., 2006a). All vessels will be required to adhere to the NSVMP which includes not deliberately approaching marine mammals as a minimum, to avoid abrupt changes in course or speed should marine mammals approach the vessel to bow-ride and to remain at safe speeds at all times (as detailed in Table 10.22   Open ▸ ) and reduce speed when a marine mammal is in the vicinity, which is therefore appropriate to reduce risk of collision for species found within the regional marine mammal study area as far as practicable. Therefore, with the designed in measures as part of the Array in place, the risk of collision is anticipated to be reduced and would only be present for transiting vessels (as opposed to stationary).
  5. Furthermore, a proportion of vessels involved in construction will be relatively small in size (e.g. tugs, vessels, support vessels, CTVs, dive boats, barges) and due to good manoeuvrability would be able to move to avoid marine mammals where detected (Schoeman et al., 2020). Larger vessels such as cargo-barges and installation 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 a marine mammal is detected. In addition, the sound emissions from vessels involved in the construction phase are likely to deter animals from the potential zone of impact. The vessel movements will likely be contained within the site boundary.
  6. The impact is predicted to be of local spatial extent in the context of the geographic frame of reference, 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. The magnitude is therefore considered to be low.
Sensitivity of the receptor
  1. In general marine mammals are largely able to detect and avoid vessels in advance due to their hearing sensitivity, particularly when conducting activities such as seismic surveys (Koski et al., 2009). Nevertheless, it remains unclear why some individuals do not always move out of the path of an approaching vessel (Schoeman et al., 2020) with analysis of data showing various interacting factors (e.g. ambient or background underwater noise) can interfere with the ability of marine mammals to detect approaching ships (Gerstein et al., 2005). It has been suggested that behaviours such as resting, foraging, nursing, and socialising could distract animals from detecting the risk posed by vessels regardless of detection abilities (Dukas, 2002, Gerstein et al., 2005). As such there can be consequences to this lack of response to disturbance for all marine mammals; behavioural habituation can result in decreased wariness of vessel traffic, which may result in an increased collision risk (Cates and Acevedo-Gutiérrez, 2017).
  2. As discussed in paragraphs 443 to 444 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), particularly for smaller marine mammals (Schoeman et al., 2020). Collisions between vessels and large whales can often lead to death or serious injury (Kraus, 1990) collisions between cetaceans and vessels are not necessarily lethal on all occasions (Van Waerebeek et al., 2007). Although all types of vessels may hit whales, most lethal and serious injuries are caused by large ships (e.g. 80 m or longer) and vessels travelling at speeds faster than 14 knots (Laist et al., 2001).
  3. Given harbour porpoise, as the most abundant cetacean species in the regional marine mammal study area, are small and highly mobile and considering their potential avoidance responses to vessel noise (see paragraph 380), it can be assumed that they will largely avoid vessel collisions. UK 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 may have resulted from vessel strikes (CSIP, 2015).
  4. Vessel strikes can result in lethal or non-lethal injuries to dolphins (Schoeman et al., 2020). Olson et al. (2022) reported that evidence from long term photo-identification data shows that only one out of a group of 277 bottlenose dolphin present within the study region exhibit marks indicative of vessel interactions. An earlier study by Van Waerebeek et al. (2007) reported that bottlenose dolphin is one of the species that may receive a moderate impact from collisions, however these may be sustainable at species level because many strikes are non-lethal.
  5. However, 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). A study in the Moray Firth, Scotland Onoufriou et al. (2016) 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 and only three instances over the 2,241 days of recorded seal activity resulted in passes at <20 m.
  6. Thus, on the basis that not all collisions that do occur are lethal, there is considered to be a medium potential for recovery. Necropsies and observations of whales surviving a vessel strike have provided information about the relationship between the severity of injury (e.g. depth of laceration, anatomical site of injury) and vessel speed (Combs, 2018, Conn and Silber, 2013, Rommel et al., 2007, Vanderlaan and Taggart, 2007, Wiley et al., 2016). Furthermore factors such as interspecific differences in bone strength may result in different risks of incurring blunt force trauma (Clifton et al., 2008) and provide further complex variability in lethality of collisions.
  7. All marine mammals are deemed to have some resilience/survivability (largely due to avoidance behaviour and that not all collisions are fatal), medium recoverability and adaptability, and high international value. 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 (particularly with the adoption of the NSVMP, volume 4, appendix 24) 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 marine mammal mitigation is considered necessary, in addition to the measures adopted as part of the array, because the likely effect in the absence of further mitigation is not significant in EIA terms.

                        Operation and maintenance phase

Magnitude of impact
  1. Vessel use during operation and maintenance phase of Array may lead to injury to marine mammals due to collision with vessels. Vessel types which will be required during the operation and maintenance phase including CTVs, SOVs, jack-up vessels, cable repair vessels, CSVs and DSVs ( Table 10.17   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 443 et seq. 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 in the context of the geographic frame of reference, long 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. 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 449 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 (particularly with the adoption of the NSVMP, volume 4, appendix 24) 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 marine mammal mitigation is considered necessary, in addition to the measures adopted as part of the array, because the likely effect in the absence of further mitigation is not significant in EIA terms.

                        Decommissioning phase

Magnitude of impact
  1. Vessel use during decommissioning phase of the Array may lead to injury to marine mammals due to collision with vessels. Vessels will be required for activities such as removal of foundation, cables and cable protection ( Table 10.17   Open ▸ ). Noise from vessels is assumed to be as per vessel activity described for construction phase, with an overview of the potential impacts described in paragraph 443 et seq. for the construction phase and have not been reiterated here for the decommissioning phase.
  2. 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. 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 449 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 (particularly with the adoption of the NSVMP, volume 4, appendix 24) 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 marine mammal mitigation is considered necessary, in addition to the measures adopted as part of the array, because the likely effect in the absence of further mitigation is not significant in EIA terms.

Effects on marine mammals due to EMFs from subsea electrical cabling in the water column

  1. The marine environment features natural magnetic and electric fields associated with both physical and biological sources, alongside anthropogenic EMFs that permeate it (Gill et al., 2014). This section involves the assessment of the LSE1 of EMFs from the dynamic inter-array cables on marine mammals.