Sensitivity of the Receptor

Auditory Injury

Harbour porpoise

  1. Scientific understanding of the biological effects of threshold shifts is limited to the results of controlled exposure studies on small numbers of captive animals (reviewed in Finneran et al., 2015) where TTS are experimentally induced (since it is unethical to induce PTS in animals) and thresholds for PTS extrapolated using TTS growth rates (see paragraph 71).
  2. Studies of auditory injury in relation to a typical piling sequence have suggested that hearing impairment as a result of exposure to piling noise is likely to occur where the source frequencies overlap the range of peak sensitivity for the receptor species rather than across the whole frequency hearing spectrum (Kastelein et al., 2013). Kastelein et al. (2013) demonstrated experimentally that for simulated piling noise (broadband spectrum), harbour porpoise’s hearing around 125 kHz (the key frequency for echolocation) was not affected. Instead, a measurable threshold shift in hearing was induced at frequencies of 4 kHz to 8 kHz, although the magnitude of the hearing shift was relatively small (2.3 dB to 3.6 dB at 4 kHz to 8 kHz) due to the lower received SELs at these frequencies. This was due to most of the energy from the simulated piling occurring in lower frequencies (Kastelein et al., 2013). Subsequently, Kastelein et al. (2017) confirmed sensitivity declined sharply above 125 kHz. The susceptibility of harbour porpoise to threshold shifts was further corroborated in a series of studies measuring temporary shifts in hearing in harbour porpoise at high amplitude frequencies of 0.5 kHz to 88.4 kHz. Here the greatest shift in mean TTS occurred at 0.5 kHz, which is very close to the lower bound of porpoise hearing (Kastelein et al., 2021). Hearing always recovered within 60 minutes after the fatiguing sound stopped.
  3. In addition to the frequency characteristics of the source, the duty cycle of fatiguing sounds is also likely to affect the magnitude of a hearing shift. Kastelein et al. (2014) suggested that hearing may recover to some extent during inter-pulse intervals. Similarly, Finneran (2015) highlighted that whilst a threshold shift can accumulate across multiple exposures, the resulting shift will be less than the shift from a single, continuous exposure with the same total SEL.
  4. There is some evidence of self-mitigation by cetaceans to minimise exposure to sound. The animal can change the orientation of its head so that sound levels reaching the ears are reduced, or it can suppress hearing sensitivity by one or more neurophysiological auditory response control mechanisms in the middle ear, inner ear, and/or central nervous system. Kastelein et al. (2020) highlighted the lack reproducibility of TTS in a harbour porpoise after exposure to repeated airgun sounds, and suggested the discrepancies may be due to self-mitigation.
  5. Extrapolating the results from captive bred studies to how animals may respond in the natural environment should, however, be treated with caution as it is not possible to exactly replicate natural environmental conditions. In addition, the small number of test subjects would not account for intraspecific differences (i.e. differences between individuals) or interspecific differences (i.e. extrapolating to other species) in response. However, based on our current understanding, since PTS is a permanent and irreversible hearing impairment it is expected that harbour porpoise is sensitive to this effect as the loss of hearing would affect key life functions (e.g. communication, predator detection, foraging, mating and maternal fitness) and could lead to a change in an animal’s health (if chronic) or vital rates (if acute) (Erbe et al., 2018). Morell et al. (2021) showed the first case of presumptive noise-induced hearing loss, based on inner ear analysis in a free-ranging harbour porpoise. Subject to the limitations of available empirical evidence a potential consequence of a disruption in key life functions is that the health of impacted animals would deteriorate and potentially lead to reduced birth rate in females and mortality of individuals (Costa, 2012).
  6. Given the uncertainty surrounding the effects of PTS on survival and reproduction and the importance of sound for echolocation, foraging and communication in all cetaceans, harbour porpoise, an IEF of international value, is deemed to be of high vulnerability and low recoverability. The sensitivity of the receptor to PTS is therefore, considered to be high.

Bottlenose dolphin and white-beaked dolphin

  1. Individual dolphins experiencing PTS would suffer a biological effect that could impact the animal’s health and vital rates (Erbe et al., 2018). Bottlenose and white-beaked dolphin are both classed as high-frequency cetaceans (Southall et al., 2019). As described for harbour porpoise in paragraph 197 et seq. there are frequency-specific differences in the onset and growth of a noise-induced threshold shift in relation to the characteristics of the noise source and hearing sensitivity of the receiving species. For example, exposure of two captive bottlenose dolphins to an impulsive noise source between 3 kHz and 80 kHz found that there was increased susceptibility to auditory fatigue between frequencies of 10 to 30 kHz (Finneran and Schlundt, 2013). The SELcum threshold incorporates hearing sensitivities of marine mammals and the magnitude of effects were considerably smaller compared to the VHF (e.g. harbour porpoise) and LF (e.g. minke whale) species, highlighting that HF species are less sensitive to the frequency components of the piling noise signal. The assessment considered the irreversibility of the effects (i.e. as noted for harbour porpoise) and importance of sound for echolocation, foraging and communication in small, toothed cetaceans.
  2. Given the uncertainty surrounding the effects of PTS on survival and reproduction and the importance of sound for echolocation, foraging and communication in all cetaceans, bottlenose dolphin and white-beaked dolphin, IEFs of international value, are deemed to be of high vulnerability and low recoverability. The sensitivity of both receptors to PTS is therefore, considered to be high.

Minke whale

  1. Although very little is known about minke whale hearing, their vocalisation frequencies are likely to overlap with anthropogenic sounds. Minke whale does not echolocate but likely use sound for communication and, like other mysticete whales, are able to detect sound via a skull vibration enabled bone conduction mechanism (Cranford and Krysl, 2015). As a baleen whale with estimated functional hearing range between 17 Hz and 35 kHz, it is likely that they rely on low frequency hearing (Ketten and Mountain, 2011). A controlled exposure study on free ranging minke whale in Iceland found that minke whales reacted strongly to a 15 kHz ADD; a frequency considered to be at the likely upper limit of their hearing sensitivity (Boisseau et al., 2021). As described for harbour porpoise in paragraph 197 et seq., there are likely to be frequency-specific differences in the onset and growth of a noise-induced threshold shift in relation to the characteristics of the noise source and hearing sensitivity of the receiving species.
  2. Given the uncertainty surrounding the effects of PTS on survival and reproduction and the importance of sound for echolocation, foraging and communication in all cetaceans, minke whale, an IEF of international value, is deemed to be of high vulnerability and low recoverability. The sensitivity of the receptor to PTS is therefore, considered to be high.

Harbour seal and grey seal

  1. Seals are less dependent on hearing for foraging than cetacean species, but may rely on sound for communication and predator avoidance (e.g. Deecke et al., 2002). Seals detect swimming fish with their vibrissae (Schulte-Pelkum et al., 2007) but, in certain conditions, they may also listen to sounds produced by vocalising fish in order to hunt for prey. Thus, the ecological consequences of a noise induced threshold shift in seals are a reduction in fitness, reproductive output and longevity (Kastelein et al., 2018a). Hastie et al., (2015) reported that, based on calculations of SEL of tagged harbour seals during the construction of the Lincs Offshore Wind Farm (Greater Wash, UK), at least half of the tagged seals would have received sound levels from pile driving that exceeded auditory injury thresholds for pinnipeds (PTS). However, population estimates indicated that the relevant population trend is increasing and therefore, although there are many other ecological factors that will influence the population health, this indicated that predicted levels of PTS did not affect a sufficient numbers of individuals to cause a decrease in the population trajectory (Hastie et al., 2015). Hastie et al. (2015), however, noted that due to paucity of data on effects of sound on seal hearing, the exposure criteria used are intentionally conservative and therefore predicted numbers of individuals likely to be affected by PTS would also have been highly conservative.
  2. There is some evidence of noise-induced PTS in harbour seals, with the first confirmed report of PTS following a known acoustic exposure event in a marine mammal (Reichmuth et al., 2019). The underwater hearing sensitivity of a trained harbour seal was evaluated before and immediately following exposure to 4.1 kHz tonal fatiguing stimulus, and rather than the expected pattern of TTS onset and growth, an abrupt threshold shift of > 47 dB was observed half an octave above the exposure frequency. While hearing at 4.1 kHz recovered within 48 h, there was a PTS of at least 8 dB at 5.8 kHz, and hearing loss was evident for more than ten years.
  3. Despite the uncertainty in the ecological effects of PTS on seals, seals rely on hearing much less than cetaceans and therefore would exhibit some tolerance (i.e. the effect is unlikely to cause a change in either reproduction or survival rates). In addition, it has been proposed that seals may be able to self-mitigate (i.e. reduce their hearing sensitivity in the presence of loud sounds in order to reduce their perceived SPL) (Kastelein et al., 2018a). Although this evidence suggests a lower sensitivity of pinnipeds to PTS, based on uncertainties a precautionary approach has been taken.
  4. The telemetry data confirmed connectivity between Firth of Tay and Eden Estuary SAC, designated for harbour seal, and the Proposed Development marine mammal study area. The population of harbour seal is mostly concentrated within the Firth of Tay and Eden Estuary SAC and Firth of Forth, however the population within the Tay SAC is continuing to decline without indication of recovery within last 20 years (see volume 3, appendix 10.2 for more information). Population modelling work conducted for the Firth of Tay and Eden Estuary SAC population has concluded that if this declining trend continues, the population may become extinct within the next 20 years (Hanson et al., 2017). Although it is unknown what is the reason for this decline, this population is deemed sensitive to any additional anthropogenic disturbance, especially during the breeding season (spring and summer). No population trajectory is available for Firth of Forth, although sporadic counts in the area indicate that the decline is localised within the SAC and may not represent the trends in the overall MU population (SCOS, 2020; Sinclair et al., 2020). Harbour seals are generalist feeders and can forage on variety of species, usually within 50 km from the coast. Individuals may be particularly sensitive to anthropogenic disturbance or changes in prey distribution especially during breeding season.
  5. Grey seal and harbour seals, 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. Studies have shown that acoustic disturbance to marine mammals may lead to the interruption of normal behaviours (such as feeding or breeding) and avoidance, leading to displacement from the area and exclusion from critical habitats (Goold, 1996; Weller et al., 2002; Castellote et al., 2010, 2012). Noise may also cause stress which in turn can lead to a depressed immune function and reduced reproductive success (Anderson et al., 2011; De Soto et al., 2013). The extent to which an animal will be behaviourally affected, however, is very much context-dependent and varies both inter- and intra-specifically as described previously (paragraph 79 et seq.). A summary of known behavioural sensitivities of different species to underwater noise from piling at other wind farm sites is provided in paragraph 213 et seq., noting that the conclusions drawn are subject to the limitations of extrapolating results from one project to another.

Harbour porpoise

  1. Harbour porpoise, as a small cetacean species, is vulnerable to heat loss through radiation and conduction. As a species with a high metabolic requirement, it needs to forage frequently to lay down sufficient fat reserves for insulation. A study of six, non-lactating, harbour porpoise found that they require between 4% and 9.5% of their body weight in fish per day (Kastelein et al., 1997). In the wild, porpoises forage almost continuously day and night to achieve their required calorific intake (Wisniewska et al., 2016). This means that they are vulnerable to starvation if their foraging is interrupted. Harbour porpoise were recorded year-round and frequently within the Proposed Development marine mammal study area and therefore could be vulnerable to piling at any time of year.
  2. The variance in behavioural responses to increased subsea noise is well documented and is context specific. Factors such as the activity state of the receiving animal, the nature and novelty of the sound (i.e. previous exposure history), and the spatial relation between sound source and receiving animal are important in determining the likelihood of a behavioural response and therefore their sensitivity (Ellison et al., 2012). Empirical evidence from monitoring at offshore wind farms during construction suggests that pile driving is unlikely to lead to 100% avoidance of all individuals exposed, and that there will be a proportional decrease in avoidance at greater distances from the pile driving source (Brandt et al., 2011). This was demonstrated at Horns Rev Offshore Wind Farm, where 100% avoidance occurred in harbour porpoises at up to 4.8 km from the piles, whilst at greater distances (10 km plus) the proportion of animals displaced reduced to < 50% (Brandt et al., 2011). A recent study on piling at the Beatrice Offshore Wind Farm suggests that harbour porpoise may adapt to increased noise disturbance over the course of the piling phase, thereby showing a degree of tolerance and behavioural adaptation (Graham et al., 2019). This study also demonstrated that the probability of occurrence of harbour porpoise (measured as porpoise positive minutes) increased exponentially moving further away from the noise source. Similarly, at a study of seven offshore wind farms constructed in the German Bight, Brandt et al., (2018) also showed that detections of harbour porpoise declined several hours before the start of pling within the vicinity (up to 2 km) of the construction site and were reduced for about one to two hours post-piling, whilst at the maximum effect distances (from 17 km out to approximately 33 km) avoidance only occurred during the hours of piling. In this study, porpoise detections during piling were found at sound levels exceeding 143 dB re 1 µPa2s and at lower received levels – at greater distances from the source – there was little evident decline in porpoise detections (Brandt et al., 2018). These studies demonstrate the dose-response relationship between received noise levels and declines in porpoise detections although noting that the extent to which responses could occur will be context-specific such that, particularly at lower received levels (i.e. 130 dB -140 dB re 1 µPa2s), detectable responses may not be apparent from region to region.
  3. A recent article by Southall et al. (2021) introduces a behavioural response severity spectrum, building on earlier work presented in Southall et al. (2007) and the expanding literature in this area. Southall et al. (2021) illustrates the progressive severity of possible responses within three response categories: survival (e.g. resting, navigation, defence), feeding (e.g. search, consumption, energetics), and reproduction (e.g. mating, parenting). For example, at the most severe end of the spectrum (scored 7 to 9), where sensitivity is highest, displacement could occur resulting in movement of animals to areas with an increased risk of predation and/or with sub-optimal feeding grounds. A failure of vocal mechanisms to compensate for noise and interruption of key reproductive behaviour including mating and socialising could occur. In these instances, there would likely be a reduction in an individual’s fitness leading to potential breeding failure and impact on survival rates.
  4. Acknowledging the limitations of the single step-threshold approach for strong disturbance and mild disturbance (i.e. does not account for inter-, or intra-specific variance or context-based variance), harbour porpoise within the area modelled as ‘strong disturbance’ would be most sensitive to behavioural effects and therefore may have a response score of seven or above according to Southall et al. (2021). At the lower end of the behavioural response spectrum, the potential severity of effects reduces. Whilst there may be some detectable responses that could result in effects on the short-term health of animals, these are less likely to impact on an animals’ survival rate. For example, mild disturbance (score four to six) could lead to effects such as changes in swimming speed and direction, minor disruptions in communication, interruptions in foraging, or disruption of parental attendance/nursing behaviour (Southall et al., 2021).
  5. Although harbour porpoise may be able to avoid the disturbed area and forage elsewhere, there may be a potential effect on reproductive success of some individuals. As mentioned previously, it is anticipated that there would be some adaptability to the elevated noise levels from piling and therefore survival rates are not likely to be affected. Due to uncertainties associated with the effects of behavioural disturbance on vital rates of harbour porpoise, the assessment is highly conservative as it assumes the same level of sensitivity for both strong and mild disturbance, noting that for the latter the sensitivity is likely to be lower.
  6. Harbour porpoise, an IEF of international value, is deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to disturbance is therefore considered to be medium.

Bottlenose dolphin and white-beaked dolphin

  1. Bottlenose dolphin and white-beaked dolphin are not thought to be as vulnerable to disturbance as harbour porpoise; with larger body sizes – and lower metabolic rates – the necessity to forage frequently is lower in comparison. White-beaked dolphin have a largely offshore distribution and their presence in the Proposed Development marine mammal study area is likely to be very seasonal. Weir et al. (2007) reported that white-beaked dolphins within the coastal North Sea area in Aberdeenshire were typically recorded only between June and August, with a peak in occurrence during August. Bottlenose dolphin is largely coastally distributed in relation to the Proposed Development marine mammal study area and are more abundant during spring and summer compared to autumn and winter months (Paxton et al., 2016). Offshore sightings during the recent DAS recorded sightings within the Proposed Development marine mammal study area during the months of October and April (see volume 3, appendix 10.2).
  2. There is limited information regarding the specific sensitivities of bottlenose dolphin and white-beaked dolphin to disturbance from piling noise as most studies have focussed on harbour porpoise. A study of the response of bottlenose dolphin to piling noise during harbour construction works at the Nigg Energy Park in the Cromarty Firth (north-east Scotland) found that there was a measurable (albeit weak) response to impact and vibration piling with animals reducing the amount of time they spent in the vicinity of the construction works (Graham et al., 2017). Another study investigating dolphin detections in the Moray Firth during impact piling at the Moray East and Beatrice Offshore Wind Farms found surprising results at small temporal scales with an increase in dolphin detections on the southern Moray coast on days with impulsive noise compared to days without (Fernadez-Betelu et al., 2021). Predicted maximum received levels in coastal areas were 128 dB re. 1 µPa2s and 141 dB re. 1 µPa2s during piling at Beatrice Offshore Wind farm Ltd (BOWL) and Moray Offshore Renewables Ltd (MORL) respectively (Fernadez-Betelu et al., 2021). The authors of this study warn that caution must be exercised in interpreting these results as increased click changes do not necessarily equate to larger groups sizes but may be due to a modification in behaviour (e.g. an increase in vocalisations during piling) (Fernadez-Betelu et al., 2021). The results of this study do, however, suggest that impulsive noise generated during piling at the offshore wind farms did not cause any displacement of bottlenose dolphins from their population range. Notably, the received levels during piling at MORL are higher than those predicted for the outer isopleths (130 dB and 135 dB re. 1 µPa2s) that overlap with the CES MU 2 m – 20 m depth contour during piling at the Proposed Development suggesting that disturbance at these lower noise levels is unlikely to lead to displacement effects.
  3. The Southall et al. (2021) severity spectrum applies across all marine mammals and therefore it is expected that, as described for harbour porpoise, strong disturbance in the near field could result in displacement whilst mild disturbance over greater ranges would result in other, less severe behavioural responses (see paragraph 215).
  4. White-beaked dolphin and bottlenose dolphin may be able to avoid the disturbed area and whilst there may some impacts on reproduction in closer proximity to the source (i.e. within the area of ‘strong disturbance’), these are unlikely to impact on survival rates as some tolerance is expected to build up over the course of the piling. It is anticipated that animals would return to previous activities once the impact had ceased.
  5. Bottlenose dolphin and white-beaked dolphin, IEFs of international value, are deemed to be of medium vulnerability and high recoverability. The sensitivity of both receptors to disturbance is therefore considered to be medium.

Minke whale

  1. Minke whale occurs seasonally within the Proposed Development marine mammal study area, moving into inshore waters during the summer months to exploit sandeel as a key prey resource (Robinson et al., 2009; Tetley et al., 2008). Minke whale is able to adopt a low energy feeding strategy by exploiting prey herded by other species, however, its reliance on sandeel as the primary energy resource (up to 70% of its diet in Scotland, Tetley et al, 2008) means that disturbance from areas that are important for sandeel could have implications on the health and survival of disturbed individuals. Sandeel habitat in the vicinity of the Proposed Development is described in volume 2, chapter 9. There are high intensity spawning grounds and low intensity nursery grounds for the lesser sandeel Ammondytes tobianus within the Proposed Development fish and shellfish ecology study area and Rait’s sandeel A. marinus is also present within the area. Therefore, displacement of minke whales could lead to reduced foraging for disturbed individuals particularly since minke whales maximise their energy storage whilst on feeding grounds (Christiansen et al., 2013a). Christiansen et al. (2013b) found that the presence of whale-watching boats within an important feeding ground for minke whale led to a reduction in foraging activity and as a capital breeder such a reduction could lead to reduced reproductive success since female body condition is known to affect foetal growth (Christiansen et al., 2014). However, it is worth noting that the study was conducted in Faxafloi Bay in Iceland where baseline noise levels (compared to the North Sea) are very low (McGarry et al., 2017). In addition, a subsequent study conducted by Christiansen and Lusseau (2015) in the same study area found no significant long-term effects of disturbance from whale watching on vital rates since whales moved into disturbed areas when sandeel numbers were lower across their wider foraging area.
  2. It is expected that for minke whale, as described by the Southall et al. (2021) strong disturbances in the nearfield could result in displacement whilst mild disturbance over larger ranges would result in other, less severe behavioural responses. In terms of context the Proposed Development is situated in region of relatively high levels of shipping, fishing and other vessel activity with up to 16 vessels on average per day recorded within a 10 nm buffer of the Proposed Development array area and 15 commercial shipping routes crossing the Proposed Development array area (volume 3, chapter 13). Therefore, minke whales that occur within the Proposed Development marine mammal study area are subject to underwater noise from existing activities and may to some extent be desensitised to increased noise levels, particularly in the far field where mild disturbance could occur. 
  3. Minke whale, an IEF of international value, is deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to disturbance is therefore considered to be medium.

Harbour seal and grey seal

  1. Strong disturbance could result in displacement of seals from an area. Whilst mild disturbance has the potential to disturb individuals, this constitutes only slight changes in behaviour, such as changes in swimming speed or direction, and is unlikely to result in population-level effects. Although there are likely to be alternative foraging sites for both harbour seal and grey seal, barrier effects as a result of installation of monopiles could either prevent seals from travelling to forage from haul-out sites or force seals (particularly harbour seal) to travel greater distances than is usual during periods of piling.
  2. A study of the movements of tagged harbour seals during piling at the Lincs Offshore Wind Farm in the Greater Wash showed significant avoidance of the wind farm by harbour seals (Russell et al., 2016). Within this study, seal abundance significantly reduced over a distance of up to 25 km from the piling activity and there was a 19% to 23% decrease in usage within this effect range. However, the displacement was limited to pile driving activity only, with seals returning rapidly to baseline levels of activity within two hours of cessation of the piling (Russell et al., 2016).
  3. Hastie et al. (2021) recently demonstrated that anthropogenic noise can influence foraging decisions in seals and such decisions were consistent with a risk/profit balancing approach. The study measured the relative influence of perceived risk of a sound (silence, pile driving, and a tidal turbine) and prey patch quality (low density versus high density), in grey seals in an experimental pool environment. Foraging success was highest under silence, but under tidal turbine and pile driving treatments success was similar at the high-density prey patch but significantly reduced under the low-density prey patch. Therefore, avoidance rates were dependent on the quality of the prey patch as well as the perceived risk from the anthropogenic noise.
  4. Recorded reactions of tracked grey seals to pile driving during construction of the Luchterduinen wind farm in 2014 and Gemini wind farm in 2015 have been diverse, and have ranged from altered surfacing and diving behaviour, changes in swimming direction, or coming to a halt (Aarts et al., 2018). In some cases, however, no apparent changes in diving behaviour or movement were observed (Aarts et al., 2018). Similar to the conclusions drawn by Hastie et al., (2021) the study at the Luchterduinen and Gemini wind farms suggested animals were balancing risk with profit. Whilst approximately half of the tracked seals were absent from the pile-driving area altogether, this may be because animals were drawn to other more profitable areas as opposed to active avoidance of the noise, although a small sample size (n=36 animals) means that no firm conclusions could be reached. It was notable that, in some cases, seals exposed to pile-driving at distances shorter than 30 km returned to the same area on subsequent trips. This suggests that the incentive to go to the area was stronger than potential deterrence effect of underwater noise from pile driving in some seals.
  5. Barrier effects and altered behaviour could affect the ability of phocid seals to accumulate the energy reserves prior to both reproduction and lactation (Sparling et al., 2006). Female seals exhibit clear patterns of increased foraging effort (including increased diving behaviour) towards the breeding season as a strategy to maximise energy allocation to reproduction. Especially during the third trimester of pregnancy, grey seals accumulate reserves of subcutaneous blubber which they use to synthesize milk during lactation (Hall et al., 2001). They may be most vulnerable to reduced foraging during this period, as maternal energy storage is extremely important to offspring survival and female fitness (Mellish et al., 1999; Hall et al., 2001). Therefore, potential exclusion from foraging grounds during this time has the potential to affect reproduction rates and probability of survival.
  6. Phocid seals may be vulnerable to disturbance during the lactation period also, although the extent to which this occurs depends on their breeding strategy. Changes in behaviour could have a particular impact on harbour seal – an income breeder – during lactating periods (June to August), when female harbour seals spend much of their time in the water with their pups, and foraging is more restricted than during other periods (Thompson and Härkönen, 2008). Consequences of disturbance may include reduced fecundity, reduced fitness, and reduced reproductive success. Although harbour seal may be able to avoid the disturbed area and forage elsewhere, there may be an energetic cost to having to move greater distances to find food, and therefore there may be a potential effect on reproductive success of some individuals. For grey seal – a capital breeder – the lactation period lasts around 17 days (Sparling et al., 2006) during which time the females remain mostly on shore, fasting. As grey seal females do not forage often during lactation, it is expected that they may exhibit some tolerance to disturbance and the effect is less likely to cause a change in both reproduction and survival rates during lactation compared to harbour seal. Note, however, that following lactation female grey seals return to the water and must forage extensively to build up lost energy reserves.
  7. Grey seal and harbour seals, IEFs of international value, are deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor is therefore considered to be medium.
Significance of the Effect

Auditory injury

Harbour porpoise

  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 and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect on harbour porpoise will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Bottlenose dolphin and white-beaked dolphin

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. Given that the potential risk of injury is reduced by appropriate designed-in measures, the effect on bottlenose dolphin and white-beaked dolphin will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Minke whale

  1. Overall, the magnitude of the impact is deemed to be medium and the sensitivity of the receptor is considered to be high. Although the risk will to some extent be reduced through appropriate designed-in measures there still remains a risk of injury (as the risk of injury may occur beyond the mitigatable zone for Marine Mammal Observer (MMOs) and PAM) and therefore the effect on minke whale will be of moderate adverse significance, which is significant in EIA terms. Secondary mitigation has been proposed to reduce the significance of this effect (see paragraph 244 et seq.).

Grey seal and harbour seal

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be high. Given that the potential risk of injury is reduced by appropriate designed-in measures, the effects on grey seal and harbour seal will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Behavioural disturbance

Harbour porpoise

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. Given that only small proportion of NS MU harbour porpoise population could be potentially disturbed at any one time and population modelling indicates that there is no potential for a long-term effect on this species, the effect on harbour porpoise will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Bottlenose dolphin

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. Given that only small proportion of CES MU bottlenose dolphin population could be potentially disturbed at any one time and population modelling indicates that there is no potential for a long-term effect on this species, the effect on bottlenose dolphin will, therefore, be of minor adverse significance, which is not significant in EIA terms

White-beaked dolphin

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. Given that only small proportion of CGNS MU white-beaked dolphin population could be potentially disturbed at any one time, the effect on white-beaked dolphin will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Minke whale

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. Given that only small proportion of CGNS MU minke whale population could be potentially disturbed at any one time and population modelling indicates that there is no potential for a long-term effect on this species, the effect on minke whale will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Harbour seal

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. Given that only small proportion of ES plus NE MU harbour seal population could be potentially disturbed at any one time and population modelling indicates that there is no potential for a long-term effect on this species, the effect on harbour seal will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Grey seal

  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. Given that only small proportion of ES plus NE MU harbour seal population could be potentially disturbed at any one time and population modelling indicates that there is no potential for a long-term effect on this species, the effect on grey seal will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary Mitigation and Residual Effect
  1. Given that potential injury impacts were predicted to be significant in EIA terms for minke whale, an IEF of international value, secondary mitigation will be applied in the form of an ADD to deter animals from the area of impact. This additional mitigation will also reduce any risk of injury (albeit very low risk) to individuals of other marine mammal species which may arise due to the inherent uncertainties in applying the standard measures (visual and acoustic approaches), for example, problems with detecting animals in high sea states or low visibility due to adverse weather conditions.
  2. ADDs have commonly been used in marine mammal mitigation at UK offshore wind farms to deter animals from potential injury zones prior to the start of piling. The JNCC (2010a) draft guidance for piling mitigation recommends their use, particularly in respect of periods of low visibility or at night to allow 24-hour working. With a number of research projects on ADDs commissioned via the Offshore Renewables Joint Industry Programme (ORJIP), the use of ADDs for mitigation at offshore wind farms has gained momentum. Indeed, for the Beatrice Offshore Wind Farm, the use of ADDs was accepted by the regulators (Marine Scotland) as the only mitigation tool applied pre-piling as it was thought to be more effective at reducing the potential for injury to marine mammals compared to standard measures (MMOs and PAM) which, as mentioned previously, has limitations with respect to effective detection over distance (Parsons et al., 2009; Wright and Cosentino, 2015).
  3. There are various ADDs available with different sound source characteristics (see McGarry et al., 2020) and a suitable device will be selected based on the key species requiring mitigation for the Proposed Development. The selected device will typically be deployed from the piling vessel and activated for a pre-determined duration to allow animals sufficient time to move away from the sound source whilst also minimising the additional noise introduced into the marine environment. The type of ADD and approach to mitigation (including activation time and procedure) will be included in the MMMP, being previously discussed and agreed with relevant stakeholders.
  4. Noise modelling was carried out to determine the potential efficacy of using this device to deter marine mammals from the injury zone (see volume 3, appendix 10.1). The results suggest that the use of an ADD for a duration of 30 minutes before the piling commences would further reduce the potential to experience injury to marine mammal receptors. For example, the maximum injury zones for species based on SPLpk metric for piling of the wind turbines and OSPs/Offshore convertor station platform foundations at a maximum hammer energy of 4,000 kJ using 1% constant conversion factor are shown in Table 10.36   Open ▸ . Assuming conservative swim speeds, it was demonstrated that activation of an ADD for 30 minutes would deter all animals beyond the maximum injury zone ( Table 10.36   Open ▸ ). This corroborates findings of other studies that reported that ADDs deter different marine mammals over several hundreds of metres or indeed several kilometres from the source (reviewed in McGarry et al., 2020).

 

Table 10.36:
Summary of Peak Pressure Injury Ranges for Marine Mammals due to Single Piling of Wind Turbine and OSPs/Offshore Convertor Station Platform at 4,000 kJ Hammer Energy Using 1% Constant Conversion Factor, Showing Whether the Individual Can Flee the Injury Range During the 30 Minutes of ADD Activation

Table 10.36: Summary of Peak Pressure Injury Ranges for Marine Mammals due to Single Piling of Wind Turbine and OSPs/Offshore Convertor Station Platform at 4,000 kJ Hammer Energy Using 1% Constant Conversion Factor, Showing Whether the Individual Can Flee the Injury Range During the 30 Minutes of ADD Activation

 

  1. The assessment found that the maximum injury zone for minke whale alone was based on SELcum metric for concurrent piling of the wind turbines foundations at a maximum hammer energy of 4,000 kJ using 4% reducing to 0.5% conversion factor. Modelling for SELcum scenario demonstrated that the use of ADD is useful for reducing PTS injury ranges, as the activation of an ADD 30 minutes prior to commencement of piling reduced PTS to a level not exceeding the injury thresholds ( Table 10.37   Open ▸ ). Thus, even assuming this very conservative range of effect (i.e. using the SELcum metric which may be an overestimate of PTS; paragraph 95), the noise modelling demonstrated that the risk of injury can be mitigated through use of an ADD.

 

Table 10.37:
Injury Ranges for Marine Mammals due to Concurrent Piling of Wind Turbine at 4,000 kJ Hammer Energy Using 4% Reducing to 0.5% Conversion Factor with and without 30 Minutes of ADD

Table 10.37: Injury Ranges for Marine Mammals due to Concurrent Piling of Wind Turbine at 4,000 kJ Hammer Energy Using 4% Reducing to 0.5% Conversion Factor with and without 30 Minutes of ADD

1 N/E = Threshold not exceeded

 

Residual Effect – Auditory Injury

  1. Overall, following application of secondary mitigation, the magnitude of the impact for all species is deemed to be low and the sensitivity of the receptors is considered to be high. Considering the significance matrix presented in Table 10.20   Open ▸ , the significance of the effect can be assessed as either minor or moderate. Given that the potential risk of injury for all species is reduced by secondary mitigation measures, the effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Residual Effect – Behavioural Disturbance

  1. Overall, the magnitude of the impact for all species is deemed to be low and the sensitivity of the receptors is considered to be medium. Given that only small proportion of each regional/national marine mammal population could be potentially disturbed, it is highly unlikely that this impact will alter the structure and functions of populations in question and population modelling suggest that there is no potential for a long-term effect on trajectories of assessed species (all species except white-beaked dolphin as iPCoD do not facilitate modelling for this species). The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Injury and Disturbance to Marine Mammals from Elevated Underwater Noise During Site Investigation Surveys

  1. Site investigation surveys during the construction phase as well as the operation and maintenance phase have the potential to cause direct or indirect effects (including injury or disturbance) on marine mammal IEFs. A detailed underwater noise modelling assessment has been carried out to investigate the potential for injurious and behavioural effects on marine mammals as a result of geophysical and geotechnical surveys, using the latest criteria (volume 3, appendix 10.1), which is drawn upon in the assessment presented in paragraph 252 et seq.

Summary of Noise Modelling

Geophysical Surveys
  1. It is understood that several sonar-based survey types will potentially be used for the geophysical surveys. That includes MBES, SSS, SBES and SBS. The equipment likely to be used can typically work at a range of signal frequencies, depending on the distance to the bottom and the required resolution. The signal is highly directional, acts like a beam and is emitted in pulses. Sonar-based sources are considered as continuous (non-impulsive) because they generally compromise a single (or multiple discrete) frequency as opposed to a broadband signal with high kurtosis, high peak pressures and rapid rise times. Unlike the sonar-based surveys, the UHRS is likely to utilise a sparker, which produces an impulsive, broadband source signal.
  2. A full description of the source noise levels for geophysical survey activities is provided in volume 3, appendix 10.1.
Geotechnical Surveys
  1. Source levels for borehole drilling ahead of standard penetration testing are in a range of 142 dB to 145 dB re 1 µPa re 1 m (rms). SEL measurements conducted during CPTs showed that it is characterised by broadband sound with levels measured generally 20 dB above the acoustic ocean noise floor (Erbe and McPherson, 2017). For the purpose of assessment of effects, these sources are considered as impulsive sounds. Measurements of a vibro-core test (Reiser et al., 2011) show underwater source SPLs of approximately 187 dB re 1 µPa re 1 m (rms). The vibro-core sound is considered to be continuous (non-impulsive).
  2. Full description of the source noise levels for geotechnical survey activities is provided in volume 3, appendix 10.1.

Construction Phase

Magnitude of Impact

Auditory injury

  1. Potential impacts of site investigation surveys will depend on the characteristic of the activity, frequency bands and water depth. The impact ranges presented in this section are rounded to the nearest 5 m. It should be noted that, for the sonar-based surveys, many of the injury ranges are limited to approximately 65 m as this is the approximate water depth in the area. Sonar based systems have very strong directivity which effectively means that there is only potential for injury when a marine mammal is directly underneath the sound source. Once the animal moves outside of the main beam, there is no potential for injury. This section provides estimated ranges for injury of marine mammals in the construction phase of the Proposed Development.
  2. The noise modelling assessment showed that ranges within which there is a potential to experience PTS by marine mammals as a result of geophysical investigation activities (based on comparison to Southall et al. (2019) SEL thresholds) are relatively low ( Table 10.38   Open ▸ ). For harbour porpoise PTS could occur out to 360 m during sub-bottom profiles surveys. However, impact ranges within which PTS could occur are smaller for other marine mammal species at maximum of 65 m.

 

Table 10.38:
PTS Impact Ranges for Marine Mammals During the Geophysical Site Investigation Surveys

Table 10.38: PTS Impact Ranges for Marine Mammals During the Geophysical Site Investigation Surveys

1 N/E = Threshold Not Exceeded

 

  1. With respect to the ranges within which there is a potential of PTS occurring to marine mammals as a result of geotechnical investigation activities, PTS threshold was not exceeded for almost all marine mammal species, except harbour porpoise and minke whale (
  2. Table 10.39   Open ▸ ). PTS is only expected to occur during cone penetration test, out to a maximum of 60 m and 5 m for harbour porpoise and minke whale, respectively.

 

Table 10.39:
PTS Impact Ranges for Marine Mammals During the Geotechnical Site Investigation Surveys

Table 10.39: PTS Impact Ranges for Marine Mammals During the Geotechnical Site Investigation Surveys

1 N/E = Threshold Not Exceeded

 

  1. The number of marine mammals potentially injured within the modelled ranges for PTS presented in Table 10.39   Open ▸ and Table 10.40   Open ▸ were estimated using the most up to date species-specific density estimates ( Table 10.13   Open ▸ ). Where ranges for density estimates have been applied (harbour porpoise, bottlenose dolphin, minke whale, grey seal and harbour seal), numbers of animals potentially injured have been based on the maximum density value as a precautionary approach. It should be noted that since sonar-based systems have strong directivity, there is only potential for injury when marine mammal is directly underneath the sound source.
  2. Due to low impact ranges, for all marine species, there is the potential for less than one animal to experience PTS (and no animals where the threshold is not exceeded) as a result of geophysical and geotechnical site investigation surveys. The site-investigation surveys are considered to be short term as they will take place over up to a period of up to three months. Standard designed in measures to reduce the risk of injury to marine mammals will be implemented for the geophysical surveys (JNCC, 2017). With such measures in place the risk is deemed to be negligible.
  3. Site investigation surveys will also involve the use of up to two geophysical/geotechnical survey vessels with up to 70 round trips. Noise impacts associated with vessel movements are identified in paragraph 385 et seq. as well as paragraph 404 et seq.
  4. The impact of site investigation surveys leading to PTS is predicted to be of very local spatial extent, short-term duration, intermittent and whilst the impact will occur during piling only, the effect of PTS will irreversible. It is predicted that the impact will affect the receptor directly. With designed-in measures in place, involving visual and/or acoustic monitoring, the risk is likely to be negligible, however, given the potential permanence of the effect (PTS) if it did occur, the magnitude is, conservatively, considered to be low.

Behavioural disturbance

  1. The estimated maximum ranges for onset of disturbance are based on exceeding the 120 dB re 1 μPa (rms) threshold applicable for all marine mammals, noting that this threshold is for ‘mild disturbance’ and therefore is not likely to result in displacement of animals. The disturbance ranges as a result of geophysical and geotechnical site-investigation surveys ( Table 10.40   Open ▸ ) will be higher than those presented for PTS. Most of the predicted ranges are within hundreds of meters, however the largest distance over which the disturbance could occur is out to approximately 7.5 km during vibro-coring.

 

Table 10.40:
Disturbance Ranges for Marine Mammals During the Geophysical and Geotechnical Site Investigation Surveys

Table 10.40: Disturbance Ranges for Marine Mammals During the Geophysical and Geotechnical Site Investigation Surveys

 

  1. The number of marine mammals potentially disturbed within the modelled ranges for behavioural response are estimated using the most up to date species specific density estimates ( Table 10.13   Open ▸ ) and presented in Table 10.41   Open ▸ . Where ranges for density estimates have been applied (harbour porpoise, minke whale, grey seal and harbour seal), numbers of animals affected have been based on the maximum density value as a precautionary approach. The number of bottlenose dolphins potentially disturbed has been assessed based on the density for offshore populations.

 

Table 10.41:
Number of Animals Potentially Likely to be Disturbed due to the Geophysical and Geotechnical Site Investigation Surveys

Table 10.41: Number of Animals Potentially Likely to be Disturbed due to the Geophysical and Geotechnical Site Investigation Surveys

 

  1. The data presented in Table 10.41   Open ▸ is considered to be conservative, especially for harbour porpoise as the number of animals likely to be disturbed is based on the peak seasonal density estimates from the Proposed Development aerial digital survey data during spring months. If these numbers were compared with estimates of the number of harbour porpoise potentially disturbed using the mean monthly density derived from the Proposed Development aerial digital survey data (0.299 animals per km2) or using the modelled density estimate for SCANS III for this area (0.599 animals per km2) these estimates would be shown to be highly precautionary. For example, based on the mean monthly density from aerial data or SCANS III data, the number of harbour porpoise affected by possible disturbance during vibro-core testing, would be 52 animals (0.02% of the NS MU) or 105 animals (0.03% of the NS MU), respectively, compared to 144 animals (0.04% of the NS MU) estimated for peak seasonal density estimates.
  2. The same applies to grey seal, where the numbers of potentially disturbed animals presented in Table 10.41   Open ▸ (based on Carter et al., 2020) were shown to be precautionary compared with estimates of the number of grey seal using the mean monthly or seasonal peak densities derived from the Proposed Development aerial digital survey data (0.276 animals per km2 and 0.321 animals per km2). For example, based on the mean monthly and seasonal peak density from aerial data, the number of grey seal affected by possible disturbance during vibro-core testing, would be 48 animals (0.11% of the ES and NE Mus) and 56 animals (0.13% of the ES and NE Mus), respectively, compared to 210 animals (0.49% of the relevant Mus) estimated by Carter et al. (2020) for mean at sea usage.
  3. The number of bottlenose dolphins that could be exposed to potential disturbance ( Table 10.41   Open ▸ ) relate to their offshore populations and accounts for 0.27% of the SCANS III Block R estimated abundance. Given that the vibro-core sampling locations are currently unknown and coastal distribution of bottlenose dolphin is spatially limited, any quantitative assessment of the disturbance to coastal populations would be an overestimation. All geotechnical and geophysical surveys will be very short duration (up to three months) and animals are expected to recover quickly after cessation of the survey activities. The magnitude of the impact could result in a negligible alteration to the distribution of marine mammals. In addition, the proportion of the MU populations affected at any one time by disturbance is likely to be very small.
  4. The impact of site investigation surveys leading to behavioural effects is predicted to be of local spatial extent, medium term duration, intermittent and the effect of behavioural disturbance is of high reversibility (with animals returning to baseline levels soon after surveys have ceased). 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 and behavioural disturbance

  1. There is no direct evidence for a causal link between geophysical survey noise and physical injury or disturbance to marine mammals, but there is some evidence for short-term behavioural responses.

Auditory injury

  1. For geotechnical surveys, injury to marine mammals is unlikely to occur beyond a few tens of metres (i.e. up to 60 m for harbour porpoise) and noise from vessels themselves is likely to deter marine mammals beyond this range. The maximum range for PTS from geophysical surveys (SBP) is 360 m. Sills et al. (2020) evaluated TTS onset levels for impulsive noise in seals following exposure to underwater noise from a seismic air gun and found transient shifts in hearing thresholds at 400 Hz were apparent following exposure to four to ten consecutive pulses (SELcum 191 dB – 195 dB re 1 µPa2s; 167 dB – 171 dB re 1 µPa2s with frequency weighting for phocid carnivores in water).
  2. Marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and low recoverability. The sensitivity of the receptor to PTS from elevated underwater noise during site investigation surveys is therefore, considered to be high.

Behavioural disturbance

  1. The transmission frequencies of many commercial sonar systems (approximately 12 kHz – 1800 kHz) overlap with the hearing and vocal ranges of many species (Richardson et al., 1995), and whilst many are high frequency sonar systems with peak frequencies well above marine mammal hearing ranges, it is possible that relatively high levels of sound are also produced as sidebands at lower frequencies (Hayes and Gough, 1992) so may elicit behavioural responses in marine mammals. Fine-scale data from porpoises equipped with high-resolution location and dive loggers when exposed to airgun pulses at ranges of 420 m – 690 m with noise level estimates of 135 dB–147 dB re 1 µPa2s (SEL) show different responses to noise exposure (van Beest, et al., 2018). One individual displayed rapid and directed movements away from the exposure site whilst two individuals used shorter and shallower dives (compared to natural behaviour) immediately after exposure. This noise-induced movement typically lasted for eight hours or less, with an additional 24-hour recovery period until natural behaviour was resumed.
  2. Results from 201 seismic surveys in the UK and adjacent waters demonstrated that cetaceans (including bottlenose dolphin, white-beaked dolphin and minke whale) can be disturbed by seismic exploration (Stone and Tasker, 2006), with small odontocetes showing strongest lateral spatial avoidance, moving out of the area, whilst mysticetes and killer whales showed more localised spatial avoidance, orienting away from the vessel and increasing distance from source but not leaving the area completely.
  3. A study by Sarnocińska et al. (2020) indicated temporary displacement or change in harbour porpoise echolocation behaviour in response to a 3D seismic survey in the North Sea. No general displacement was detected from 15 km away from any seismic activity but decreases in echolocation signals were detected up to 8 km – 12 km from the active airguns. Taking into account findings of other studies (Dyndo et al., 2015; Tougaard et al., 2015) harbour porpoise disturbance ranges due to airgun noise are predicted to be smaller than to pile driving noise at the same energy. The reason for this is because the perceived loudness of the airgun pulses is predicted to be lower than for pile driving noise due to less energy at the higher frequencies where porpoise hearing is better (Sarnocinska et al., 2020). Similarly, Thompson et al. (2013) used PAM and digital aerial surveys to study changes in the occurrence of harbour porpoises across a 2,000 km2 study area during a commercial two-dimensional seismic survey in the North Sea and found acoustic detections decreased significantly during the survey period in the impact area compared with a control area, but this effect was small in relation to natural variation. Animals were typically detected again at affected sites within a few hours, and the level of response declined through the ten-day survey suggesting exposure led to some tolerance of the activity (Thompson et al., 2013). This study suggested that prolonged seismic survey noise did not lead to broader-scale displacement into suboptimal or higher-risk habitat. Likewise, a ten month study of overt responses to seismic exploration in humpback whales Megaptera novaeangliae, sperm whales Physeter macrocephalus and Atlantic spotted dolphins Stenella frontalis, demonstrated no evidence of prolonged or large-scale displacement of each species from the region during the survey (Weir, 2008).
  4. Hastie et al. (2014) carried out behavioural response tests to two sonar systems (200 kHz and 375 kHz systems) on grey seals at SMRU seal holding facility. Results showed that both systems had significant effects on the seals’ behaviour. Seals spent significantly more time hauled out during the 200 kHz sonar operation and although seals remained swimming during operation of the 375 kHz sonar, they were distributed further from the sonar.
  5. It is expected that, to some extent, marine mammals will be able to adapt their behaviour to reduce impacts on survival and reproduction rates and tolerate elevated levels of underwater noise during site investigation surveys. Marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to PTS and disturbance from elevated underwater noise during site investigation surveys is therefore considered to be medium.
Significance of the Effect
  1. Overall, the magnitude of the impact of PTS 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 and the scale of effect (injury radius and number of animals affected) was predicted to be very small. The effect on marine mammals will, therefore, be of minor adverse significance, which is not significant in EIA terms.
  2. Overall, the magnitude of the impact of disturbance 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.