Behavioural disturbance
  1. It has been demonstrated 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 (Castellote et al., 2010, Castellote et al., 2012, Goold, 1996, Weller et al., 2002). Elevated underwater 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 220 et seq.). A summary of known behavioural sensitivities of key IEFs to underwater noise from piling at other wind farm sites is provided in paragraph 29 et seq., noting that the conclusions drawn are subject to the limitations of extrapolating results from one project to another.

Harbour porpoise

  1. As a small cetacean species, harbour porpoise are vulnerable to heat loss through radiation and conduction. They have a high metabolic requirement, with a need to forage frequently to lay down sufficient fat reserves for insulation. Kastelein et al. (1997) found in a study of six, non-lactating, harbour porpoise that they require between 4% and 9.5% of their body weight in fish per day. In the wild, porpoises forage almost continuously day and night to achieve their required calorific intake (Wisniewska et al., 2016), meaning they are vulnerable to starvation if foraging is interrupted. Harbour porpoise were recorded year-round (in 21 out of 24 survey months) in the Array marine mammal study area and therefore could be vulnerable to piling at any time of year (volume 3, appendix 10.2, annex A).
  2. It is well documented that there is variance in behavioural responses to increased underwater noise and it 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 piling 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 piling source (Brandt et al., 2011). (Brandt et al., 2011). (Graham et al., 2019) demonstrated this dose-response 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%). More recently Graham et al. (2019) studied responses of harbour porpoise to piling at the Beatrice Offshore Wind Farm, and suggested 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) 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, (Brandt et al., 2018) at a study of seven offshore wind farms constructed in the German Bight (Brandt et al., 2018) it has been shown that detections of harbour porpoise declined several hours before the start of piling within the vicinity (up to 2 km) of the construction site and were reduced for about one to two hours post-piling. At the maximum effect distances (from 17 km out to approximately 33 km) avoidance only occurred during the hours of piling. Brandt et al. (2018) found harbour porpoise detections during piling were found at sound levels exceeding 143 dB re 1 µPa2s and at lower received levels (i.e. at greater distances from the source) there was little evident decline in porpoise detections. 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 re 1 µPa2s to 140 dB re 1 µPa2s), detectable responses may not be apparent from region to region.
  3. Building on earlier work presented in Southall et al. (2007) and the mounting literature in this area, Southall et al. (2021) introduced a concept of behavioural response severity spectrum with 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 point of the spectrum rated seven to nine (where sensitivity is highest) displacement is likely to 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 sound can result in interruption of key reproductive behaviour including mating and socialising, causing a reduction in an individual’s fitness leading to potential breeding failure and impact on survival rates.
  4. There are limitations of the single step-threshold approach for strong disturbance and mild disturbance as it does not account for inter-, or intraspecific variance or context-based variance. However, according to Southall et al. (2021), 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. 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). Therefore, at the lower end of the behavioural response spectrum, the potential severity of effects is reduced and 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 the survival rate of the animal.
  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 in paragraph 235, it is anticipated that there would be some adaptability to the elevated sound levels from piling and therefore survival rates are not likely to be affected. The assessment is highly conservative due to uncertainties associated with the effects of behavioural disturbance on vital rates of harbour porpoise, 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. Therefore, harbour porpoise is 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.

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 Array 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 Array marine mammal study area and are more abundant during spring and summer compared to autumn and winter months (Paxton et al., 2016). No bottlenose dolphin were observed in the Array marine mammal study area during any of the aerial surveys over 24 months, but white-beaked dolphin accounted for the second highest number of sightings and was recorded in seven months over the 24-month survey period (see volume 3, appendix 10.2).
  2. Limited information is available regarding the specific sensitivities of bottlenose dolphin and white-beaked dolphin to disturbance from piling noise as most studies have concentrated 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 Firth coast on days with impulsive noise compared to days without (Fernandez-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 and Moray Offshore Renewables Ltd (MORL), respectively (Fernandez-Betelu et al., 2021). The authors of this study warn caution these results as increased click changes do not necessarily equate to larger groups sizes but may result from modification in behaviour (e.g. an increase in vocalisations during piling). The results, however, do 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 re 1 µPa2s and 135 dB re 1 µPa2s) that overlap with the CES2 MU during piling at the Array, suggesting that disturbance at these lower noise levels is unlikely to lead to displacement effects.
  3. The Southall (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 237).
  4. White-beaked dolphin and bottlenose dolphin may be able to avoid the disturbed area and whilst there may be 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. Therefore, bottlenose dolphin and white-beaked dolphin 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.

Minke whale and humpback whale

  1. Minke whale occurs seasonally within the Array marine mammal study area, moving into inshore waters during the summer months with peak numbers from July to September, depending on the region (Evans et al., 2003), 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. Volume 2, chapter 9 details the sandeel habitat in the vicinity of the Array. There are low intensity spawning grounds and high intensity nursery grounds for the lesser sandeel Ammondytes tobianus within the Array site boundary fish and shellfish ecology study area. Modelling from Langton et al. (2021) demonstrated the whole site boundary has extremely low probability of sandeel presence, with areas where predicted density is high closer to the coasts or towards the Firth of Forth. 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., 2013b). Christiansen et al. (2013a) 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 in the same study area (Christiansen and Lusseau, 2015) 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 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 context, the Array is situated in a region of relatively high levels of shipping, fishing and other vessel activity with up to nine vessels on average per day in the winter survey period and 11 vessels per day within the summer survey period recorded within the shipping and navigation study area (volume 2, chapter 13). Therefore, minke whale that occur within the Array 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 is 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.

Grey seal

  1. Mild disturbance has the potential to disturb seals, however 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 piling could either prevent seals from travelling to forage from haul-out sites or force seals to travel greater distances than is usual during periods of piling. Strong disturbance could result in displacement of seals from an area.
  2. Hastie et al. (2021) 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 seal in an experimental pool environment. The study found 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 sound and therefore it can be anticipated such decisions are consistent with a risk/profit balancing approach.
  3. Seal behaviour during offshore wind farm installation has been studied based on empirical data (Russell et al., 2016). Movements of tagged harbour seal during piling at the Lincs Offshore Wind Farm in the Greater Wash showed significant avoidance of the offshore wind farm by harbour seal (Russell et al., 2016). Within this study, seal abundance significantly reduced from the piling activity over a distance of up to 25 km and there was a 19% to 23% decrease in usage within this range. Nevertheless, displacement was limited to pile driving activity only, and harbour seal returned rapidly to baseline levels of activity within two hours of cessation of the piling (Russell et al., 2016). Diverse reactions of tracked grey seal to pile driving during construction of the Luchterduinen and Gemini wind farms was reported in (Aarts et al., 2018). Reactions ranged from altered surfacing and diving behaviour, changes in swimming direction, or coming to a halt. 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 indicated animals were balancing risk with profit. Approximately half of the tracked grey seal were absent from the pile driving area altogether, but this may be because animals were drawn to other more profitable areas as opposed to active avoidance of the sound, although a small sample size (n = 36 animals) means that no firm conclusions could be reached. It was notable that, in some cases, grey seal exposed to pile driving at distances shorter than 30 km returned to the same area on subsequent trips suggesting that the incentive to go to the area was stronger than potential deterrence effect of underwater sound from pile driving in some animals.
  4. Changes in behaviour and subsequent barrier effects have the potential to affect the ability of pinnipeds to accumulate the energy reserves prior to both reproduction and lactation (Sparling et al., 2006). Female seals increase their foraging effort (including increased diving behaviour) before the breeding season, maximising energy allocation to reproduction. Especially during the third trimester of pregnancy, grey seal accumulate reserves of subcutaneous blubber which they use to synthesise milk during lactation (Hall and Thompson, 2009). Therefore, grey seal foraging at-sea may be most vulnerable in this period, as maternal energy storage is extremely important to offspring survival and female fitness (Ailsa J et al., 2001, Mellish et al., 1999). Potential exclusion from foraging grounds during this time could affect reproduction rates and probability of survival.
  5. Pinnipeds may also be vulnerable to disturbance during the lactation period, depending on the breeding strategy of particular species. The lactation period for grey seal lasts around 17 days (Sparling et al., 2006) with females remaining mostly on shore, fasting. Furthermore, as grey seal females do not forage often during lactation, it is expected that they may exhibit some tolerance to disturbance as they would not spend as much time at-sea, where they can be affected by underwater sound. Following lactation however female grey seal return to the water and must forage extensively to build up lost energy reserves. Consequences of disturbance may include reduced fecundity, reduced fitness, and reduced reproductive success. Although grey 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.
  6. Grey seal is deemed to have some resilience to behavioural disturbance, high recoverability and international value. The sensitivity of the receptor to behavioural disturbance is therefore, considered to be medium.
Significance of the effect
Auditory injury (PTS)

Harbour porpoise

  1. Overall, the magnitude of the impact is deemed to be negligible 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.

Bottlenose dolphin and white-beaked dolphin

  1. Overall, the magnitude of the impact is deemed to be negligible 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.

Minke whale and humpback whale

  1. Overall, the magnitude of the impact is deemed to be negligible 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.

Grey seal

  1. Overall, the magnitude of the impact is deemed to be negligible 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

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. The effect 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 medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.

Minke whale (and humpback whale)

  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.

Grey seal

  1. Overall, the magnitude of the impact is deemed to be negligible and the sensitivity of the receptor is considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
Auditory injury (PTS)
  1. No marine mammal mitigation is considered necessary (beyond measures adopted) because the likely effect in the absence of mitigation is not significant in EIA terms.
Behavioural disturbance
  1. No marine mammal mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Injury and disturbance from underwater noise generated during Unexploded Ordnance (UXO) clearance

  1. Clearance of UXOs before construction begins could lead to effects from high order detonation of UXO. This action has the capacity to produce some of the most elevated peak sound pressures among all human-made underwater sound sources and is recognised as a high-energy, impulsive sound source (von Benda-Beckmann et al., 2015). The effects of this impact will vary based on the characteristics of the sound source, the species affected, proximity to the sound source and the degree of sound attenuation within the surrounding environment.
  2. Further detail on underwater noise modelling of UXO clearance is provided in volume 3, appendix 10.1. In the case of high order detonation, acoustic modelling was conducted following the approach outlined in (Soloway and Dahl, 2014). The estimates are conservative, assuming the charge is freely positioned in mid-water, unlike a UXO resting on the seabed, which could experience burial, degradation, or significant attenuation. Additionally, the explosive material is likely to have deteriorated over time, making maximum sound levels probable overestimations of actual sound levels. Frequency-dependent weighting functions were applied to facilitate comparison with marine mammal hearing weighted thresholds.
  3. As per Robinson et al. (2020), low order deflagration yields a considerably lower amplitude of peak sound pressure compared to high order detonations. Therefore, underwater noise modelling has been based on the methodology outlined in paragraph 265 but with a smaller donor charge size (as described in volume 3, appendix 10.1).

                        Construction phase

Magnitude of impact
  1. Potential impacts of underwater noise resulting from UXO clearance on marine mammals could include mortality, physical injury or auditory injury. The duration of impact (elevated noise) for each UXO detonation is very short (seconds) therefore behavioural effects are considered to be negligible in this context. As such, TTS represents a temporary auditory injury but can be also considered as a threshold for strong behavioural disturbance (for the onset of a moving away response) (see paragraph 285). A detailed underwater noise modelling assessment was carried out to investigate the potential PTS and TTS to occur, using the latest assessment criteria (volume 3, appendix 10.1). A project-specific outline MMMP (volume 4, appendix 22) will be developed to mitigate the potential for injury.
  2. It is anticipated that up to 15 UXOs within the site boundary may require clearance. The maximum UXO size is assumed to be 698 kg NEQ and the most realistic maximum size is 227 kg NEQ ( Table 10.17   Open ▸ ). A low order clearance donor charge of 0.25 kg NEQ is assumed for each clearance event and up to 0.5 kg NEQ clearance shot may be required for neutralisation of residual explosive material at each location. The clearance activities will be tide and weather dependent. The aim is to enable clearance of at least one UXO per tide, during the hours of daylight and good visibility.
  3. Whilst the clearance of UXO can result in the high order detonation, in line with the UK Government et al. (2022) joint interim position statement, the Applicant commits to prioritise low order clearance techniques ( Table 10.17   Open ▸ ). To ensure a precautionary approach, the assessment of significance for auditory injury (PTS, paragraph 270 et seq.) and strong behavioural disturbance (using TTS onset as a proxy, paragraph 285 et seq.) is based on the high order clearance of maximum UXO (698 kg NEQ), however noting that the realistic maximum case NEQ of 227 kg is considered the more likely scenario.
Auditory injury (PTS)
  1. It is considered that there is a small risk that a low order clearance could result in high order detonation of UXO and therefore the assessment considered both high order and low order techniques. With regard to UXO detonation (low order techniques as well as high order events), due to a combination of physical properties of high frequency energy, the sound is unlikely to still be impulsive in character once it has propagated more than a few kilometres (volume 3, appendix 10.1 for more details). The precise range at which this transition occurs is unknown, however the NMFS (2018) guidance suggests an estimate of 3 km for transition from impulsive to continuous. Hastie et al. (2019) suggest that some measures of impulsiveness change markedly within approximately 10 km of the source (for seismic surveys and piling). as such, caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres as the PTS ranges are likely to be significantly lower than those predicted.
  2. PTS ranges for low order clearance donor charge and clearance shot are presented in Table 10.33   Open ▸ and high order clearance of UXO presented in Table 10.34   Open ▸ . The number of animals predicted to experience PTS due to low order clearance donor charge and clearance shot is presented in Table 10.35   Open ▸ and high order clearance in Table 10.36   Open ▸ .
  3. A high order clearance of 698 kg NEQ yielded the largest PTS ranges for all species, with the greatest injury range (14,540 m) seen for harbour porpoise (SPLpk) ( Table 10.34   Open ▸ ). The PTS range as a result of the high order detonation of the realistic maximum case (227 kg NEQ) is reduced to 10,000 m for harbour porpoise (SPLpk). Conservatively, the number of harbour porpoise that could be potentially injured, based on the site-specific seasonal peak density of 0.651 animals per km2, was estimated as 433 animals for 698 kg NEQ UXO high order explosion (SPLpk) equating to 0.12% of the North Sea MU ( Table 10.36   Open ▸ ). Predicted numbers are smaller for the realistic maximum case UXO (227 kg NEQ) with up to 205 animals potentially experiencing PTS (SPLpk) equating to 0.06 % of the North Sea MU Table 10.36   Open ▸ ). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the PTS ranges of 1,050 m and 1,320 m were predicted ( Table 10.33   Open ▸ ), which could injure up to three and four harbour porpoises, respectively ( Table 10.35   Open ▸ ).
  4. The underwater noise assessment found that the maximum injury (PTS) range estimated for bottlenose dolphin and white-beaked dolphin using the SPLpk metric is 840 m for the high order detonation of 698 kg NEQ, but this is reduced to 577 m for the realistic maximum case (227 kg NEQ) ( Table 10.34   Open ▸ ). Given relatively low densities of both species within the Array marine mammal study area, the high order detonation of 698 kg and 227 kg could result in injury for no more than one individual ( Table 10.36   Open ▸ ). With reference to the wider populations of these species, this equated to small proportions of the relevant MUs (less than 0.01%). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the injury ranges were considerably lower with a maximum of 61 m and 77 m respectively ( Table 10.33   Open ▸ ), and there would be no more than one animal potentially injured within these ranges ( Table 10.35   Open ▸ ).
  5. For minke whale, the underwater noise assessment found that the maximum injury (PTS) range estimated is 3,925 m (using the SELcum metric) for the high order detonation of 698 kg NEQ, but this is reduced to 2,305 m for 227 kg NEQ ( Table 10.34   Open ▸ ). Using the SPLpk metric, the maximum PTS range estimated is 2,575 m for the high order detonation of 698 kg NEQ, but this is reduced to 1,770 m for 227 kg NEQ ( Table 10.34   Open ▸ ). The number of individuals that could be potentially injured was estimated at up to two animals for 698 kg NEQ using the SELcum (and less than one using the SELpk metric), which equates to 0.01% of the CGNS MU, and less than one animal for 227 kg NEQ UXO ( Table 10.36   Open ▸ ). For low order techniques, the maximum range predicted was up to 234 m (SPLpk) (0.25 kg NEQ) ( Table 10.33   Open ▸ ) and there would be no more than one animal potentially injured within this range ( Table 10.35   Open ▸ ).
  6. The maximum injury (PTS) range estimated for grey seal was 2,850 m using the SPLpk metric, for the high order detonation of 698 kg NEQ, but this was reduced to 1,960 m for 227 kg NEQ ( Table 10.34   Open ▸ ). The number of individuals that could be potentially injured, based on average densities within the Array marine mammal study area from Carter et al. (2022), was estimated at up to five animals for 698 kg NEQ ( Table 10.36   Open ▸ ), which equates to 0.01% of the East Scotland plus North-east England seal MUs, and up to three animals for the realistic maximum scenario (227 kg NEQ). For low order clearance donor charge (0.25 kg NEQ) and clearance shot (0.5 kg NEQ), the injury ranges were considerably lower with a maximum of 50 m and 259 m (SPLpk), respectively ( Table 10.33   Open ▸ ) and there would be no more than one animal potentially injured within these ranges ( Table 10.35   Open ▸ ).
  7. The auditory injury (PTS) ranges do 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 10.33:
Maximum Potential PTS Ranges For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded). Bold Number Represents the Maximum Potential PTS Range For All Species

Table 10.33: Maximum Potential PTS Ranges For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded). Bold Number Represents the Maximum Potential PTS Range For All Species

 

Table 10.34:
Maximum Potential PTS Ranges for High Order Detonation of Maximum and Realistic Maximum Case. Bold Number Represents the Maximum Potential PTS Range For All Species

Table 10.34: Maximum Potential PTS Ranges for High Order Detonation of Maximum and Realistic Maximum Case. Bold Number Represents the Maximum Potential PTS Range For All Species

 

Table 10.35:
Maximum Potential Number of Animals With the Potential to Experience PTS Due to Low Order Clearance Donor Charge and Clearance Shot (N/A = Not Applicable As the Threshold Was Not Exceeded)

Table 10.35: Maximum Potential Number of Animals With the Potential to Experience PTS Due to Low Order Clearance Donor Charge and Clearance Shot (N/A = Not Applicable As the Threshold Was Not Exceeded)


Table 10.36:
Maximum Potential Number of Animals With the Potential to Experience PTS Due to High Order Detonation of Maximum and Realistic Maximum Case (Prior to Any Mitigation)

Table 10.36: Maximum Potential Number of Animals With the Potential to Experience PTS Due to High Order Detonation of Maximum and Realistic Maximum Case (Prior to Any Mitigation)

 

  1. With primary mitigation (i.e. using low order techniques, Table 10.22   Open ▸ ) in place the assessment found that there would be a risk of injury over a range of 1,050 m for harbour porpoise using the SPLpk metric ( Table 10.33   Open ▸ ) for a 0.25 kg NEQ. The injury range for clearance shot of 0.5 kg NEQ was predicted across a range of 1,320 m ( Table 10.33   Open ▸ ).
  2. However, if low order clearance is not feasible or accidentally results in high order detonation, there is a maximum risk of injury (predicted for harbour porpoise) out to 14,540 m during detonation of 698 kg NEQ and 10,000 km for a 227 kg NEQ. Therefore, in line with standard industry practice (JNCC, 2010b), mitigation will be applied as a part of the outline MMMP (volume 4, appendix 22) ( Table 10.22   Open ▸ ). In line with stakeholder advice provided in response to Marine Mammal Consultation Note 2 (see Table 10.10   Open ▸ ; volume 3, appendix 5.1, annex E) the assessment of significance with respect to PTS from UXO clearance will be based on both SPLpk and SELcum, and assumes designed in measures (30 minute ADD and soft-start) ( Table 10.22   Open ▸ ).
  3. The maximum injury ranges presented in Table 10.33   Open ▸ and Table 10.34   Open ▸ are larger than the standard 1,000 m mitigation zone recommended for UXO clearance (JNCC, 2010b). The mitigation zone cannot be excessively large (e.g. a few km) as there may be difficulties in detecting marine mammals (particularly harbour porpoise) over large ranges (McGarry et al., 2017) with a significant decline in visual detection rate with increasing sea state (Embling et al., 2010, Leaper et al., 2015).
  4. Mitigation set out in the outline MMMP will therefore include the use of ADDs (up to 30 minutes) and soft start (very small scare charges) to deter animals from the injury zone ( Table 10.22   Open ▸ ). The efficacy of such deterrence will depend upon the device selected and reported ranges of effective deterrence vary. The reported effective deterrence range for harbour porpoise vary from 2.5 km out to 12 km (Brandt et al., 2013, Dähne et al., 2017, Kyhn et al., 2015, Olesiuk et al., 2002). A full review of available devices is provided in McGarry et al. (2022). In addition to the ADD use, deterrence can also be achieved through the use of soft start charges (JNCC, 2010b). Details of these and other appropriate mitigation are discussed in the outline MMMP (volume 4, appendix 22) and will be discussed with consultees post-consent when further details of the size and type of potential UXOs are understood.
  5. Designed in measures include up to 30 minutes of ADD. Table 10.37   Open ▸ presents indicative displacement distances per species, based upon conservative swim speeds presented in Table 10.24   Open ▸ . With 30 minutes of ADD, all species except for harbour porpoise will be deterred beyond the maximum injury zone (using the maximum injury range from either the SPLpk or SELcum metric). With the inclusion of 20 minute of soft start, in addition to 30 minutes of ADD, all species except for harbour porpoise be deterred beyond the maximum injury zone.
  6. For harbour porpoise, to illustrate what this may entail for high order clearance of the realistic maximum case (227 kg NEQ), based on a swim speed of 1.5 m/s ( Table 10.24   Open ▸ ), a total of 112 minutes of deterrence activities would be required to allow animals to flee the injury range. This potential further mitigation is discussed in paragraph 318.

 

Table 10.37:
Indicative Displacement Distances based upon Designed in ADD (30 minutes) for Marine Mammal Receptors, based upon Conservative Swim Speeds

Table 10.37: Indicative Displacement Distances based upon Designed in ADD (30 minutes) for Marine Mammal Receptors, based upon Conservative Swim Speeds

 

  1. The impact is predicted to be of local (for all species except harbour porpoise) to regional (harbour porpoise) spatial extent in the context of the relevant geographic frame of reference, very short term duration, intermittent and the effect of injury is permanent. It is predicted that the impact will affect the receptor directly. With designed in mitigation applied it is anticipated that all species except harbour porpoise would be deterred from the injury zone and therefore the likelihood of PTS and population-level effects would be unlikely. The magnitude is therefore considered to be low for bottlenose dolphin, white-beaked dolphin, minke whale, humpback whale and grey seal.
  2. For harbour porpoise the ranges of effect are large for high order clearance, and it is likely that following designed-in mitigation measures there will be a residual risk of PTS to a number of individuals ( Table 10.36   Open ▸ ). Therefore conservatively, the magnitude is considered to be medium.
Behavioural disturbance (TTS as a proxy)
  1. As discussed in paragraph 267, the duration of effect for each UXO detonation is less than one second and therefore behavioural effects are considered to be negligible in this context. The assessment for behavioural disturbance uses the onset of TTS as a proxy. Although the effect would be a potential temporary loss in hearing and some ecological functions would be inhibited in the short term due to TTS, these are reversible on recovery of the animal’s hearing and therefore not considered likely to lead to any long term effects on the individual. The onset of TTS corresponds to a moving away or ‘fleeing response’ as this is the threshold at which animals experience disturbance and are likely to move away from the ensonified area. The onset of TTS is also considered to represent the boundary between the most severe disturbance levels and the start of physical auditory impacts on animals. Considering the above, the results of underwater noise modelling based on TTS onset as a proxy, will be hereinafter referred to as ‘strong behavioural disturbance’.
  2. Strong behavioural disturbance ranges for low order clearance donor charge and clearance shot are presented in Table 10.38   Open ▸ and high order clearance of UXO presented in Table 10.39   Open ▸ . The largest ranges using SPLpk metric were predicted for clearance of the 698 kg NEQ with potential strong disturbance over a distance of up to 26,790 m for harbour porpoise ( Table 10.39   Open ▸ ). Ranges predicted for other species using SPLpk only slightly exceeded 5 km for all other species, with the largest strong behavioural disturbance range predicted for grey seal at 5,250 m ( Table 10.39   Open ▸ ). However, based on the SELcum metric, the strong behavioural disturbance ranges are much larger with a maximum of 32,735 m predicted for minke whale ( Table 10.39   Open ▸ ). It should be noted that impulsive noise thresholds (TTS onset) were used in the underwater noise modelling for strong behavioural disturbance as a result of UXO clearance. As previously described in paragraph 270, the sound is unlikely to be impulsive in character once it has propagated more than a few kilometres and it is particularly important when interpreting results for disturbance within ranges larger than 10 km as these are likely to be significantly lower than predicted see (Hastie et al., 2019) (see volume 3, appendix 10.1 for more details).

 

Table 10.38:
Maximum Potential Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded)

Table 10.38: Maximum Potential Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) For Low Order Clearance Donor Charge and Clearance Shot (N/E = Threshold Not Exceeded)

 

Table 10.39:
Maximum Potential Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) for High Order Detonation of Maximum and Realistic Maximum Case

Table 10.39: Maximum Potential Strong Behavioural Disturbance Ranges (TTS Used As a Proxy) for High Order Detonation of Maximum and Realistic Maximum Case

 

  1. The number of animals predicted to experience strong behavioural disturbance due to low order clearance donor charge and clearance shot is presented in Table 10.40   Open ▸ and high order clearance in Table 10.41   Open ▸ .
  2. Given the largest strong behavioural disturbance ranges ( Table 10.39   Open ▸ ) 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 1,467 animals could experience strong disturbance as a result of high order detonation of a 698 kg NEQ (based on SPLpk metric, 0.42% of the North Sea MU population). The second largest number of animals disturbed was predicted for minke whale based on SELcum metric with up to 96 individuals potentially experiencing strong disturbance (0.47% of the CGNS MU) as a result of high order detonation of 698 kg NEQ. Based on SELcum, the number of grey seals at risk of experiencing strong behavioural disturbance within a predicted 6,120 m disturbance range was estimated as 22 animals (0.06% of the East Scotland MU plus the North-east England seal MU). For bottlenose dolphin and white-beaked dolphin the number of animals predicted to be disturbed was very small with no more than one animal within the predicted effect zones ( Table 10.40   Open ▸ and Table 10.41   Open ▸ ).
  3. The strong 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 10.40:
Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to Low Order Clearance Donor Charge and Clearance Shot

Table 10.40: Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to Low Order Clearance Donor Charge and Clearance Shot

 

Table 10.41:
Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to High Order Detonation of Maximum and Realistic Maximum Case

Table 10.41: Maximum Number of Animals With the Potential to Experience Strong Disturbance (TTS Used as a Proxy) Due to High Order Detonation of Maximum and Realistic Maximum Case

 

  1. Strong behavioural effects are reversible and therefore animals are anticipated to fully recover following cessation of the activity. It is, however, recognised that where designed in mitigation applies to reduce the risk of auditory injury (PTS), the deterrence measures (i.e. ADD and soft start charges) by their nature would contribute to, rather than reduce, the moving away response.
  2. For all species a small proportion of the relevant MUs is predicted to be affected by strong behavioural disturbance ( Table 10.41   Open ▸ ). As such, 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.
  3. As previously described in paragraph 269, the assessment considered the magnitude of a high order detonation for the MDS of 698 kg NEQ. The impact (high order detonation) is predicted to be of regional spatial extent in the context of the relevant geographic frame of reference, very short term duration, intermittent and both the impact itself (i.e. the elevation in underwater noise during detonation event) and effect of disturbance is reversible (TTS represents a non-trivial disturbance but not permanent injury). The magnitude is therefore considered to be low for all species.
Sensitivity of the receptor
Auditory injury (PTS)
  1. The main characteristic of the acoustical properties of explosives is a short shock wave, comprising a sharp rise in pressure followed by an exponential decay with a time constant of a few hundred microseconds (volume 3, appendix 10.1). The interactions of the shock and acoustic waves create a complex pattern in shallow water, and this was investigated further by von Benda-Beckmann et al. (2015).
  2. Scientific literature often focuses on harbour porpoises due to their high sensitivity to noise. von Benda-Beckmann et al. (2015) studied the range of effects of explosives on harbour porpoise in the southern North Sea; measures of SEL and peak overpressure (in kPa) were taken at distances up to 2 km from the explosions of seven aerial bombs detonated at approximately 26 m to 28 m depth, on a sandy substrate. Six bombs had a charge mass of 263 kg (580 lb) and one had a charge mass of 121 kg (267 lb). von Benda-Beckmann et al. (2015) investigated the potential for injury to occur as an ear trauma caused by the blast wave at a peak overpressure of 172 kPa (190 dB re. 1 µPa). In addition, the potential for noise-induced PTS to occur was based on a threshold of 190 dB re. 1 µPa2s (PTS ‘very likely to occur’) and an onset threshold of 179 dB re. 1 µPa2s (SEL) (PTS ‘increasingly likely to occur’) (Lucke et al. (2009) criteria). Results demonstrated the largest distance at which a risk of ear trauma could occur was at 500 m. They also found that noise-induced PTS was likely to occur greater than the 2 km range that was measured during the study since the SEL recorded at this distance was 191 dB re. 1 µPa2s, i.e. 1 dB above the ‘very likely to occur’ threshold.
  3. The study also modelled possible effect ranges for 210 explosions (of up to 1,000 kg charge mass) that had been logged by the Royal Netherland Navy and the Royal Netherlands Meteorological Institute over a two year period (2010 and 2011) (von Benda-Beckmann et al., 2015). Validating the model using the empirical measurements of SEL out to 2 km (see paragraph 294), von Benda-Beckmann et al. (2015) found that the effect distances ranged between hundreds of metres to just over 10 km (for charges ranging from 10 kg up to 1,000 kg). Porpoises are known to spend a large proportion of time near the surface (e.g. 55% based on Teilmann et al. (2007)) where the SELs were predicted to be lower, with effect distances for the onset of PTS just below 5 km. The authors caveat these results as, whilst the model could provide a reasonable estimate of the SEL within 2 km (given empirical measurements were made out to this point), estimates above this distance required further validation since the uncorrected model systematically overestimates SEL. More recently, Salomons et al. (2021) analysed sound measurements performed near two detonations of UXO (with charge masses of 325 kg and 140 kg). Subsequently a PTS effect distance in the range 2.5 km to 4 km was derived (Salomons et al., 2021), using the weighted SEL values and threshold levels from Southall et al. (2019). When comparing the experimental data and model predictions, the same study concluded that harbour porpoise are at risk of permanent hearing loss at distances of several kilometres, i.e. distance between 2 km and 6 km based on 140 kg and 325 kg charge masses, respectively (Salomons et al., 2021).
  4. In 2019, 24 harbour porpoise were found dead following clearance of ground mines in the Baltic Sea in along the German coastline (Siebert et al., 2022). The post-mortem examination found that in ten cases the cause of death was associated with a blast injury, however the charge masses of the explosives in this study are unknown.
  5. Not much is known about sensitivity of bottlenose dolphin, white-beaked dolphin and minke whale to blasting. However, during a clearance of relatively small explosive (35 kg charge) at an important feeding area for a resident community of bottlenose dolphin in Portugal, acoustic pressure levels in excess of 170 dB re 1 µPa were measured. No adverse effects were recorded in the behaviour or appearance of the resident community (dos Santos et al., 2010), even with pressure levels 60 dB higher than ambient noise. Nonetheless, other studies reported that external injuries consistent with inner ear damage have been found in dolphins subjected to explosives, with little change in surface animal behaviour near blast areas (Ketten et al., 1993).
  6. Robinson et al. (2020) described a controlled field experiment and compared the sound produced by high order detonations with a low order disposal method, i.e. deflagration. The study found that using low order techniques offers a substantial reduction in acoustic output over traditional high order methods, with the peak SPLpk and SELcum observed being typically >20 dB lower for the deflagration of the same sized munition (therefore a reduction factor of just over ten in SPLpk and 100 in acoustic energy). It was also demonstrated the acoustic output depends on the size of the shaped charge, rather than the size of the UXO itself. Considering the above, compared to high order methods, the study provided the evidence that low order techniques offers the potential for greatly reduced acoustic noise exposure of marine mammals (Robinson et al., 2020).
  7. The sensitivity of the receptors to the injury from impulsive underwater noise has been described previously in detail for piling and is presented in paragraphs 217 to 232.
  8. Therefore, all receptors, are deemed to have limited resilience to PTS, low recoverability and adaptability and are of high international value. The sensitivity of the receptor is therefore considered to be high.