10.10. Measures Adopted as Part of the Array

  1. As part of the Array design process, a number of designed in measures have been proposed to reduce the potential for impacts on marine mammal receptors (see Table 10.22   Open ▸ ). They are considered inherently part of the design of the Array and, as there is a commitment to implementing these measures, these have been considered in the assessment presented in section 10.11 (i.e. the determination of magnitude and therefore significance assumes implementation of these measures). These designed in measures are considered standard industry practice for this type of development.

 

Table 10.22:
Designed In Measures Adopted as Part of the Array

Table 10.22: Designed In Measures Adopted as Part of the Array

 

10.11. Assessment of Significance

  1. Table 10.17   Open ▸ summarises the potential impacts arising from the construction, operation and maintenance and decommissioning phases of the Array, as well as the MDS against which each impact has been assessed. An assessment of the likely significance of the effects of the Array on the marine mammal receptors as a result of each identified impact is provided within this section..
  2. Given that many of the impacts identified for marine mammals relate to underwater noise ( Table 10.17   Open ▸ ), the assessment has been informed by underwater noise modelling, the scope of which was agreed through consultation (see volume 2, chapter 5). An overview of the relevant thresholds for onset of significant effects alongside the evidence base used to derive them in provided in section 10.11.1. Further detail about noise modelling is provided in volume 3, appendix 10.1.

10.11.1. Marine Mammals and Underwater Noise

  1. Marine mammals, in particular cetaceans, are capable of generating and detecting sound and are dependent on sound for many aspects of their life, including prey identification, predator avoidance, communication and navigation (Au et al., 1974, Bailey et al., 2010). Increases in anthropogenic sound may consequently lead to a potential effect within the marine environment (Bailey et al., 2010, Parsons et al., 2008). Underwater noise influence may then subsequently affect marine mammals in a number of ways and vary with the distance from the noise source (Marine Mammal Commission, 2007). It can compete with important signals (masking) and alter behaviour (by inducing changes in foraging or habitat-use patterns, separation of mother-calf pairs). Underwater noise can also cause temporary hearing loss or, if the exposure is prolonged or intense, permanent hearing loss. It can also cause damage to tissues other than the ear if noise is sufficiently intense (Marine Mammal Commission, 2007).
  2. Given that there is sparse scientific evidence to properly evaluate masking (e.g. no relevant threshold criteria to enable a quantitative assessment), the assessment of effects associated with underwater noise on marine mammals presented in this section will consider auditory injury (temporary and permanent hearing loss) and behavioural response.

                        Injury

  1. Auditory injury in marine mammals can be either temporary, also referred to as Temporary Threshold Shift (TTS), where an animal’s auditory system recovers over time, or permanent, referred to as PTS, where there is no hearing recovery in the animal. The ‘onset’ of TTS is deemed to be where there is a 6 dB shift in a hearing threshold, defined by NMFS (2016) as a “the minimum threshold shift clearly larger than any day to day or session to session variation in a subject’s normal hearing ability”, and which “is typically the minimum amount of threshold shift that can be differentiated in most experimental conditions”. The acoustic threshold that would result in the PTS-onset in marine mammals have not been directly measured, largely as it is considered unethical to conduct experiments measuring PTS in animals. Therefore PTS-onset must be extrapolated from available TTS-onset measurements, including early studies on TTS growth rates in chinchillas (Henderson, 1983). The PTS onset is therefore conservatively considered to occur where there is 40 dB of TTS (Southall et al., 2007). TTS exceeding 40 dB requires a longer recovery time than smaller shifts (e.g. of 6 dB), suggesting a higher probability of irreversible damage or different underlying mechanisms (Kryter, 1994, Ward, 1970).
  2. Whether such shifts in hearing would lead to loss of fitness will depend on several factors including the frequency range of the shift and the duty cycle of impulsive sounds. For example, if a shift occurs within a frequency band that lies outside of the main hearing sensitivity of the receiving animal there may be a ‘notch’ in this band, but potentially no effect on the animal’s ability to survive. Further discussion on the sensitivity of marine mammals to hearing shifts is provided later in this assessment. Potential auditory injury is assessed in terms of PTS given the irreversible nature of the effect, unlike TTS which is temporary and reversible.
  3. Marine mammals exposed to sound levels that could induce TTS are likely to respond by moving away from (fleeing) the ensonified area and therefore avoiding potential injury. It is considered there is a behavioural response (disturbance) that overlaps with potential TTS ranges. Since derived thresholds for the onset of TTS are based on the smallest measurable shift in hearing, TTS thresholds are likely to be very precautionary and could result in overestimates of TTS ranges. In addition, the conservative assumptions applied in the underwater sound modelling (e.g. use of impulsive sound thresholds at large ranges; see paragraph 116 et seq.) may also result in the overestimation of ranges.
  4. Hastie et al. (2019) found that during piling there were range dependent changes in signal characteristics with received sound losing its impulsive characteristics at ranges of several kilometres, especially beyond 10 km. Therefore, where TTS ranges exceed 10 km it is not considered a useful predictor of the effects of underwater sound on marine mammals. As such, although TTS ranges were modelled for completeness for all sound-related impacts and are presented in volume 3, appendix 10.1, these are not included in the assessment of significance of auditory injury presented in this section (aligning with the proposed approach in the Array EIA Scoping Report (Ossian OWFL, 2023)). Alternatively, the assessment of potential auditory injury is assessed in terms of PTS and accounts for the irreversible nature of the effect.
  5. For marine mammals, auditory injury thresholds are based on both SPLpk (i.e. unweighted) and marine mammal hearing-weighted SELcum as per the latest guidance (Southall et al., 2019) ( Table 10.23   Open ▸ ). NatureScot was content with the proposed criteria and metrics for the assessment of injury using Southall et al. (2019) criteria for auditory injury to marine mammals (see Table 10.10   Open ▸ ). The marine mammal hearing-weighted categories are based on the frequency characteristics (bandwidth and sound level) for each group within which acoustic signals can be perceived and therefore assumed to have auditory effects ( Table 10.23   Open ▸ ). To calculate distances using the SELcum metric the sound modelling assessment made a simplistic assumption that an animal would be exposed over the duration of the piling activity and that there would be no breaks in activity during this time. It was assumed that an animal would swim away from the sound source at the onset of activity at a constant rate. The conservative species-specific swim speeds, as agreed with the NatureScot (see Table 10.10   Open ▸ ), were incorporated into the model ( Table 10.24   Open ▸ ).


  1. Marine mammal hearing groups are described in the latest guidance (Southall et al., 2019) as follows:
  • Low frequency (LF) cetaceans (i.e. marine mammal species such as mysticetes with an estimated functional hearing range between 7 Hz and 35 kHz); minke whale and humpback whale are marine mammal IEF in the LF cetacean group.
  • High frequency (HF) cetaceans (i.e. marine mammal species such as dolphins, toothed whales, beaked whales and bottlenose whales with an estimated functional hearing range between 150 Hz and 160 kHz); bottlenose dolphin and white-beaked dolphin are the marine mammal IEFs in the HF cetacean group.
  • Very High frequency (VHF) cetaceans (i.e. marine mammal species such as true porpoises, Kogia, river dolphins and cephalorhynchid with an estimated functional hearing range between 275 Hz and 160 kHz); harbour porpoise is the marine mammal IEF in the HF cetacean group.
  • Phocid Carnivores In Water (PCW) (i.e. true seals with an estimated functional hearing range between 50 Hz and 86 kHz); grey seal is the marine mammal IEF in the PCW group.

 

Table 10.23:
Summary of Acoustic Thresholds for PTS Onset in Relevant Hearing Groups (Southall et al., 2019)

Table 10.23: Summary of Acoustic Thresholds for PTS Onset in Relevant Hearing Groups (Southall et al., 2019)

 

Table 10.24:
Swim Speeds Used in the Underwater Noise Modelling

Table 10.24: Swim Speeds Used in the Underwater Noise Modelling

 

                        Disturbance

  1. As sound intensity decreases beyond the injury threshold zone, sound levels have the potential to disrupt the behavioural patterns of marine mammals. Behavioural reactions can vary in severity, from sustained vigilance, to interruptions in foraging, to active avoidance or displacement (NRW, 2023b). Responses may not necessarily directly scale with received sound level (Gomez et al., 2016). The reaction of a marine mammal to disturbance is dependent upon individual factors and contextual considerations (Southall et al., 2019), with prior experience and acclimatisation playing crucial roles in determining whether an individual will manifest an aversive response to sound (Ellison et al., 2012, Popper et al., 2014), especially in regions characterised by elevated underwater sound levels associated with human activities.
  2. Brandt et al. (2018) for example investigated disturbance in harbour porpoise during construction of the seven offshore wind farms in the German Bight, and found there was a clear gradient in the decline of porpoise detections after piling, depending on both the noise level and distance to piling activity. Within the local vicinity of the construction site (up to 2 km), porpoise detections declined several hours before the start of piling and were reduced for about one to two days after cessation of piling. Declines in harbour porpoise detections were found up to 17 km from piling when no noise mitigation system was used, with detections declining strongly during unmitigated piling. When mitigation was used, the maximum distance of effect was 14 km and the decline in detections was not as marked. Other studies at other wind farms (e.g. Nysted Offshore Wind Farm, Horns Rev Offshore Wind Farm, eight wind farms in the German Bight) which investigated the distances over which harbour porpoise are disturbed found effects up to 15 to 20 km from the piling site (Brandt et al., 2011, Carstensen et al., 2006, Dähne et al., 2013, Tougaard et al., 2006), though methodologies are not directly comparable and some did not involve acoustic measurements of piling noise, introducing uncertainty in transmission loss of noise over distance. Several studies demonstrated pronounced effects on harbour porpoise behaviour during construction but complete recovery once piling ceased (in the operation and maintenance phase) (Tougaard et al., 2009a).
  3. However, some studies have reported positive effects. (Scheidat et al., 2011) reported an overall increase in harbour porpoise activity from baseline to operation at Dutch wind farm Egmond aan Zee, with acoustic activity significantly higher inside the wind farm than in the surrounding reference areas, indicating that the occurrence of porpoises in this particular array area increased (potentially due to the reef effect or sheltering effect).
  4. Furthermore, the way in which disturbance is assessed in EIAs can vary considerably (NRW, 2023b). Key methods include dose-response curves, fixed noise thresholds and area-based thresholds (termed effective deterrent ranges (EDRs)). A summary of the approaches applied to this assessment is given in Table 10.25   Open ▸ , with further detail on dose-response and thresholds used below.

 

Table 10.25:
Summary of Criteria Used in The Impact Assessment of Behavioural Disturbance for Different Marine Mammal Species

Table 10.25: Summary of Criteria Used in The Impact Assessment of Behavioural Disturbance for Different Marine Mammal Species

 

Thresholds
  1. For impulsive sound sources other than piling (e.g. UXO clearance, some geotechnical and geophysical surveys), this assessment adopts the NMFS (2005) Level B harassment threshold of 160 dB re 1 μPa rms for impulsive sound, which is defined as: “having the potential to disturb a marine mammal or marine mammal stock in the wild by causing disruption of behavioural patterns, including, but not limited to, migration, breathing, nursing, breeding, feeding, or sheltering but which does not have the potential to injure a marine mammal or marine mammal stock in the wild”. This definition is similar to the JNCC (2010) description of non-trivial (significant) disturbance. The United States (US) NMFS (2005) guidelines also suggest a precautionary threshold of 140 dB re 1 μPa (rms) to indicate the onset of low level marine mammal disturbance effects for all mammal groups for impulsive sound, although this is not considered likely to lead to a ‘significant’ disturbance response and is therefore hereinafter referred to as “mild disturbance”.
  2. The assessment of significance for behavioural disturbance during piling will be based on the dose-response approach described in more detail in paragraph 105 et seq. The unweighted noise threshold value of 143 dB re 1µPa2s SELss was recently recommended in the position statement on assessing behavioural disturbance of harbour porpoise from underwater sound published by (NRW, 2023b). Acoustic recordings of the piling noise were utilised alongside harbour porpoise monitoring to derive a threshold for harbour porpoise reactions to the noise from piling. Declines were found at sound levels exceeding an unweighted SELss of 143 dB re 1 µPa2s and up to 17 km from piling. This means that harbour porpoises may react with avoidance only when exposure exceeds a threshold value of 143 dB re 1 µPa2s. It is worth noting that the noise threshold of 143 dB re 1 µPa2s was derived from modelled average of six different studies of full-scale pile driving operation and thereby represents a large amount of empirical data (Tougaard, 2021). As such, this threshold is also referred to in this assessment to provide context. It is particularly relevant to the HRA as a designated area-based approach and has also been applied to the HRA in reference to harbour porpoise SAC only.
  3. The NMFS (2005) guidance sets the marine mammal Level B harassment threshold (analogous to disturbance) for continuous sound at 120 dB re 1 μPa (rms). This threshold has therefore been adopted in the assessment of effects as a result of continuous noise, such as noise originating from drilling and vessels.
Dose-response
  1. The data collected during 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). During monitoring at Horns Rev Offshore Wind Farm, harbour porpoise demonstrated 100% avoidance at distances 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). Graham et al. (2019) analysed data collected during piling at the Beatrice Offshore Wind Farm (Moray Firth, Scotland) to demonstrate that the probability of occurrence of harbour porpoise (measured as porpoise positive minutes) increased exponentially moving further away from the noise source. The study demonstrated that the response of harbour porpoise to piling diminished over the piling phase such that, for a given received sound level or at a given distance from the source, there were more detections of animals at the last piling location compared to the first piling location (Graham et al., 2019) ( Figure 10.5   Open ▸ ). For harbour porpoise, as a representative approach, the dose-response curve was applied from the first location modelled as shown by Graham et al. (2019) where the probability of response approaches zero at circa 120 dB SELss. In the absence of species-specific data for other cetacean species, the same dose-response curve was assumed to apply to all cetacean species in this assessment ( Figure 10.5   Open ▸ ) and represents a precautionary approach to assessment as other cetacean species are likely to be less sensitive than harbour porpoise to behavioural disturbance as noted in the literature (Tougaard, 2021).
  2. Whyte et al. (2020) used tracking data from 24 harbour seal to estimate the effects of pile driving sounds on this species. The study used predictions of seal density during pile driving made by Russell et al. (2016) compared to distance from the wind farm and predicted single-strike SEL (SELss) by multiple approaches. The study reported predictions of seal density, and changes in seal density during piling, averaged across all water depths and piling events (Whyte et al., 2020). Predicted seal density significantly decreased within 25 km or above 145 dB re 1 µPa2 SELss (averaged across depths and pile installations). Other studies have reported similar avoidance reactions for both grey seal and harbour seal to the same sound source (Aarts et al., 2018, Götz and Janik, 2010) and therefore harbour seal dose-response curve is considered as appropriate to be used as a proxy for grey seal. As such, the dose-response curve derived from Whyte et al. (2020) ( Figure 10.24   Open ▸ ) was applied to the grey seal assessment to determine the number of animals that may potentially respond behaviourally to received sound levels during piling.
  3. To obtain the numbers of animals disturbed during piling, SELss contours from underwater noise modelling were plotted by 5 dB isopleths in GIS for all modelled locations. The areas within each isopleth were calculated from the spatial GIS map and a proportional expected response (derived from the dose-response curve for each isopleth area) was used to calculate the number of animals potentially disturbed. These numbers were subsequently summed across all isopleths to estimate the total number of animals disturbed during piling at any given time. The number of animals predicted to respond are based on species-specific densities derived from site-specific surveys and desktop data ( Table 10.13   Open ▸ ), as agreed with NatureScot ( Table 10.10   Open ▸ ). For each species the location taken forward for assessment was that which resulted in the greatest number of animals affected, thereby representing the MDS. For cetaceans (except bottlenose dolphin, where an average density to represent distribution within the CES2 MU population was used to estimate the number of animals) this was represented by the location with the largest spatial extent of the noise contours, whilst for bottlenose dolphin and grey seal (where the numbers of animals were calculated from the mean density derived from the Lacey et al. (2022) and Carter et al. (2022) density maps, respectively) it was the modelled contour that coincided with higher density areas. A full account of the approach to estimating marine mammal density for assessment is presented in volume 2, appendix 10.1).

Figure 10.4:
The Probability of a Harbour Porpoise Response (24 hrs) in Relation to the Partial Contribution of Unweighted Received SELss for the First Location Piled (Purple Line), the Middle Location (Green Line) and the Final Location Piled (Grey Line) (Graham et al., 2019)

Figure 10.4: The Probability of a Harbour Porpoise Response (24 hrs) in Relation to the Partial Contribution of Unweighted Received SELss for the First Location Piled (Purple Line), the Middle Location (Green Line) and the Final Location Piled (Grey Line) (Graham et al., 2019)

Figure 10.5:
Predicted Decrease in Seal Density as a Function of Estimated Sound Exposure Level, Error Bars Show 95% Confidence Interval (CI) (Whyte et al., 2020)

Figure 10.5: Predicted Decrease in Seal Density as a Function of Estimated Sound Exposure Level, Error Bars Show 95% Confidence Interval (CI) (Whyte et al., 2020)

 

Assumptions and limitations
  1. By applying the fixed-threshold based and dose-response criteria, the magnitude of impact can be quantified with respect to the spatial extent of disturbance, and subsequently the number of animals potentially disturbed based on available density information. However, Southall et al. (2021) noted that it is challenging to develop a comprehensive set of empirically derived criteria for such a diverse group of animals. The study identified data gaps, as for example, measurements of the effects of elevated sound on mysticetes have never been conducted and extrapolation from other species has been necessary. Since there are broad differences in hearing across the frequency spectrum for different marine mammal hearing groups, sounds that disturb one species may be irrelevant or inaudible to other species. Variance in responses even across individuals of the same species are well documented to be context and sound-type specific (Ellison et al., 2012). In addition, the potential interacting and additive effects of multiple stressors (e.g. reduction in prey, sound and disturbance, contamination, etc.) is likely to influence the severity of responses (Lacy et al., 2017).
  2. As such, the recent recommendations by Southall et al. (2021) steer away from a single overarching approach. Instead, the study proposes a framework for developing probabilistic response functions for future studies (Southall et al., 2021). The paper suggests different contexts for characterising marine mammal responses for both free-ranging and captive animals with distinctions made by sound sources (i.e. active sonar, seismic surveys, continuous/industrial sound and pile driving). Three parallel categories have been proposed within which a severity score from an acute (discrete) exposure can be allocated:
  • survival – defence, resting, social interactions and navigation;
  • reproduction – mating and parenting behaviours; and
  • foraging – search, pursuit, capture and consumption.
  1. Although some studies have been able to assign responses to these categories based on acute exposure, there is still limited understanding of how longer-term (chronic) exposure could translate into population-level effects. The potential for behavioural disturbance to lead to population consequences has been considered for this assessment using the iPCoD approach and is described in detail in paragraph 131 et seq. and in volume 3, appendix 10.3.
  2. Southall et al. (2021) reported observations from long term whale-watching studies and suggested that there were differences in the ability of marine mammals to compensate for long term disturbance which related to their breeding strategy. For example, mysticetes as ‘capital breeders’ accumulate energy in their feeding grounds and transfer it to calves in their breeding ground, whilst other species such as harbour porpoise, bottlenose dolphin and harbour seal are ‘income breeders’ as they balance the costs of pregnancy and lactation by increased food intake, rather than depending on fat stores. Reproductive strategy can impact the energetic consequences of disturbance and cause variation in an individual’s vulnerability to disturbance based on both its reproductive strategy and stage (Harwood et al., 2020).
  3. Marine mammal ability to compensate for chronic exposure to sound will also depend on a range of ecological factors, including the relative importance of the disturbed area and prey availability within their wider home range, the distance to and quality of other suitable sites, the relative risk of predation or competition in other areas, individual exposure history, and the presence of concurrent disturbances in other areas of their range (Gill et al., 2001). Animals may be able to compensate for short term disturbances by feeding in other areas, for example, which would reduce the likelihood of longer-term population consequences. Booth (2019) reported that although minimising the anthropogenic disturbance is an important factor to animal’s health, if animals can find suitable high-energy-density prey they may be capable of recovering from some lost foraging opportunities. Christiansen and Lusseau (2015) studied the effect of whale-watching on minke whale in Faxafloi Bay, Iceland and found no significant long term effects on vital rates, although years with low sandeel density led to increased exposure to whale-watching as whales were forced to move into disturbed areas to forage. Odontocetes may be more vulnerable to whale-watching compared to mysticetes due to their more localised, and often, coastal home ranges. Bejder et al. (2006) documented a decrease in local abundance of bottlenose dolphin which was associated with an increase in whale-watching in a tourist area compared to a control area. Studies of changes in abundance as a result of disturbance should be considered in light of findings presented in Gill et al. (2001) who reported that if there is no suitable habitat nearby animals may be forced to remain in an area despite the disturbance, regardless of whether or not it could affect survival or reproductive success.
  4. The marine mammal receptors considered in this assessment vary biologically and therefore have different ecological requirements that may affect their sensitivity to disturbance. This point is illustrated by the differences between marine mammals identified as key biological receptors in the baseline. Humpback whales and grey seals are capital breeders and store energy for reproduction and survival, while harbour porpoise (and other cetaceans whose ecology is well studied, e.g. bottlenose dolphin) are income breeders and they use energy that is acquired on a continual basis, including during the reproductive period (Stephens et al., 2009).
  5. Recognising the inherent uncertainty in the quantification of effects using threshold and dose-response approaches, this assessment has adopted a precautionary approach at all stages of assessment, including additional conservative assumptions in the:
  • marine mammal baseline (e.g. use of seasonal density peaks for harbour porpoise densities);
  • MDS for the project parameters ( Table 10.17   Open ▸ , e.g. use of high order UXO clearance as the MDS); and
  • underwater noise modelling (see paragraph 116 et seq. for summary and volume 3, appendix 10.1 for more details).
  1. These assumptions have been referred to throughout this chapter, illustrating that the systematic incorporation of layers of conservatism is likely to result in a very precautionary assessment.

                        Conservatism in the underwater noise modelling

  1. In order to ensure that the assessment is precautionary, a number of conservative assumptions were adopted in the underwater noise model. These measures of conservatism are summarised in this section and highlight that both PTS (and TTS onset ranges) predicted using the SELcum threshold are likely to lead to overestimates in the ranges and therefore should be interpreted with caution. For more details refer to volume 3, appendix 10.1.
  2. The underwater noise modelling assumed that the maximum hammer energy would be reached and maintained at all locations, whereas this is unlikely to be the case based on examples from other offshore wind farms, e.g. Beatrice Offshore Wind Farm, where the mean actual hammer energy averages were considerably lower than the maximum assessed in the Environmental Statement and only six out of 86 asset locations reached maximum hammer energy (Beatrice Offshore Wind Farm Ltd (BOWL), 2018).
  3. Additionally, the piling procedure simulated in the model does not allow for short pauses in piling (e.g. for realignment) and therefore the modelled SELcum is likely to be an overestimate since, in reality, these pauses would reduce the sound exposure that animals experience whilst moving away.
  4. The underwater noise modelling assessment also assumed that animals swim directly away from the sound source at constant and conservative average speeds based on published values. Whilst this buffers the uncertainty with respect to the directionality of their movement, it may lead to overestimates of the potential range of effect as animals are likely to exceed these speeds. For example, Otani et al. (2000) reported horizontal speed for harbour porpoise can be significantly faster than vertical speed and cite a maximum speed of 4.3 m/s (compared to 1.5 m/s used in the underwater noise model). Similarly, McGarry et al. (2017) reported minke whale speeds of up to 4.2 m/s during acoustic deterrent exposure experiments on free-ranging animals, compared to swim speed of 2.3 m/s used in the underwater noise model.
  5. The underwater noise model accounts for the SELcum metric as an equal-energy rule, where exposures of equal-energy are assumed to produce the same sound-induced threshold shift regardless of how the energy is distributed over time. Since for intermittent sound (such as piling) the quiet periods between sound exposures will allow some recovery of hearing compared to continuous sound, the equal-energy rule is likely to overestimate the extent of impact. Additionally, modelling of concurrent piling assumed piling will occur at exactly the same time and strike piles simultaneously, whereas in reality this is highly unlikely and could lead to overestimates in the injury and/or disturbance ranges.
  6. The impulsive sound is likely to undergo transition into non-impulsive sound at distance from the sound source due to a combination of factors (e.g. dispersion of the waveform, multiple reflections from sea surface and seafloor, and molecular absorption of high frequency energy). The empirical evidence suggest that such shifts in impulsivity could occur within 10 km from the sound source (Hastie et al., 2019). However, since the precise range at which this transition occurs is unknown, the underwater noise model adopted the impulsive thresholds at all ranges. This is likely to lead to an overly precautionary estimate of injury ranges at larger distances (tens of kilometres) from the noise source.

10.11.2. Assessment of Effects

Injury and disturbance from underwater noise generated during piling

                        Summary of piling scenarios

  1. Piling during the construction phase of the Array has the potential to result in higher levels of underwater sound when compared to background levels and could result in auditory injury and/or potential behavioural effects on marine mammal receptors. A detailed underwater noise modelling assessment was carried out to investigate the potential for such effects to occur, using the latest assessment criteria as presented in paragraph 96 (and discussed in detail in volume 3, appendix 10.1).
  2. As first recommended by stakeholders during the pre-Scoping workshop (see Table 10.10   Open ▸ ), only the SPLpk is used to inform the appropriate mitigation zone. However, both metrics (SPLpk and SELcum) are presented in the impact assessment of PTS for the Array. More recent advice from NatureScot (see Table 10.10   Open ▸ ) advised that pre-piling mitigation should be based on SPLpk but the assessment of effects itself should use the dual metric approach (SPLpk and SELcum). Therefore, the assessment of effects is based upon the dual metric approach following the latest advice from NatureScot.
  3. The measures adopted to mitigate impacts within this mitigation zone, defined by SPLpk, are detailed in the outline MMMP (volume 4, appendix 22). During piling, with respect to the SPLpk metric, the soft start initiation is the most relevant period, as this is when animals may potentially experience injury from underwater sound emitted by the initial strike of the hammer, after which point it is assumed that they will move away from the noise source. However, to ensure a precautionary approach, the injury ranges for SPLpk are based on the sound from the maximum hammer energy over the entire installation (which is highly conservative, as discussed in paragraph 117).
  4. The scenarios modelled were based on the maximum hammer energies (of 3,000 kJ or 4,400 kJ, see Table 10.17   Open ▸ ) for the longest possible duration, noting that piling is unlikely to reach and maintain the absolute maximum hammer energy at all locations. The assessment of potential effects on marine mammal receptors from piling considered a maximum spatial and maximum temporal scenario ( Table 10.17   Open ▸ ).
  5. Maximum spatial scenarios assume concurrent piling of piles at OSPs and wind turbine (anchors), leading to the largest area of effect at any one time. Maximum temporal scenarios, leading to the greatest number of days of piling, is based on single piling of piles at wind turbines (anchors) and OSPs (jackets).
  6. Underwater sound modelling modelled concurrent piling at:
  • wind turbines (anchors) with a maximum hammer energy of 3,000 kJ; and
  • wind turbines (anchors) and OSP with a maximum hammer energy of 3,000 kJ and 4,400 kJ, respectively.
  1. For the concurrent piling scenarios modelled, the following assumptions were identified:
  • minimum separation distance of 950 m between concurrent piling events as a MDS for potential injury; and
  • maximum separation distance of up to 30 km as a MDS for potential disturbance based on the Project Description and site bathymetry (volume 1, chapter 3).
  1. The modelled locations ( Figure 10.6   Open ▸ ) were species-specific, e.g. those that were likely to generate noise contours with the highest potential to overlap with sensitive areas for a given species (e.g. density hotspots). The modelling locations were as follows (detailed in volume 3, appendix 10.1):
  • a point at the northern end of the site boundary (closest point to land to capture potential overlap with the coastal distribution of bottlenose dolphins);
  • the central point of the site boundary (to capture potential overlap with the coastal distribution of bottlenose dolphins and potential effects on grey seal density hotspots within the Berwickshire and North Northumberland Coast SAC); and
  • a point at the southern end of the site boundary (to assess potential effects on grey seal density hotspots within the Berwickshire and North Northumberland Coast SAC and the Southern North Sea SAC designated for harbour porpoise.
  1. For the maximum temporal scenario, the assessment focussed on the longest duration of piling and the greatest number of days over which piling could occur. The longest duration of piling per pile for wind turbines (anchors) or OSPs (jackets) is eight hours per pile. Therefore, piling activities can take place over a maximum of 602 days (530 days at wind turbines and 72 days at OSPs) ( Table 10.17   Open ▸ ). For a realistic scenario, the average number of piles that can be installed over 24 hours is more likely to be four for wind turbines and three for OSPs, and this would reduce the temporal scenario to 397.5 days.


'

Figure 10.6:
Locations Modelled Within the Ossian Array (Red Line)

Figure 10.6: Locations Modelled Within the Ossian Array (Red Line)

 

                        Summary of interim population consequences of disturbance (iPCoD) modelling

  1. To aid with the assessment of magnitude for piling, the potential for population-level consequences of behavioural disturbance has been considered using the iPCoD approach for harbour porpoise, bottlenose dolphin, minke whale and grey seal. The results of population modelling are presented in the relevant magnitude sections of disturbance for each species, following estimations of the number of animals disturbed.
  2. There is limited understanding of how behavioural disturbance and auditory injury affect survival and reproduction in individual marine mammals and consequently how this translates into potential effects at the population-level. The iPCoD framework was developed by SMRU consulting and the University of St Andrews using a process of expert elicitation to determine how physiological and behavioural changes affect individual vital rates (i.e. the components of individual fitness that affect the probability of survival, production of offspring, growth rate and offspring survival). The iPCoD framework applies simulated changes in vital rates to infer the number of animals that may be affected by disturbance as a means to iteratively project the size of the population. The expert elicitation process has not been undertaken for white-beaked dolphin (only five key species have been included), and as such the current version of iPCoD does not allow modelling of population trajectories for this species. Relevant MUs for modelling were informed by baseline characterisation in volume 3, appendix 10.3.
  3. For bottlenose dolphin, the CES2 MU was used as the relevant reference population. Given the importance of the Moray Firth SAC for bottlenose dolphin in this area, the sensitivity of this population and its known ranging behaviour further south towards St Andrews Bay and the Tay Estuary, and inshore in north-east English waters, it is important to capture the potential impact on this important coastal ecotype which may experience potential barrier effects. Whilst there is an abundance estimate for the Greater North Sea MU (2,022 animals (IAMMWG, 2023)) this large MU extends the entire length of the east coast of the UK and east to Scandinavia, so apportioning numbers of the offshore ecotype to the east coast of Scotland is not possible. It is also unlikely that the Array will create significant barrier effects for this offshore ecotype, given the extent of the MU along the east coast of Scotland. Therefore, the assessment has focussed on the impacts for bottlenose dolphin within the CES2 MU and Moray Firth SAC.
  4. For harbour porpoise and minke whale, only one MU for each species occurs in the vicinity of the Array marine mammal study area (IAMMWG, 2023), and the respective population estimates for these MUs have been used for iPCoD modelling: the North Sea MU for harbour porpoise and the CGNS MU for minke whale. The site boundary coincides with the boundary between two seal MUs, so for grey seal, the reference population comprises the sum of the East Scotland seal MU and the North-east England seal MU (SCOS, 2023).
  5. The population estimates used to parameterise iPCoD models were taken from IAMMWG (2023) for cetacean species and from SCOS (2023) for grey seal (summarised in Table 10.26   Open ▸ ), alongside vital rates taken from Sinclair et al. (2020), presented in Table 10.27   Open ▸ .

 

Table 10.26:
Management Units and Population Estimates for Species Included in iPCoD Models

Table 10.26: Management Units and Population Estimates for Species Included in iPCoD Models

Table 10.27:
Marine Mammal Vital Rates Used to Parameterise iPCoD Models (from Sinclair et al. (2020))

Table 10.27: Marine Mammal Vital Rates Used to Parameterise iPCoD Models (from Sinclair et al. (2020))

 

  1. The SPLpk metric has been used to inform the appropriate mitigation range (see Table 10.10   Open ▸ ) although the dual metric approach is presented in underwater noise modelling and informs the impact assessment. Therefore, the number of animals that may experience PTS to be inputted into the iPCoD models were derived from calculations based upon the maximum numbers of animals experiencing PTS from modelling of SPLpk or SELcum, so that the assessment of magnitude (which is based on the dual metric approach) aligns with modelling of population effects, even though SPLpk will be used to define the mitigation zone.
  2. Furthermore, calculation of the number of animals that may experience PTS assumed a 30 minute implementation of ADD, as per standard industry practice. The numbers of animals for injury taken forward to iPCoD modelling therefore was based upon those with implementation of 30 minute of ADD. This was agreed with NatureScot following Ossian Array Marine Mammal Consultation Note 2 (volume 3, appendix 5.1, annex E), which confirmed that the auditory injury assessment should be based on numbers of animals remaining following 30 minutes of ADD usage ( Table 10.10   Open ▸ ).
  3. Both the maximum temporal scenario (e.g. the single piling scenario with fewer animals impacted per day, but over more days) and the maximum spatial scenario (e.g. the concurrent piling scenario with more animals impacted per day, but for fewer days) were modelled.
  4. Results of population modelling are presented in full in volume 3, appendix 10.3 the relevant magnitude sections for harbour porpoise, bottlenose dolphin, minke whale and grey seal: key species for which iPCoD functionality is currently available.
    Construction phase
Magnitude of impact
Auditory injury (PTS)
  1. The summary of potential PTS ranges (for both SPLpk and SELcum) (without use of an ADD) for single pile installation is presented in Table 10.28   Open ▸ , and for concurrent piling is presented in Table 10.29   Open ▸ .
  2. The maximum spatial effect was predicted for concurrent piling at wind turbines and OSPs with a hammer energy of 3,000 kJ and 4,400 kJ, respectively ( Table 10.29   Open ▸ ). Whilst the effect of PTS is considered to result in permanent injury to animals, the risk of animals being exposed to sound levels leading to auditory injury would occur during piling only. As shown in Table 10.17   Open ▸ , piling will be intermittent over an eight-year construction piling phase and will occur up to a maximum of 602 days.
  3. The instantaneous injury (based on SPLpk metric) could occur out to a maximum range of 1,600 m across all species during single pile installation at OSPs, with the maximum range predicted for harbour porpoise ( Table 10.28   Open ▸ ). Considering cumulative exposure using the SELcum metric, the risk of PTS was estimated to occur out to a maximum range of 7,200 m and was predicted for minke whale during single pile installation at OSPs ( Table 10.28   Open ▸ ).
  4. The maximum spatial effect was estimated using two different concurrent piling scenarios, at wind turbines with a hammer energy of 3,000 kJ with either a wind turbine with a hammer energy of 3,000 kJ or an OSP with hammer energy of 4,400 kJ, respectively ( Table 10.29   Open ▸ ). Given that the potential injury range for the concurrent scenarios based on the SPLpk metric would remain the same as the injury ranges for the single installation scenario (as detailed in volume 3, appendix 10.1) ( Table 10.28   Open ▸ ), these were omitted from the results presented in Table 10.29   Open ▸ . Considering cumulative exposure using the SELcum metric, the risk of PTS was estimated to occur out to a maximum range of 9,740 m and was predicted for minke whale during concurrent pile installation at wind turbine and OSP ( Table 10.29   Open ▸ ).
  5. Designed-in mitigation in the form of an outline MMMP (volume 4, appendix 22) will be implemented to reduce the likelihood of PTS. Such mitigation will include deployment of an ADD as recommended in the guidelines (JNCC, 2010a). Pre-scoping advice from NatureScot (see Table 10.10   Open ▸ ) was to consider ranges predicted using the SPLpk metric only with respect to application of the JNCC (2010) guidance on defining a mitigation zone. The conclusions of significance in the impact assessment with respect to PTS, however, required consideration of both SPLpk and SELcum ranges as clarified by NatureScot in a subsequent advice note (volume 3, appendix 5.1, annex E). Subsequently, the efficacy of ADD specifically as a mitigation tool was explored with respect to both metrics by applying a 30 minute deployment time prior to hammer initiation (see paragraph 137). The exact duration of ADD activation will, however, be discussed and agreed with consultees as part of the outline MMMP to be submitted post-consent and in respect of any refinements in the Project Description that may be available at a later stage and included within the outline MMMP (volume 1, chapter 3; volume 4, appendix 22).
  6. The assessment of magnitude with respect to auditory injury is presented below (paragraph 152 et seq.) on a species-specific basis, where the MDS is identified for each species. Humpback whale is considered qualitatively in the same section as minke whale given that both species fall within the low frequency hearing group (Southall et al., 2019).

Table 10.28:
Summary of Potential PTS Ranges for Single Pile Installation at Wind Turbines (3,000 kJ) and OSPs (4,400 kJ) Using Both Metrics – SPLpk and SELcum (N/E = Threshold Not Exceeded)

Table 10.28: Summary of Potential PTS Ranges for Single Pile Installation at Wind Turbines (3,000 kJ) and OSPs (4,400 kJ) Using Both Metrics – SPLpk and SELcum (N/E = Threshold Not Exceeded)

 

Table 10.29:
Summary of Potential PTS Ranges for Concurrent Pile Installation at Wind Turbines (3,000 kJ) and at Wind Turbines (3,000 kJ) and OSPs (4,400 kJ) Using SELcum (N/E = Threshold Not Exceeded)

Table 10.29: Summary of Potential PTS Ranges for Concurrent Pile Installation at Wind Turbines (3,000 kJ) and at Wind Turbines (3,000 kJ) and OSPs (4,400 kJ) Using SELcum (N/E = Threshold Not Exceeded)

Given that the potential injury range for the concurrent scenarios based on the SPLpk metric remain the same as the injury ranges for the single installation ( Table 10.28   Open ▸ ), these were omitted from the results presented in Table 10.29   Open ▸ .

 

  1. 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. It is considered to be more effective at reducing the potential for injury to marine mammals compared to actions informed by standard mitigation measures (MMOs2 and PAM) which may have limitations with respect to effective detection over distance (Parsons et al., 2009, Wright and Cosentino, 2015).
  2. There are various ADDs available with different sound source characteristics (McGarry et al., 2022) and a suitable device will be selected based on the key species requiring mitigation for the Array. 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 reducing the additional sound introduced into the marine environment as far as practicable.
  3. Therefore, sound modelling was carried out to determine the efficacy of using ADDs to deter marine mammals from the injury zone for a duration of 30 minutes (see volume 3, appendix 10.1) for both the SPLpk and SELcum metrics.
  4. Using SPLpk metric (which has been used to define the mitigation zone), the maximum potential injury ranges were predicted for single pile installation at OSPs with a hammer energy of 4,400 kJ ( Table 10.28   Open ▸ ). Please note that although humpback whale has not been considered quantitively in the assessment, mean swim speeds during control measurements (0.3 m/s) published by Sprogis et al. (2020) were used to assess whether it will be able to move away from the injury zone before the commencement of piling. Assuming conservative swim speeds listed in Table 10.30   Open ▸ , it was demonstrated that activation of an ADD for 30 minutes would deter all animals beyond the maximum injury zones using the SPLpk metric.

 

Table 10.30:
Summary of Maximum Potential PTS Ranges due to Single Pile Installation (at OSPs, Hammer Energy 4,400 kJ) Using SPLpk Metric, Indicating Whether the Individual Can Move Beyond the Injury Range During the 30 minutes of ADD Activation

Table 10.30: Summary of Maximum Potential PTS Ranges due to Single Pile Installation (at OSPs, Hammer Energy 4,400 kJ) Using SPLpk Metric, Indicating Whether the Individual Can Move Beyond the Injury Range During the 30 minutes of ADD Activation

 

  1. The maximum injury ranges using SELcum metric were predicted for concurrent pile installation at wind turbine and OSP with hammer energies of 3,000 kJ and 4,400 kJ, respectively ( Table 10.29   Open ▸ ). Activation of an ADD 30 minutes prior to commencement of piling reduced injury ranges to a level which does not exceed injury thresholds for all species except minke whale ( Table 10.31   Open ▸ ). Based on the underwater noise modelling, there is a residual risk of injury for minke whale across the range of 5,610 m.
  2. Initial stakeholder advice provided during the pre-Scoping Workshop (see Table 10.10   Open ▸ ), suggested to base the assessment of significance for PTS on SPLpk metric only as this aligns with the approach to defining the mitigation zone (see paragraph 123 above for further detail on assessment of injury from underwater noise from piling). However, in response to the Marine Mammal Consultation Note 2 (volume 3, appendix 5.1, annex E), NatureScot clarified that whilst the mitigation should based on the SPLpk metric, the assessment of significance should consider the dual metric approach (i.e. both SPLpk and SELcum). Therefore, the Applicant highlights that, whilst the risk of injury to all species can be fully mitigated based on the SPLpk metric, the deployment of an ADD does not fully remove the potential for injury to minke whale and humpback whale if considering the SELcum metric. Given the very precautionary nature of modelled predictions using the SELcum metric and the low probability of encountering either species (particularly humpback whale) in the zone of influence (due to low densities) the potential for injury is considered to be very low and therefore no additional mitigation is proposed. As part of the post-consent process, final details of mitigation will be discussed and agreed in consultation with stakeholders and will be fully informed by the final project design.  

 

Table 10.31:
Summary of Maximum Potential PTS Ranges due to Concurrent Pile Installation (at Wind Turbine and OSP, Hammer Energies of 3,000 kJ and 4,400 kJ) Using SELcum Metric With and Without 30 Minutes of ADD Activation (N/E = Threshold Note Exceeded)

Table 10.31: Summary of Maximum Potential PTS Ranges due to Concurrent Pile Installation (at Wind Turbine and OSP, Hammer Energies of 3,000 kJ and 4,400 kJ) Using SELcum Metric With and Without 30 Minutes of ADD Activation (N/E = Threshold Note Exceeded)

 

Harbour porpoise

  1. Based on SPLpk metric, the maximum potential range for injury to harbour porpoise was estimated as 1,600 m during pile installation at OSPs ( Table 10.28   Open ▸ ). Based on the density value of 0.651 animals per km2, up to six animals would be at risk of experiencing PTS. However, with designed in measures applied ( Table 10.22   Open ▸ ) which includes ADD, it is predicted that no animals would be affected by peak pressure (SPLpk) as they would be able to flee the potential injury range (1,600 m) during the 30 minute period of ADD activation ( Table 10.30   Open ▸ ). The maximum potential injury range for harbour porpoise is also not exceeded using the SELcum metric, when including ADD ( Table 10.31   Open ▸ ).
  2. The injury range is predicted to be localised to within the Array marine mammal study area and therefore there is no potential for spatial overlap with the Southern North Sea SAC, the closest site designated for harbour porpoise, which is located south at a distance of 130.7 km ( Table 10.15   Open ▸ ).
  3. Harbour porpoise typically live between 12 and 24 years and give birth once a year (Lockyer, 2013). The duration of piling is up to 602 days, within an eight-year piling programme (see Table 10.17   Open ▸ ), and therefore could potentially overlap with a maximum of eight breeding cycles. It should be noted that piling at OSPs with the hammer energy of 4,400 kJ resulting in maximum injury range of 1,600 m would take place over only a fraction of the total piling days (72 days). The total duration (602 days) of the impact in the context of the life cycle of harbour porpoise is classified as long term, as animals will be at the risk of potential injury (albeit very small) over a notable proportion of their lifespan.
  4. The impact (elevated underwater noise during piling) is predicted to be of local (small) spatial extent within the relevant geographic range of reference, medium-term duration, intermittent and the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Since injury is assumed to be fully mitigated via designed in measures ( Table 10.22   Open ▸ ), there is considered to be no residual risk of injury and therefore no population-level effects. The magnitude is therefore considered to be negligible.

Bottlenose dolphin and white-beaked dolphin

  1. Based on SPLpk metric, the maximum range for injury to bottlenose dolphin and white-beaked dolphin was estimated as 171 m during pile installation at OSPs ( Table 10.28   Open ▸ ). Based on the density values of 0.00303 and 0.120 animals per km2 for bottlenose dolphin and white-beaked dolphin, respectively, no more than one animal of each species would be at risk of experiencing PTS. However, with designed in measures applied ( Table 10.22   Open ▸ ), it is predicted that no animals would be affected by peak pressure (SPLpk) as they would be able to flee the potential injury range (171 m) during the period of ADD activation ( Table 10.30   Open ▸ ).
  2. The injury range is predicted to be localised to within the Array marine mammal study area and therefore there is no potential for spatial overlap with the Moray Firth SAC, the closest site designated for bottlenose dolphin, which is located north west at a distance of 176.5 km ( Table 10.15   Open ▸ ).
  3. Bottlenose dolphin typically live between 20 and 30 years. The gestation period is 12 months with calves suckling for 18 to 24 months with females reproducing every three to six years (Mitcheson, 2008). Less is known about reproductive behaviour of white-beaked dolphins, however, it has been reported that females are pregnant for about 11 months and give birth to a single calf (Reid et al., 2003), although the typical life expectancy of the white-beaked dolphin is largely unknown. The duration of piling is up to 602 days, within an eight-year piling programme (see Table 10.17   Open ▸ ), and therefore could potentially overlap with a maximum of three bottlenose dolphin and eight white-beaked dolphin breeding cycles. It should be noted that piling at OSPs with the hammer energy of 4,400 kJ resulting in maximum injury range of 171 m would take place over only a fraction of the total piling days (72 days). The total duration of the impact in the context of the life cycle of bottlenose dolphin and white-beaked dolphin is classified as long term, as animals will be at the risk of potential injury (albeit very small) over a notable proportion of their lifespan.
  4. The impact (elevated underwater noise during piling) is predicted to be of local (small) spatial extent within the geographic range of reference, medium-term duration, intermittent and the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Since injury is assumed to be fully mitigated via designed in measures ( Table 10.22   Open ▸ ), there is considered to be no residual risk of injury and therefore no population-level effects. The magnitude is therefore considered to be negligible.

Minke whale and humpback whale

  1. Based on SPLpk metric (which defines the mitigation zone, as per consultation guidance), the maximum range for potential injury to minke whale and humpback whale was estimated as 353 m during pile installation at OSPs ( Table 10.28   Open ▸ ). For minke whales, based on the density value of 0.0284 animals per km2, no more than one animal would be at risk of experiencing PTS. However, with designed in measures applied ( Table 10.22   Open ▸ ), it is predicted that no minke whales or humpback whales (see paragraph 149) would be affected by peak pressure (SPLpk) as they would be able to flee the potential injury range (353 m) during the period of ADD activation ( Table 10.30   Open ▸ ).
  2. However, based on SELcum metric with the inclusion of 30 minutes ADD, the maximum range for injury to minke whale and humpback whale was estimated as 5,610 m during concurrent pile installation ( Table 10.31   Open ▸ ). For minke whales, with designed in measures applied and based on the density value of 0.0284 animals per km2, up to three animals would be at risk of experiencing PTS. However, as discussed in paragraph 116 and acknowledged by NatureScot in their response to Marine Mammal Consultation Note 2 (volume 3, appendix 5.1, annex E), there are caveats to using the SELcum metric given the layers of conservatism in the assessment which may lead to overestimates in injury ranges.
  3. The injury range is predicted to be localised to within the Array marine mammal study area and therefore there is no potential for spatial overlap with the Southern Trench ncMPA, the closest site designated for protection of minke whale, which is located north-west at a distance of 66.9 km ( Table 10.15   Open ▸ ).
  4. Minke whale typically lives up to 60 years and females give birth to a calf every 12 to 14 months, with gestation period believed to last up to ten months (Sea Watch Foundation, 2012). Humpback whale calves are born in low latitudes after a gestation period lasting between 11 and 12 months (Mann et al., 2000). Reliable data on life expectancy of humpback whales are lacking with the oldest individuals aged 48 recorded off western Australia (Mann et al., 2000). Interbirth intervals among mature female humpback whales vary from a single year to several years (Mann et al., 2000).
  5. The duration of piling is up to 602 days, within an eight-year piling programme (see Table 10.17   Open ▸ ), and therefore could potentially overlap with a maximum of eight breeding cycles for both species. It should be noted that piling at OSPs with the hammer energy of 4,400 kJ resulting in maximum injury range of 171 m would take place over only a fraction of the total piling days (72 days). Additionally (as described in volume 1, chapter 3) there is anticipated to be limited piling activities carried out during winter months (due to inclement weather) over the eight year construction phase. Based on humpback whale sightings off eastern Scotland, although there are some records of individuals photographed in July and August, most of the sightings took place between December and March (Hague, 2023, O’Neil et al., 2019, Scottish Humpback, 2023). Therefore, given that that humpback whale sightings on the east coast of Scotland are seasonal, the presence of humpback whales within the injury zones is highly unlikely for a notable proportion of the piling days.
  6. The residual number of minke whales from SELcum predicted to experience PTS were carried forward to the iPCoD modelling assessment (see paragraph 183) alongside disturbance to understand the implications at a population-level, following advice from NatureScot in response to Marine Mammal Consultation Note 2 to input whichever metric results in the largest numbers of animals effected to the population model ( Table 10.10   Open ▸ ; volume 3, appendix 5.1, annex E).
  7. Overall, the total duration of the impact in the context of the life cycle of minke whale and humpback whale is classified as medium term, as animals will be at the risk of potential injury (albeit very small) over a notable proportion of their lifespan.
  8. For minke whale and humpback whale, as discussed in paragraph 160, potential injury can be fully mitigated using the SPLpk metric and therefore the magnitude would likely be negligible. However, given there is a small residual risk to minke whale using the SELcum metric, and noting this does not define the mitigation zone, the magnitude for minke whale and humpback whale has conservatively been assessed as low (rather than negligible).
  9. For minke whale and humpback whale, the impact (elevated underwater noise during piling) is predicted to be of local (small) spatial extent within the relevant geographic range of reference, medium-term duration, intermittent and the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Since injury is assumed to be fully mitigated via designed in measures (Table 10.28), there is considered to be a small residual risk of injury albeit no population-level effects. The magnitude is therefore considered to be low.

Grey seal

  1. Based on SPLpk metric, the maximum range for injury to grey seal was estimated as 379 m during pile installation at OSPs ( Table 10.28   Open ▸ ). Based on the density value of 0.180 animals per km2, no more than one animal of each species would be at risk of experiencing PTS. However, with designed in measures applied ( Table 10.22   Open ▸ ), it is predicted that no animals would be affected by peak pressure (SPLpk) as they would be able to flee the potential injury range (379 m) during the period of ADD activation ( Table 10.30   Open ▸ ).
  2. The injury range is predicted to be localised to within the Array marine mammal study area and therefore there is no potential for spatial overlap with the Berwickshire and North Northumberland Coast SAC, the closest site considered in this assessment designated for grey seal, which is located south-west at a distance of 114 km ( Table 10.15   Open ▸ ).
  3. Grey seal typically live between 20 to 30 years with gestation lasting between ten to 11 months (SCOS, 2023). The duration of piling is up to 602 days, within an eight-year piling programme (see Table 10.17   Open ▸ ), and therefore could potentially overlap with a maximum of eight breeding cycles. It should be noted that piling at OSPs with the hammer energy of 4,400 kJ resulting in maximum injury range of 379 m would take place over only a fraction of the total piling days (72 days). The total duration of the impact in the context of the life cycle of grey seal is classified as medium term, as animals will be at the risk of potential injury (albeit very small) over notable proportion of their lifespan.
  4. The impact (elevated underwater noise during piling) is predicted to be of local (small) spatial extent within the geographic frame of reference, medium-term duration, intermittent and the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Since injury is assumed to be fully mitigated via designed in measures, there is considered to be no residual risk of injury and therefore no population-level effects. The magnitude is therefore considered to be negligible.