Behavioural disturbance (TTS as a proxy)
  1. While underwater sound as a result of UXO clearance has the potential to produce behavioural disturbance, there are no agreed thresholds for the onset of a behavioural response generated as a result of a single UXO explosion. Thresholds for the onset of behavioural disturbance from detonation of explosives exist (Finneran and Jenkins, 2012), which follow the proposed approach by Southall et al. (2007), but these are intended for repeated detonations over a 24 hour period and therefore are not suitable for single detonations of a UXO. Finneran and Jenkins (2012) states for these single detonations, behavioural disturbance is likely to be limited to ‘a short-lived startle reaction’ and therefore does not use any unique behavioural disturbance thresholds for marine mammals exposed to single explosive events.
  2. Southall et al. (2007) recommended that the use of TTS onset as an auditory effect may be most appropriate for single pulses (such as UXO detonation) and therefore it has been applied to inform the assessment.
  3. As TTS is a temporary and reversible hearing impairment, it is anticipated that any animals experiencing this shift in hearing would recover after they have moved beyond the injury zone and are no longer exposed to elevated sound levels. Whilst the implication of animals experiencing TTS, leading to potential displacement, is not fully understood, it is likely that aversive responses to anthropogenic sound could temporarily affect life functions as described for PTS.
  4. Therefore, in this respect animals exposed to sound levels that could induce TTS have similar susceptibility as those exposed to sound levels that could induce PTS. There is an important distinction, however, given that TTS is only temporary hearing impairment, it is less likely to lead to acute effects and will largely depend on recoverability. The degree and speed of hearing recovery will depend on the characteristics of the sound the animal is exposed to and the degree of shift in hearing experienced.

Harbour porpoise

  1. Recovery rates of harbour porpoise were measured following exposure to a piling playback sound source of 175 dB re 1 μPa2s (SEL) over 120 minutes (SEAMARCO, 2011). SEAMARCO (2011) found that recovery to the pre-exposure threshold was estimated to be complete within 48 minutes following exposure and the higher the hearing threshold shift, the longer the recovery.
  2. Kastelein et al. (2021) found that the susceptibility to TTS depends on the frequency of the fatiguing sound causing the shift and the greatest TTS depends on the SPL (and related SEL). In a series of studies reviewed in Finneran (2015), which measured TTS occurrence in harbour porpoise at a range of frequencies typical of high-amplitude anthropogenic sounds, the greatest shift in mean TTS occurred at 0.5 kHz with hearing recovery within 60 minutes after the fatiguing sound stopped. Scientific understanding of the biological effects of TTS is limited to the results of controlled exposure studies on small numbers of captive animals. Extrapolating these results to how animals may respond in the natural environment should be treated with caution as it is not possible to exactly replicate natural environmental conditions, and the small number of test subjects would not account for intraspecific differences (i.e. differences between individuals) or interspecific differences (i.e. extrapolating to other species) in response.

Bottlenose dolphin and white-beaked dolphin

  1. Finneran et al. (2000) investigated the behavioural and auditory responses of two captive bottlenose dolphin to sounds that simulated distant underwater explosions. The animals were exposed to an intense sound once per day and no auditory shift (i.e. TTS) greater than 6 dB in response to levels up to 221 dB re 1 µPa peak-to-peak (p-p) was observed. Behavioural shifts, such as delaying approach to the test station and avoiding the ‘start’ station, were recorded at 196 dB re 1 µPa Pk-Pk and 209 dB re 1 µPa Pk-Pk for the two bottlenose dolphin and continued at higher levels. However, there are several caveats to this study as discussed in (Nowacek et al., 2007), with the signals used in this study distant and the study measured masked-hearing signals. The animals used in the experiment were also trained and rewarded for tolerating high levels of sound and subsequently, it can be anticipated that behavioural disruption would likely be observed at lower levels in other contexts.
  2. Whilst there are no available species-specific recovery rates for high frequency cetaceans to TTS, there is no evidence to suggest that recovery will be significantly different to harbour porpoise recovery rates therefore animals can recover their hearing after they are no longer exposed to elevated sound levels. It can be anticipated that both bottlenose dolphin and white-beaked dolphin would be able to tolerate the effect without any impact on reproduction or survival rates with the ability to return to previous behavioural states or activities once the impacts had ceased.

Minke whale and humpback whale

  1. There are no species-specific recovery rates for minke whale/humpback whale to TTS, however there is no evidence to suggest that recovery will be significantly different to harbour porpoise recovery rates. A recent study by Boisseau et al. (2021) reported that minke whale avoided a 15 kHz ADD and clearly react to signals at the likely upper limit of their hearing sensitivity. It is anticipated that minke whale would be able to tolerate the effect without any impact on reproduction or survival rates and is expected to return to previous behavioural states or activities once the impacts had ceased.

Grey seal

  1. Kastelein et al. (2018) measured recovery rates of harbour seal following exposure to a sound source of 193 dB re 1 μPa2s (SELcum) over 360 minutes and found that recovery from TTS to the pre-exposure baseline was estimated to be complete within 72 minutes following exposure. These results are in line with findings reported in SEAMARCO (2011), which showed that for small TTS values, recovery in seal species was very fast (around 30 minutes) and the higher the hearing threshold shift, the longer the recovery. Kastelein et al. (1995) also reported relatively fast recovery, with full hearing recovery within two hours following exposure.
  2. Considering the above, in most cases, impaired hearing for a short time is anticipated to have little effect on the total foraging period of a seal. If hearing is impaired for longer periods (hours or days) the impact has the potential to be ecologically significant (SEAMARCO, 2011). Nevertheless, the findings of studies presented in this section indicate that seal species are less vulnerable to TTS than harbour porpoise for the sound bands tested. It is also expected that animals would move beyond the injury range prior to the onset of TTS. The assessment considered that both grey seal and harbour seal are likely to be able to tolerate the effect without any impact on either reproduction or survival rates and would be able to return to previous behavioural states or activities once the impacts had ceased.
  3. All species considered are deemed to have some resilience to behavioural disturbance, high recoverability, and high international value. The sensitivity of the receptor to TTS is therefore, considered to be low.
Significance of the effect
Auditory injury (PTS)
  1. Although the preferred approach is the use of low order techniques to clear UXO ( Table 10.22   Open ▸ ), in the case that a low order technique results in a high order detonation (as per paragraph 270) conclusions presented in paragraph 314 et seq. are based on the assessment for high order clearance, which therefore presents a conservative assumption of project parameters (as discussed in paragraph 114).
  2. For bottlenose dolphin, white-beaked dolphin, minke whale, humpback whale and grey seal, overall, the magnitude of the impact (auditory injury) is deemed to be negligible and the sensitivity of all receptors is considered to be high. The effect will therefore be of minor adverse significance, which is not significant in EIA terms.
  3. For harbour porpoise the magnitude of the impact is deemed to be medium and the sensitivity of all receptors is considered to be high. The effect will therefore be of moderate adverse significance, which is significant in EIA terms. Secondary mitigation and residual significance is discussed in paragraph 318 et seq.
Behavioural disturbance (TTS as a proxy)
  1. As described for PTS in paragraph 313, the preferred approach is the use of low order techniques to clear UXOs, however in the case that a low order technique results in a high order detonation, the conclusion presented in paragraph 317 is based on the assessment for high order clearance.
  2. Overall, for all species the magnitude of the impact (behavioural disturbance) is deemed to be low and the sensitivity of all receptors is considered to be low. 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. If required, secondary mitigation (i.e. ADD with a duration over 30 minutes) will be applied to further reduce the potential for injury to harbour porpoise occurring during UXO clearance (detailed in Table 10.22   Open ▸ ). Final mitigation required will be addressed post consent, in consultation with stakeholders, following more detailed information such as the size, number and quality of UXOs to be cleared (following site-investigation surveys), noting that it may be possible to reduce the ADD activation period and soft start procedure depending on the size and number of UXOs located within the Array. Paragraph 320 et seq. therefore details a worked example for mitigation based on the most significant predicted effect, and focused on harbour porpoise (as this is the species with a potential residual risk of injury), which considers the different timescales that would be required to clear the injury zone if ADD and soft-start is required.
  2. As described in paragraph 269 et seq., low order techniques will be applied as the intended methodology for clearance of UXO, however there is a small risk that a low order clearance could result in high order detonation of UXO (as per paragraph 270). The secondary mitigation has been therefore tailored based on the size of the UXO and high order detonation scenario.
  3. A range of UXO munitions sizes have been considered for the purpose of determining effective mitigation measures, up to a maximum scenario of a UXO size of 698 kg. This approach follows a similar strategy to that which was taken for Seagreen 1 Offshore Wind Farm EPS Risk Assessment and MMMP (volume 4, appendix 22) (Seagreen Wind Energy Ltd, 2021).
  4. An outline MMMP (volume 4, appendix 22) has been developed for the purpose of mitigating the risk of auditory injury (PTS) to marine mammals from the proposed UXO clearance activities at the Array. This has been provided as a stand-alone document; however, this section provides an overview of the procedures for ADD and soft start, prior to making conclusions on the potential for residual effects and requirement for secondary mitigation.
  5. The designed in measures included as a part of the outline MMMP (volume 4, appendix 22) ( Table 10.22   Open ▸ ) are in line with JNCC guidelines for minimising the risk of injury to marine mammals from using explosives (JNCC, 2010b). Details of ADD use and soft start charge application are specific for each of the anticipated UXO sizes. As discussed in paragraph 318, prior to the commencement of UXO clearance works, a more detailed assessment will be produced including an evaluation of the most appropriate measures to employ particularly with respect to emerging evidence on the use of scare charges as the most widely applied approach alongside ADDs. The approach to mitigating injury to marine mammals involves the monitoring of a 1 km radius mitigation zone in line with current guidance (JNCC, 2010b). Monitoring will be carried out by suitably qualified and experienced personnel within a mitigation team, comprising of two dedicated MMOs2 and one dedicated PAM operator. The purpose of this monitoring is to clear the mitigation zone of marine mammals prior to detonation.
  6. Given the potential for auditory injury from high-order detonations for several marine mammal receptors (harbour porpoise, minke whale, humpback whale and grey seal) is at a greater range than can be mitigated by monitoring the 1 km zone ( Table 10.34   Open ▸ ), an ADD will be deployed to deter marine mammals to a greater distance before any detonation. The assessment of effects provided in paragraph 270 et seq. determine the auditory injury range based on high order detonation of a 698 kg NEQ UXO ( Table 10.34   Open ▸ ). At the time of writing, the actual number and size of the UXOs within the Array are unknown and therefore, the example secondary mitigation has been designed for a range of UXO munitions sizes so that the most appropriate approach can be applied to balance the risk of injury from UXO detonation with any additional noise introduced into the marine environment as deterrent measures. Details of ADD duration of activation is presented in the outline MMMP (volume 4, appendix 22).
  7. Swim speeds are summarised in Table 10.24   Open ▸ along with the source papers for the assumptions. Therefore, the duration of the application of the ADD prior to UXO detonation will determine whether the animal can move out of the injury zone prior to UXO detonation ( Table 10.24   Open ▸ ). Activation of an ADD will commence within the 60 minutes pre-detonation search, providing no marine mammals have been observed within the mitigation zone for a minimum of 20 minutes.
  8. Example deterrence distances are provided for all marine mammal IEFs in Table 10.42   Open ▸ . Summaries provided in this paragraph refer to harbour porpoise only, as the species with the largest PTS ranges ( Table 10.34   Open ▸ ).
  9. Based on the UXO clearance flow chart ( Figure 10.23   Open ▸ ; informed by Seagreen Wind Energy Ltd, 2021), for low order UXO size up to 0.25 kg NEQ, the required time of ADD activation is 12 minutes and this is expected to displace harbour porpoise to 1,080 m (exceeding the PTS distance of 1,050 m). If UXO size of up 0.5 kg NEQ is identified during the survey, then ADD will be activated for 15 minutes and this is expected to deter harbour porpoise to 1,350 m. For all other species, three minutes of ADD would be sufficient to deter the animals from the injury zone.
  10. However, for high order UXO clearance, injury ranges are larger. Assuming the ADD is activated for an indicative 60 minutes ( Table 10.42   Open ▸ ), the displacement distance for harbour porpoise would be 5,400 m, meaning there is a need to deter harbour porpoise from larger ranges that cannot be achieved using an ADD for 60 minutes duration alone (i.e. the injury zone exceeds 5,400 m). However, for all other species, a duration of 60 minutes ADD activation will be sufficient to deter animals from the injury zone up to the 698 kg NEQ ( Table 10.42   Open ▸ ).
  11. For high order UXO, to reduce the risk of PTS, there is a need to deter animals from larger ranges than can achieved using an ADD alone. Therefore, following an ADD activation period of up to 60 minutes, a ‘soft start’ will be undertaken, using a sequence of small explosive charges, detonated at five minutes intervals, over a total of maximum 20 minutes ( Table 10.42   Open ▸ , Figure 10.23   Open ▸ ). It is expected that up to 80 minutes of combined ADD/soft start procedure (up to 60 minutes of ADD and 20 minutes of soft start) will displace harbour porpoise to ranges of 7,200 m. Whilst this secondary mitigation is considered to be sufficient to deter most animals (noting that use of ADD alone deterred all other species from the injury zone), there may be a residual effect for harbour porpoise for this largest UXO size, as the maximum predicted PTS impact range for this species was 10,000 for the 227 kg NEQ and 14,580 m for 698 kg NEQ ( Table 10.34   Open ▸ ).

Table 10.42:
Recommended ADD Duration for Low Order and High Order UXO Clearance and Sizes, and Associated Displacement Distance

Table 10.42: Recommended ADD Duration for Low Order and High Order UXO Clearance and Sizes, and Associated Displacement Distance

Figure 10.23:
High Order UXO Clearance Mitigation Flow Chart for the Array (based upon Seagreen Wind Energy Ltd, 2021)

Figure 10.23: High Order UXO Clearance Mitigation Flow Chart for the Array (based upon Seagreen Wind Energy Ltd, 2021)


  1. The indicative analysis presented in Table 10.42   Open ▸ suggests that for UXO sizes of up to 698 kg, pre-detonation search and use of 30 minutes of ADD will be sufficient to reduce the potential of experiencing PTS by bottlenose dolphin, white-beaked dolphin, minke whale, and grey seal to negligible magnitude.
  2. However, for harbour porpoise, it has been estimated that harbour porpoises could potentially experience an auditory injury at distances that cannot be fully mitigated by application of ADD and soft start charges. The maximum mitigation zone has been assessed as 7,200 m and PTS range for this species has been modelled as 14,580 m.
  3. To assess the residual effect, the average and maximum number of animals that may potentially be present within an area of 501 km2 (difference between the area across which effects could be mitigated and area of effect) could be calculated using harbour porpoise density range ( Table 10.34   Open ▸ ). However, this approach is considered likely to lead to an overestimate and may result in unrealistic predictions for the numbers of animals potentially injured. For example, for highly impulsive sounds such as piling, at ranges from the source in the order of tens of kilometres, the sound changes from being impulsive in character to being non-impulsive 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). Empirical evidence has suggested such shifts in impulsivity could occur markedly within 10 km from the sound source (Hastie et al., 2019). Since the precise range at which this transition occurs is unknown (not least because the transition also depends on the response of the marine mammals’ ear), sound models still adopt the impulsive thresholds at all ranges and this is likely to lead to an overly precautionary estimate of injury ranges at larger distances (tens of kilometres) from the sound source. It is noted defining this transition range is an active area of research and scientific debate, with a number of other potential methods being investigated. Furthermore, at even greater ranges, the sound will not only be non-impulsive but can be characterised as being continuous (i.e. each pulse will merge into the next one ad therefore is considered that any predicted injury ranges in the tens of kilometres are almost certainly an overly precautionary interpretation of existing criteria (Southall et al., 2021)
  4. There is also a likelihood that the range over which the animals are anticipated to be displaced during 60 minutes of ADD plus application of soft start charges is underestimated ( Table 10.42   Open ▸ ). Firstly, strong and far-reaching responses to an ADD have been recorded by Thompson et al. (2020) at approximately 10 km to the ADD source. Moreover, to assess the range of 7,200 m, an average harbour porpoise swim speed has been applied (i.e. 1.5 m/s). However various scientific papers provided significantly faster speeds with a maximum speed of 4.3 m/s and 6.2 m/s cited by Otani et al. (2000) and Leatherwood et al. (1988), respectively.
  5. For harbour porpoise, it is expected that small numbers of animals could potentially be exposed to PTS. Given that details about UXO clearance technique to be used and charge sizes will not be available until after the consent is granted (following a pre-construction UXO survey), it is not possible to quantify the effects of UXO detonations and therefore a residual number of animals potentially impacted is not presented within this chapter. At a later stage, when details about UXO sizes and specific clearance techniques to be used become available, it will be possible to tailor the secondary mitigation to specific UXO sizes following the UXO survey and species to reduce the risk of injury.
  6. Therefore, prior to the commencement of UXO clearance works, appropriate secondary mitigation measures will be discussed with stakeholders and proposed as a part the final MMMP for UXO clearance works (refer to volume 4, appendix 22 for outline MMMP). It is therefore anticipated that following receipt of more detail regarding size and number of UXO (and tailoring of secondary mitigation measures as described above), the magnitude of this impact will be reduced to low for harbour porpoise.
Auditory injury (PTS)
  1. For all species excluding harbour porpoise, no marine mammal mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.
  2. For harbour porpoise, following secondary mitigation measures, tailored once a more detailed understanding of the size and number of UXO is available, will be discussed with stakeholders and proposed as part of the final MMMP (volume 4, appendix 22), the magnitude of the impact is deemed to be low and the sensitivity of the receptors is considered to be high. Given that only a small proportion of the population could be potentially injured (PTS), the effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Behavioural disturbance (TTS as a Proxy)
  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 Due to Site-Investigation surveys (including geophysical surveys)

  1. Site-investigation surveys during the construction and operation and maintenance phases have the potential to cause direct or indirect effects (including injury or disturbance) on marine mammal receptors ( Table 10.17   Open ▸ ).
  2. A detailed underwater noise modelling assessment has been carried out to investigate the potential for injurious and behavioural effects on marine mammals as a result of geophysical and geotechnical surveys, using the latest criteria (volume 3, appendix 10.1). Several sonar-like sources will potentially be used for the geophysical surveys, including MBES, SSS, SBP and UHRS. The equipment likely to be used can typically work at a range of signal frequencies, depending on the distance to the seabed and the required resolution. For sonar-like sources the signal is highly directional, acts like a beam and is emitted in pulses. Sonar-based sources are considered by the NMFS (2018) as continuous (non-impulsive) because they generally comprise a single (or multiple discrete) frequency. Unlike the sonar-like survey sources, the UHRS is likely to utilise a sparker, which produces an impulsive, broadband source signal. Additionally, MAG will be used to measure and detect anomalies within the existing magnetic field. The survey parameters, such as source SEL, used in the underwater noise modelling are presented in detail in volume 3, appendix 10.1. For geotechnical surveys, potential equipment to be used may include CPT, vibrocore, piston core, box core and borehole ( Table 10.17   Open ▸ ).
  3. The site-investigation surveys as listed in Table 10.17   Open ▸ for the construction phase will involve the use of up to four survey vessels with up to 50 vessel movements in total. The site-investigation surveys will be carried out over five months within a three year period.

                        Construction phase

Magnitude of impact
Auditory injury (PTS)
  1. As detailed in volume 3, appendix 10.1, Injury ranges for impulsive survey sources (UHRS, CPT) are based on a comparison to the Southall et al. (2019) thresholds for impulsive noise (with the distances presented in brackets for SPLpk thresholds) whereas non-impulsive survey sources (MBES, SSS, SBP, borehole, vibrocore) results are compared against the non-impulsive thresholds. Please note that for impulsive noise, the injury ranges were larger for the SELcum metric compared to SPLpk ( Table 10.43   Open ▸ , Table 10.44   Open ▸ ).
  2. The maximum injury (PTS) range across all geophysical surveys was estimated as 310 m for harbour porpoise due to SBP activity ( Table 10.43   Open ▸ ). For bottlenose dolphin, white-beaked dolphin, minke whale, humpback whale and grey seal the maximum PTS is expected to occur out to 75 m ( Table 10.43   Open ▸ ). However, it should be noted that as sonar-like sources have very strong directivity (as detailed in volume 3, appendix 10.1), there is only potential for injury when a marine mammal is directly underneath the noise source. Once the animal moves outside of the main beam, there is no potential for injury.
  3. With respect to the ranges within which there is a potential of PTS occurring to marine mammals as a result of geotechnical investigation activities, PTS threshold was not exceeded for all marine mammal species, except harbour porpoise ( Table 10.44   Open ▸ ). Harbour porpoises are at risk of potential injury within 45 m from the noise source during the CPT activity ( Table 10.44   Open ▸ ).
  4. The number of marine mammals potentially injured within the modelled PTS ranges ( Table 10.43   Open ▸ , Table 10.44   Open ▸ ) were estimated using species-specific density estimates ( Table 10.45   Open ▸ ). Given that the potential PTS ranges are relatively low, no more than one animal of each species is deemed to be at risk of experiencing PTS across all types of geophysical and geotechnical surveys ( Table 10.45   Open ▸ ). The auditory injury (PTS) 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 10.43:
Potential Injury (PTS) Impact Ranges (m) For Geophysical Site-Investigation Surveys (N/E = Threshold Not Exceeded, Comparison to Ranges for SPLpk Where Threshold was Exceeded Shown in Brackets)

Table 10.43: Potential Injury (PTS) Impact Ranges (m) For Geophysical Site-Investigation Surveys
(N/E = Threshold Not Exceeded, Comparison to Ranges for SPLpk Where Threshold was Exceeded Shown in Brackets)

 

Table 10.44:
Potential Injury (PTS) Impact Ranges (m) For Geotechnical Site-Investigation Surveys (N/E = Threshold Not Exceeded, Comparison to Ranges for SPLpk Where Threshold was Exceeded Shown in Brackets)

Table 10.44: Potential Injury (PTS) Impact Ranges (m) For Geotechnical Site-Investigation Surveys
(N/E = Threshold Not Exceeded, Comparison to Ranges for SPLpk Where Threshold was Exceeded Shown in Brackets)

 

Table 10.45:
Estimated Number of Animals With the Potential To Experience Injury (PTS) During Geophysical and Geotechnical Site-Investigation Surveys (Number of Animals Based on SPLpk Where Threshold was Exceeded Shown in Brackets)

Table 10.45: Estimated Number of Animals With the Potential To Experience Injury (PTS) During Geophysical and Geotechnical Site-Investigation Surveys (Number of Animals Based on SPLpk Where Threshold was Exceeded Shown in Brackets)

 

  1. The site-investigation surveys are considered to be short term as they will take place over a period of five months. In line with good practice guidance, designed in measures during geophysical surveys will involve the use of MMOs2 and PAM so that the risk of injury over the defined mitigation zone is reduced (JNCC, 2017). The largest PTS range was estimated as 310 m for SBP and it is considered that standard industry measures will be effective at reducing the risk of injury over this distance (JNCC, 2017).
  2. The impact (elevated underwater noise during site-investigation surveys) is predicted to be of local spatial extent, short term duration, intermittent and, although the impact itself is reversible (i.e. the elevation in underwater noise only occurs during surveys), the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Since the 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 for all receptors.
Behavioural disturbance
  1. For impulsive noise sources (UHRS, CPT) the underwater noise modelling adopted the NMFS (2005) thresholds of 140 dB re 1 µPa for mild disturbance and 160 dB re 1 µPa for strong disturbance. For non-impulsive noise sources (MBES, SSS, SBP, borehole, vibrocore) the underwater noise modelling used the NMFS (2005) threshold of 120 dB re 1 µPa. The underwater noise modelling predicted that the behavioural effects as a result of site-investigation surveys can occur within a range of between 27 m for borehole drilling and up to 9,101 m for vibrocoring ( Table 10.46   Open ▸ ).
  2. For impulsive noise sources (UHRS, CPT) the strong behavioural disturbance ranges vary from 80 m during UHRS to 140 m during CPT ( Table 10.46   Open ▸ ). Qualitatively, no more than one animal of each species would be at risk of experiencing strong behavioural disturbance. Mild disturbance may occur within 565 m during UHRS to 1,330 m during CPT and for all species ( Table 10.46   Open ▸ ), except harbour porpoise, no more than one animal could be affected ( Table 10.47   Open ▸ ). Up to four harbour porpoises could experience mild behavioural disturbance during CPT ( Table 10.47   Open ▸ ), however, such low level disturbance could lead to mild disruptions of normal behaviours, but prolonged or sustained behavioural effects, including displacement are unlikely to occur.
  3. For non-impulsive noise sources (MBES, SSS, SBP, borehole drilling, vibrocore), the maximum behavioural disturbance ranges vary from 27 m to the maximum 9,101 m for vibrocoring ( Table 10.46   Open ▸ ). Qualitatively, no more than one animal is predicted to be disturbed during MBES, SSS and borehole drilling. With the use of SBP, up to four harbour porpoises are at risk of experiencing disturbance and up to two grey seals. Due to relatively large disturbance ranges predicted for vibrocoring, based on conservative species-specific densities, up to 170 harbour porpoises could experience disturbance ( Table 10.47   Open ▸ ). Vibrocoring may also lead to disturbance of up to one bottlenose dolphin, 32 white-beaked dolphins, eight minke whales and 47 grey seals ( Table 10.47   Open ▸ ). However, for those animals disturbed, there is likely to be a proportional response, e.g. not all animals will be disturbed to the same extent. There is no dose-response curve available to apply in the context of site-investigation surveys, however, Joy et al. (2019) derived a dose-response for killer whales and underwater noise from vessels, indicating that marine mammals display a proportional response to non-impulsive noise. It is important to note that the life history of an individual and the context will also influence the likelihood of an individual to exhibit an aversive response to noise. Furthermore, this threshold does not take into account of ambient sound levels in the area, which may be already be above the 120 dB re 1 μPa (Farcas et al., 2020). Considering that the underwater noise modelling used a single threshold that does not take into account the ambient noise, the numbers of animals potentially disturbed presented for vibrocore and other site-investigation surveys are likely to be an overestimate.
  4. The behavioural disturbance ranges will not overlap with any known important areas for any of the species, e.g. Southern North Sea SAC (harbour porpoise), CES2 MU (bottlenose dolphin), Southern Trench ncMPA (minke whale), Berwickshire and North Northumberland SAC (grey seal) ( Table 10.15   Open ▸ , Figure 10.3   Open ▸ ).

 

Table 10.46:
Potential Disturbance Ranges For Geophysical and Geotechnical Site-Investigation Surveys

Table 10.46: Potential Disturbance Ranges For Geophysical and Geotechnical Site-Investigation Surveys

 

Table 10.47:
Estimated Number of Animals With the Potential To Be Disturbed During Geophysical and Geotechnical Site-Investigation Surveys

Table 10.47: Estimated Number of Animals With the Potential To Be Disturbed During Geophysical and Geotechnical Site-Investigation Surveys

 

  1. The impact (elevated underwater noise during site-investigation surveys) is predicted to be of local to regional spatial extent, short term duration, intermittent and the effect of behavioural disturbance is of high reversibility (with animals returning to baseline levels soon after surveys have ceased). It is predicted that the impact will affect the receptor directly. Whilst there may be effects at an individual level, these are not predicted to be at a scale that would lead to any population-level effects. The magnitude was therefore considered to be low.
Sensitivity of the receptor
Auditory injury
  1. For geotechnical surveys, injury to marine mammals is unlikely to occur beyond a few tens of metres ( Table 10.44   Open ▸ ) and sound from vessels themselves is likely to deter marine mammals beyond this range. For geophysical surveys, the maximum range for PTS (SBP) is 310 m ( Table 10.43   Open ▸ ). Sills et al. (2020) evaluated TTS onset levels for impulsive sound in seals following exposure to underwater sound from a seismic air gun. The study found that transient shifts in hearing thresholds at 400 Hz were apparent following exposure to four to ten consecutive pulses (SELcum 191 dB dB re 1µPa2s to 195 dB re 1µPa2s; 167 dB dB re 1µPa2s to 171 dB re 1µPa2s with frequency weighting for PCW). Matthews et al. (2020) used a modelling approach to compare potential effects of a non-impulsive sound source (marine vibriosis (MV)) and impulsive seismic sources (air gun) on marine mammals and found few marine mammals could be expected to be exposed to potentially injurious sound levels for either source type, but fewer were predicted for MV arrays than air gun arrays. The estimated number of animals exposed to sound levels depended on the choice of evaluation criteria. When using Sound Pressure Level (SPL), more behavioural disturbance was predicted for MV arrays compared to air gun arrays. However, the opposite was observed when using frequency-weighted sound fields and a multiple-step, probabilistic, threshold function. Matthews et al. (2020) therefore highlighted the two metrics relate to different characteristics of both impulsive and continuous sound (e.g. SELcum looks at accumulative exposure over a set duration whilst SPLpk measures acute exposure to high-amplitude sound).
  2. More recently, Ruppel et al. (2022) categorised marine acoustic sources into four tiers based on their potential to injure marine mammals using physical criteria about the sources (e.g. source level, transmission frequency, directionality, beamwidth, and pulse repetition rate). Those in Tier Four were considered unlikely to result in ‘incidental take’ (i.e. loss of individuals) of marine mammals and therefore termed de minimis, and included most high-resolution geophysical sources (MBES, SSS, SBP, low powered sparkers). For context, Tier 1 refers to high-energy airgun surveys with a total volume larger than 1500 in3 or arrays with more than 12 airguns, Tier 2 covers the remaining low/intermediate energy airgun and Tier 3 covers most non-airgun seismic sources, which either have characteristics that do not meet the de minimis category (e.g., some sparkers) or could not be fully evaluated in Ruppel et al. (2022) (e.g., bubble guns, some boomers).The study also suggested surveys that simultaneously deploy multiple, non-impulsive de minimis sources are unlikely to result in incidental take of marine mammals.
  3. All receptors are deemed to have limited resilience to PTS, low recoverability and high international value. The sensitivity of the receptor is therefore considered to be high.
Behavioural disturbance
  1. It is widely recognised that the transmission frequencies of commercial sonar systems (approximately 12 kHz to 1800 kHz) overlap with the hearing ranges of many marine mammal species (Richardson et al., 1995). Many frequencies associated with sonar systems are very high and have peak frequencies well above marine mammal hearing ranges, however it is possible that relatively high levels of sound are also produced as sidebands at lower frequencies (Hayes and Gough, 1992) and therefore may result in behavioural responses.
  2. A study undertaken by (Hermannsen et al., 2015) confirmed that there are substantial medium-to-high frequency components in airgun pulses, when reporting the source characteristics and propagation of broadband pulses (10 Hz up to 120 kHz) from a small airgun. These findings suggest that small odontocetes and seals could be affected by even a single airgun. However, Ruppel et al. (2022) reported that in response to sonar-like sound sources (e.g. MBES, singlebeam echosounder (SBES)) marine mammals may show subtle behavioural responses although species, behavioural context, location, and prey availability are likely to play more of a role than the acoustic signals themselves. In a study undertaken by MacGillivray et al. (2014) seven acoustic sources (including air guns, SBP, MBES and SSS) were compared and documented the sound level above hearing threshold as a function of horizontal distance. Weighting sounds according to hearing sensitivity allows assessment of relative risks associated with exposure and whilst this analysis did not directly relate to potential for behavioural responses, it allowed comparison of modelled acoustic sources. The modelling undertaken in MacGillivray et al. (2014) suggested that odontocetes were most likely to hear sounds from mid-frequency sources (such as fisheries, communication, and hydrographic systems), whilst mysticetes, were most likely to hear sounds from low frequency sources (SBP and airguns), and pinnipeds from both mid and low frequency sources. For all species included within the study, modelled sensation levels were lowest for the high frequency sources (e.g. SSS and MBES) which operate at the upper limits of the audible spectrum.
  3. A recent study by Kates Varghese et al. (2021) on MBES surveys showed that the only marine mammal metric that was identified as changing was vocalisation rate, with neither changes in displacement nor foraging being observed. Similarly, Quick et al. (2017) reported that tagged short-finned pilot whale Globicephala macrorhynchus that were exposed to a SBES did not change their foraging behaviour, but variance in directionality of movement was observed, suggesting increased vigilance while the SBES was active. It was however stated that the range of behaviours exhibited could not be directly attributed to SBES operation, and that changes in behaviour were unlikely to be biologically significant. A study by Cholewiak et al. (2017) investigated the impact of SBES on toothed whales and reported that fewer beaked whale vocalisations were recorded when the source was actively transmitting. This suggested that animals either move away from the area or reduced foraging activity (although findings were not statistically significant).
  4. Many studies to date have focussed on the effects of multi-array seismic surveys on marine mammals, and therefore there is less widely available evidence for behavioural responses to seismic sources (e.g. MBES, SSS, SBPs). Multi-array impulsive sound sources are broadband in character (i.e. produce sound across a wide range of frequencies), unlike seismic sources which typically produce more tonal sound either at a discrete frequency or a range of discrete frequencies. However, findings from studies of multi-array impulsive sources may be useful in supporting predictions of behavioural responses of marine mammals to geophysical survey sources in general, given the overlap of parameters that typically characterise sound sources (i.e. transmission frequency; source level; pulse duration) (see MacGillivray et al. (2014), Ruppel et al. (2022)). Whilst evidence on the behavioural responses to MBES is limited, an Independent Scientific Review Panel deemed a 12 kHz MBES to be the most plausible trigger for an extreme behavioural response in melon-headed whale Peponocephala electra, which resulted in a mass group stranding in a shallow lagoon in Madagascar in 2008 (Southall et al., 2013) (an area where such open-ocean species would not usually frequent). Whilst an unequivocal cause and effect relationship between MBES and the strandings cannot be concluded, the paper states that intermittent, repeated sounds of this nature could present a salient and potential aversive stimulus and suggests potential for such behavioural responses (or indirect injury) from MBES should be considered in environmental assessments (Southall et al., 2013).
  5. van Beest et al. (2018) used fine-scale data from harbour porpoise equipped with high-resolution location and dive loggers when exposed to airgun pulses at ranges of 420 m to 690 m with sound level estimates of 135 dB re 1µPa2s to 147 dB re 1µPa2s (SEL). They showed different responses to sound exposure, with one individual displayed rapid and directed movements away from the exposure site whilst two individuals used shorter and shallower dives (compared to natural behaviour) immediately after exposure. This sound-induced movement typically lasted for eight hours or less, with an additional 24 hour recovery period until natural behaviour was resumed (van Beest et al. (2018)).
  6. Results from 201 seismic surveys in the UK and adjacent waters demonstrated that cetaceans (including bottlenose dolphin, white-beaked dolphin and minke whale) can be disturbed by seismic exploration (Stone and Tasker, 2023), with small odontocetes showing strongest lateral spatial avoidance, moving out of the area, whilst mysticetes and killer whale showed more localised spatial avoidance, orienting away from the vessel and increasing distance from source but not leaving the area completely.
  7. A recent study by Sarnocińska et al. (2020) indicated temporary displacement or change in harbour porpoise echolocation behaviour in response to a 3D seismic survey in the North Sea. No general displacement was detected from 15 km away from any seismic activity but decreases in echolocation signals were detected up to 8 km – 12 km from the active airguns. Considering findings of other studies ((Dyndo et al., 2015, Tougaard et al., 2015) harbour porpoise disturbance ranges due to airgun sound are predicted to be smaller than to piling sound at the same energy. The reason for this is that the perceived loudness of the airgun pulses is predicted to be lower than for piling sound due to less energy at the higher frequencies where porpoise hearing is better (Sarnocińska et al., 2020). Likewise, Thompson et al. (2013) used PAM and DAS to study changes in the occurrence of harbour porpoise across a 2,000 km2 study area during a commercial two-dimensional seismic survey in the North Sea. The study found acoustic detections decreased significantly during the survey period in the impact area compared with a control area, but this effect was small in relation to natural variation. Animals were typically detected again at affected sites within a few hours, and the level of response declined through the survey period (ten days) suggesting exposure led to some tolerance of the activity (Thompson et al., 2013). Thompson et al. (2013) therefore suggested that prolonged seismic survey sound did not lead to broader-scale displacement into sub-optimal or higher risk habitat. Similarly, a ten-month study of overt responses to seismic exploration in humpback whale, sperm whale Physeter macrocephalus and Atlantic spotted dolphin Stenella frontalis, demonstrated no evidence of prolonged or large scale displacement of each species from the region during the survey (Weir, 2008).
  8. Regarding grey seal, behavioural response tests to two sonar systems (200 kHz and 375 kHz systems) have been carried out on grey seal at the SMRU seal holding facility (Hastie et al., 2014). Results showed that both systems had significant effects on seal behaviour, with significantly more time spent hauled out during the 200 kHz sonar operation and although animals remained swimming during operation of the 375 kHz sonar, they were distributed further from the sonar.
  9. Aside from displacement or avoidance, other behavioural responses have been demonstrated (Wright and Cosentino, 2015). Responses to seismic surveys have included cessation of singing (Melcón et al., 2012) and alteration of dive and respiration patterns which may lead to energetic burdens on the animals (Gordon et al., 2003). In some cases, behavioural responses may lead to greater effects, such as strandings (Cox et al., 2001, Tyack et al., 2006) or interruptions to migration (Heide-Jørgensen et al., 2013). However such responses are highly context-dependent and variable, contingent on factors such as the activity of the animal at the time (Robertson, 2014), prior experience to exposure (Andersen et al., 2012), extent or type of disturbance (Melcón et al., 2012), environment in which they inhabit (Heide-Jørgensen et al., 2013) and the type of survey.
  10. It is expected that, to some extent, marine mammals will be able to withstand temporary elevated levels of underwater sound during site-investigation surveys and behavioural responses are highly species and context specific (as evidenced in paragraphs 359 to 363).
  11. All receptors are deemed to have some resilience to behavioural disturbance, high recoverability and high international value. The sensitivity of the receptor is therefore, considered to be medium.
Significance of the effect
Auditory injury
  1. Overall, for all IEFs, 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
  1. Overall, for all IEFs, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
  1. The PTS thresholds are not exceeded for most surveys and for most species. This is with the exception of cone penetration testing where the PTS range is so small (45 m predicted for harbour porpoise only) that it is considered that animals are likely to be deterred beyond this range (i.e. out to 3,259 m) by the vessel noise itself (see Table 10.49   Open ▸ ). Additionally, as a part of designed in measures ( Table 10.22   Open ▸ ) standard mitigation from JNCC (2017) will be adhered to for the geophysical surveys, which will involve the use of MMOs2/PAM monitoring of a standard 500 m mitigation zone for a period of up to 30 minutes prior to the start of surveys ( Table 10.22   Open ▸ ). Soft starts will be applied for electromagnetic equipment (such as SBP and SSS) as well as seismic sources (UHRS).
  2. No secondary marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in paragraph 368 and in Table 10.22   Open ▸ is not significant in EIA terms.

                        Operation and maintenance phase

Magnitude of impact
  1. Elevated underwater noise generated during the site-investigation surveys may lead to injury and/or disturbance to marine mammals during the operation and maintenance phase. The MDS comprises of routine geophysical surveys such as MBES and SBP ( Table 10.17   Open ▸ ), which will take place once every 24 months for wind turbines and OSP foundations, as well as wind turbines interior and exterior and annually for the first 3 years, then every 24 months for inter-array cables and interconnector cables ( Table 10.17   Open ▸ ). Duration of each geophysical survey campaign will be up to 3 months ( Table 10.17   Open ▸ ).
  2. The potential impacts from auditory injury due to elevated underwater noise during site-investigation surveys is described in paragraph 342 et seq. for the construction phase and has not been reiterated here for the operation and maintenance phase. Similarly, the magnitude of potential impacts for behavioural disturbance to marine mammals is described in paragraph 347 et seq. In terms of behavioural disturbance, although the underwater noise from geophysical surveys could result in a negligible alteration to the distribution of marine mammals, these surveys are anticipated to be short term in nature, targeted to localised areas and occur intermittently over the operation and maintenance phase. Therefore, the impact is likely to be the same or less (due to highly targeted short surveys), than the impact assessed in the construction phase.
  3. For injury, the impact (elevated underwater noise during the geophysical surveys) is predicted to be of local spatial extent within the relevant geographic range of reference, short term duration, intermittent and the effect of PTS is permanent. It is predicted that the impact will affect the receptor directly. Since the 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 for PTS was therefore considered to be negligible.
  4. For the behavioural disturbance, the impact (elevated underwater noise during the geophysical surveys surveys) is predicted to be of local to regional spatial extent within the relevant geographic range of reference, short term duration, intermittent and the effect of behavioural disturbance is reversible (with animals returning to baseline levels soon after surveys have ceased). It is predicted that the impact will affect the receptor directly. Whilst there may be effects at an individual level, these are not predicted to be at a scale that would lead to any population-level effects. The magnitude was therefore considered to be low.
Sensitivity of the receptor
  1. The sensitivity of the receptors during the operation and maintenance phase is not expected to differ from the sensitivity of the receptors during the construction phase. Therefore, the sensitivity of marine mammal receptors to elevated underwater noise during site-investigation surveys (auditory injury and behavioural disturbance) is as described previously in paragraph 352 et seq., where it has been assessed as high for auditory injury and medium for behavioural disturbance.
Significance of the effect
Auditory injury (PTS)
  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
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be medium. The effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
  1. No marine mammal mitigation is considered necessary therefore because the likely effect in the absence of mitigation (beyond designed in measures) is not significant in EIA terms.

Injury and disturbance from underwater noise generated during vessel use and other noise producing activities

  1. Increased vessel movements and other noise producing activities during the construction, operation and maintenance, and decommissioning phases have the potential to result in a range of effects to marine mammals such as injury, avoidance behaviour or displacement and masking of vocalisations or changes in vocalisation rate.
  2. The assessment of LSE1 from elevated underwater noise due to vessel use and other (non-piling) sound producing activities is based on a vessel and/or activity basis, considering the maximum injury/disturbance range as modelled in volume 3, appendix 10.1. However, it should be noted that several activities could be potentially occurring at the same time and therefore ranges of effects may extend from several vessels/locations where the activity is carried out and potentially overlap.