5.3. Assessment of the Adverse Efects of the Array Alone

5.3.1. Underwater Noise Generated during Piling and UXO Clearance

152. The LSE2 assessment during the HRA Stage One process identified that during the construction phase, LSE2 could not be ruled out for the impact of underwater noise generated during piling and UXO clearance. This relates to the following sites and relevant Annex II diadromous fish features:

• River Dee SAC;

–           Atlantic salmon; and

–           freshwater pearl mussel.

• River South Esk SAC;

–           Atlantic salmon; and

–           freshwater pearl mussel.

• Tweed Estuary SAC;

–           sea lamprey.

• River Tweed SAC;

–           Atlantic salmon; and

–           sea lamprey.

• River Tay SAC;

–           Atlantic salmon; and

–           sea lamprey.

• River Spey SAC;

–           Atlantic salmon;

–           freshwater pearl mussel; and

–           sea lamprey.

• Berriedale and Langwell Waters SAC;

–           Atlantic salmon.

• River Teith SAC; and

–           Atlantic salmon; and

–           sea lamprey.

• River Oykel SAC;

–           Atlantic salmon; and

–           freshwater pearl mussel.

153. The MDS and designed in measures considered for the assessment of underwater noise during piling and UXO clearance are shown in Table 5.3   Open ▸ and Table 5.4   Open ▸ , respectively.

Table 5.3: MDS Considered for the Assessment of Potential Impacts to Annex II Diadromous Fish due to Underwater Noise Generated during Piling and UXO Clearance in the Construction Phase of the Array Alone

Table 5.4: Designed In Measures Considered for the Assessment of Potential Impacts to Annex II Diadromous Fish to Underwater Noise Generated during Piling and UXO Clearance in the Construction Phase

                        Information to support the assessment

                        Hearing sensitivity of Annex II diadromous fish

154. Underwater noise can potentially have an adverse impact on various fish species ranging from physical injury and mortality to behavioural effects. Peer reviewed guidelines have been published by the Acoustical Society of America (ASA) and provide directions and recommendations for setting criteria (including injury and behavioural criteria) for fish. These guidelines (Popper et al., 2014) provide the most relevant and best available guidelines for impacts of underwater noise on fish species (see volume 3, appendix 10.1 of the Array EIA Report for further detail).
155. The Popper et al. (2014) guidelines broadly group fish into the following categories according to the presence or absence of a swim bladder and on the potential for that swim bladder to improve the hearing sensitivity and range of hearing:

• Group 1: Fishes lacking swim bladders (e.g. elasmobranchs and flatfish). These species are only sensitive to particle motion, not sound pressure and show sensitivity to only a narrow band of frequencies;
• Group 2: Fishes with a swim bladder but the swim bladder does not play a role in hearing (e.g. salmonids and some Scombridae). These species are considered to be more sensitive to particle motion than sound pressure and show sensitivity to only a narrow band of frequencies;
• Group 3: Fishes with swim bladders that are close, but not connected, to the ear (e.g. gadoids and eels). These fishes are sensitive to both particle motion and sound pressure and show a more extended frequency range than Groups 1 and 2, extending to about 500 Hz; and
• Group 4: Fishes that have special structures mechanically linking the swim bladder to the ear (e.g. clupeids such as herring Clupea harengus, sprat Sprattus sprattus and shads Alosa spp.). These fishes are sensitive primarily to sound pressure, although they also detect particle motion. These species have a wider frequency range, extending to several kHz and generally show higher sensitivity to sound pressure than fishes in Groups 1, 2 and 3.

156. Sea lamprey are considered to be a Group 1 species, and therefore has relatively low sensitivity to underwater noise (Popper et al., 2014). Lamprey species are known to have relatively simple ear structures (Popper et al., 1987), with very few responses to auditory stimuli noted overall (Popper, 2005), except a slight swimming speed increase and decrease in resting behaviour when exposed to continuous low frequency noise of 50 to 200 Hz (Mickle et al., 2018). This suggests a low vulnerability to impacts of noise overall. In contrast, Group 4 hearing specialist fish, such as herring, possess an otic bulla; a gas filled sphere that is connected to the swim bladder, which enhances hearing ability. This anatomy is not present in sea lamprey, although the gas filled swim bladder in Atlantic salmon may be involved in their hearing capability (Popper et al., 2014). While there is no direct link to the inner ear, Atlantic salmon are able to detect lower noise frequencies and as such are considered to be a Group 2 species, and therefore have a higher hearing sensitivity to sea lamprey, but comparatively low with respect to Group 3 and Group 4 species (Popper et al., 2014).
157. Freshwater pearl mussel may only be indirectly affected by underwater noise as they are a freshwater-resident species, and piling will only occur within the site boundary, which is around 80 km offshore. Therefore, the effects of underwater noise upon freshwater pearl mussel assessed in this section are limited to indirect effects, due to potential disruption to Atlantic salmon migration.

                        Overview of underwater noise modelling conducted for the Array

158. Piling and UXO clearance activities may lead to injury and/or disturbance to Annex II diadromous fish species. The MDS ( Table 5.3   Open ▸ ) considers the reasonable worst case scenario from underwater noise generated during piling based on the greatest hammer energy. This scenario is represented by the installation of up to 265 semi-submersible floating wind turbine foundations, with up to six anchors per foundation and one 4.5 m diameter pile per anchor (1,590 piles) for wind turbines, and up to three large and 12 small jacket foundations (total 216 piles) for OSPs, with all piles assumed to be installed via impact piling as the most precautionary scenario.
159. For wind turbines, piling was assumed to take place over a period of up to eight hours per pile with up to eight piles installed in each 24 hour period. OSP foundations will take place over 25 hours for up to three piles (maximum duration of up to eight hours per pile) with up to eight piles installed in each 24 hour period. A maximum duration of 1,728 hours of piling activity, over a maximum of 72 months over eight years, may take place during the construction phase, based on the maximum duration of the piling phase.
160. Underwater noise modelling was undertaken for both single piling and concurrent piling (i.e. piling at more than one location simultaneously). To ensure a precautionary assessment, modelling of a concurrent piling scenario based on a 3,000 kJ hammer energy for the wind turbine foundation piles and 4,400 kJ hammer energy for the OSP jacket piles has been undertaken, alongside single piling scenarios, using the maximum 4,400 kJ hammer energy for the OSP jacket piles. These are discussed further below in relation to injurious effects with relevant contours also presented and discussed in the context of potential behavioural effects on Annex II diadromous fish (specifically disruption and barriers to migration). 
161. If required, UXO clearance (including detonation) will be completed prior to the construction phase (pre-construction). The MDS ( Table 5.3   Open ▸ ) assumes clearance of up to 15 UXOs within the site boundary, with a maximum of 698 kg NEQ. The UXO clearance campaign will involve subsonic combustion with a single donor charge of up to 0.025 kg NEQ for each clearance event, and up to 0.5 kg NEQ to neutralise residual explosive material at each location. Total duration of UXO clearance campaigns is eight days, with up to two detonations within 24 hours although it is noted that this may not always be required.
162. To understand the magnitude of noise emissions from piling and UXO clearance during construction activity, underwater noise modelling has been undertaken considering the key design parameters summarised above. Compared to piling, UXO detonations will be single, isolated events of very short duration; as such, potential behavioural effects upon Annex II diadromous fish will be extremely short lived and reversible. Full detail on the underwater noise modelling is provided in volume 3, appendix 10.1 of the Array EIA Report, and is summarised in terms of injury and disturbance in paragraphs 163 to 185.

                        Injury from piling (permanent and temporary)

163. The Popper et al. (2014) guideline criteria for the onset of mortality, recoverable injury, and Temporary Threshold Shift (TTS) due to impulsive piling are presented in Table 5.5   Open ▸ . A dual criteria approach has been adopted in the guidelines to account for the uncertainties associated with the effects of underwater noise on fish. This includes two parameters for assessment: Cumulative Sound Exposure Level (SELcum) and SPLpk.
164. It should be noted that the SPLpk thresholds for mortality and potential mortal injury and recoverable injury are the same ( Table 5.5   Open ▸ ). The data on mortality and recoverable injury used by Popper et al. (2014) are derived from Halvorsen et al. (2011), Halvorsen et al. (2012a), and Halvorsen et al. (2012b), based on 960 sound events at 1.2 second intervals. The same Single Strike SEL (SELss) was used throughout these Halvorsen et al. piling studies, therefore, the same peak level was derived (SPLpk) as part of the criteria by Popper et al. (2014).

Table 5.5: Criteria for the Onset of Mortality, Recoverable Injury, and TTS due to Impulsive Piling for Relevant Annex II Diadromous Fish Species (Popper et al., 2014)

165. To inform the assessment on Annex II diadromous fish, predicted injury ranges associated with the installation of one 4.5 m diameter pile have been presented. This modelling resulted in the greatest predicted injury ranges and therefore forms the focus of the assessment for injury, noting that in most cases, the maximum hammer energy would not be reached during piling. The metrics presented are for SELcum for fleeing fish and static fish ( Table 5.6   Open ▸ ) and SPLpk ( Table 5.7   Open ▸ ). A swim speed of 0.5 m/s was used to model fleeing fish (Popper et al., 2014).
166. For the SELcum metric, the injury ranges presented indicate that mortality and recoverable injury may occur out to ranges of tens of metres, based on the MDS for fleeing receptors (e.g. 15 m to 20 m for sea lamprey and 32 m to 110 m for Atlantic salmon ( Table 5.6   Open ▸ ). If modelled as static receptors, the mortality and recoverable injury ranges increased to the low hundreds of metres for sea lamprey and up to 2,300 m for Atlantic salmon ( Table 5.6   Open ▸ ). For both species, the TTS ranges were 8,380 m as fleeing receptors and 13,200 m as static receptors ( Table 5.6   Open ▸ ). Practically, the risk of injury will be considerably lower due to the hammer energies being lower than the absolute maximum modelled (3,000 kJ). The expected fleeing behaviour of fish when exposed to high levels of noise and the implementation of soft starts mean that it is likely that fish will have ample time to vacate the areas in which injury may occur prior to noise levels reaching the maximum modelled; however there are uncertainties as to whether all fish species will flee from piling noise and as such static receptors were also modelled, noting these are likely to be highly precautionary ranges.
167. For peak pressure noise levels when piling energy is at its maximum for the wind turbine foundation pile installation, mortality and recoverable injury may occur within approximately 266 m and 414 m of the piling source for sea lamprey and Atlantic salmon, respectively ( Table 5.7   Open ▸ ).
168. When piling for OSP foundations (i.e. maximum hammer energy of 4,400 kJ; Table 5.8   Open ▸ ), greater injury ranges are predicted than for single piling of wind turbine foundations. Using the SELcum metric for fleeing fish, mortality and recoverable injury may occur out to ranges between 25 m and 31 m for sea lamprey and 112 m and 1,440 m for Atlantic salmon ( Table 5.8   Open ▸ ). If modelled as static receptors, the mortality and recoverable injury ranges increase to 855 m and 1,220 m, respectively, for sea lamprey and 2,440 m and 5,120 m for Atlantic salmon ( Table 5.8   Open ▸ ). For both species, the TTS ranges were 21,100 m as fleeing receptors and 26,960 m as static receptors ( Table 5.8   Open ▸ ). Modelling using the peak SPL metric showed a similar pattern with mortality and recoverable injury to ranges of up to 615 m for sea lamprey and up to 1,055 m for Atlantic salmon under the maximum hammer energy of 4,400 kJ ( Table 5.9   Open ▸ ).
169. Based on the two noise criteria (SELcum and SPLpk), injury will occur in the range of tens to hundreds of metres ( Table 5.6   Open ▸ to Table 5.9   Open ▸ ), with larger injury ranges predicted for the maximum hammer energy of 4,400 kJ used during OSP jacket pile installation. However, the modelling has been informed by the maximum hammer energies within the MDS, which, in most cases, will not be reached. Additionally, injury ranges at the start of each piling sequence will be much smaller than those presented here, due to soft starts; at 660 kJ for OSP foundations and 450 kJ for foundation piles.

Table 5.6: Potential Mortality, Injury, and TTS Ranges for Single Wind Turbine Foundation Pile Installation at 3,000 kJ based on the SELcum Metric for Fleeing and Static Annex II Diadromous Fish

Table 5.7: Potential Mortality and Injury Ranges for Single Wind Turbine Foundation Pile Installation at 3,000 kJ based on the SPLpk Metric for Annex II Diadromous Fish

Table 5.8: Potential Mortality, injury, and TTS Ranges for Single OSP Jacket Pile Installation at 4,400 kJ Based on the SELcum Metric for Fleeing and Static Annex II Diadromous Fish

Table 5.9: Potential Mortality and Injury Ranges for Single OSP Jacket Pile Installation at 4,400 kJ based on the SPLpk Metric for Annex II Diadromous Fish

170. The MDS considers the potential for up to two pile installation vessels operating concurrently ( Table 5.3   Open ▸ ). The potential SELcum injury ranges for Annex II diadromous fish due to impact driving of piles have been modelled as following the same piling plans with all phases starting at the same time. For injury, the MDS is that of two adjacent piles, separated by a distance of 950 m in order to assume the maximal overlap of noise propagation contours leading to the maximum generated noise levels. Conversely, for disturbance, the maximum separation between two piling locations would lead to the larger area ensonified at any one time and therefore the greatest disturbance (discussed in paragraph 176 et seq.).
171. As per the MDS, there is potential for two vessels to be piling concurrently at one wind turbine and one OSP foundation ( Table 5.3   Open ▸ ). Injury ranges for concurrent piling of OSP jacket installation at 4,400 kJ and wind turbine foundation installation at 3,000 kJ at each site are given in Table 5.10   Open ▸ . The peak metric will remain the same as the single installation case ( Table 5.7   Open ▸ and Table 5.9   Open ▸ ). For all other piling scenarios, injury ranges would be smaller; the full range of modelled scenarios are given in volume 3, appendix 10.1 of the Array EIA Report. As expected, these show that for this precautionary cumulative piling scenario, injury ranges are similar or slightly larger than the single piling scenarios for fleeing fish, but considerably larger (e.g. double the ranges) for static fish.

Table 5.10: Potential Mortality, Recoverable Injury, and TTS Ranges for Concurrent OSP Jacket Piling (4,400 kJ) and Wind Turbine Foundation Piling (3,000 kJ) based on the SELcum­ Metric for Fleering and Static Annex II Diadromous Fish

                        Injury from UXO clearance

172. Underwater noise modelling was undertaken for UXO clearance. The criteria used in this underwater noise assessment for explosives are given in Table 5.11   Open ▸ following Popper et al. (2014). The recoverable injury and TTS criteria are categorised in relative terms as ‘high’, ‘moderate’, or ‘low’ at three distances from the source: ‘near’ (i.e. in the tens of metres), ‘intermediate’ (i.e. in the hundreds of metres), or ‘far’ (i.e. in the thousands of metres), as shown in Table 5.11   Open ▸ . It is important to note that these criteria are qualitative rather than quantitative.

Table 5.11: Criteria for the Onset of Mortality, Recoverable Injury, and TTS due to UXO Clearance for Relevant Annex II Diadromous Fish Species (Popper et al., 2014)

173. Modelling was undertaken for a range of orders of detonation, from the maximum high order detonation (698 kg) to low order detonations (e.g. deflagration and clearance shots), which will be used as mitigation to reduce noise levels. Table 5.12   Open ▸ details the injury ranges in relation to various orders of detonation. The method of low order has been committed to ( Table 5.4   Open ▸ ), and as such will be the dominant method of UXO clearance, although higher order detonations may also occur if low order is not successful or unintentionally as part of the low order process.
174. The predicted injury ranges for low and high order disposal order detonations of UXOs are presented in Table 5.12   Open ▸ and demonstrate the effectiveness of the low order methods to reduce the risk of injury (i.e. injury ranges of tens of metres for low order, but up to 930 m for high order detonations).
175. Due to a combination of dispersion (i.e. where the waveform elongates), multiple reflections from the sea surface, and seabed and molecular absorption of high frequency energy, the noise is unlikely to still be impulsive once it has propagated more than a few kilometres. Consequently, caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres. Furthermore, the modelling assumes that the UXO acts like a charge suspended in open water whereas it is likely to be partially buried in the sediment. In addition, it is possible that the explosive material will have deteriorated over time meaning that the predicted noise levels are likely to be over-estimated. Overall, these factors mean that the results should be treated as precautionary potential impact ranges and are likely to be significantly lower than predicted.

Table 5.12: Potential Impact Ranges for UXO Clearance Activities, based on the Criteria Presented in Table 5.11   Open ▸

                        Behavioural disturbance (including TTS as a proxy)

176. Behavioural reactions of fish to underwater noise have been found to vary between species and depend on hearing sensitivity. Typically, fish sense noise via particle motion in the inner ear which is detected from noise-induced motions in the fish’s body. The detection of sound pressure is restricted to those fish which have air filled swim bladders; however, particle motion (induced by noise) can be detected by fish without swim bladders (e.g. sea lamprey). Further, the presence of a swim bladder does not necessarily mean that the fish can detect pressure. Some fish have swim bladders that are not involved in the hearing mechanism and can only detect particle motion (e.g. Atlantic salmon).
177. Popper et al. (2014) provides qualitative behavioural criteria for fish from a range of noise sources. The behavioural criteria categorise the risks of effects as ‘high’, ‘moderate’, or ‘low’ at three distances from the source: ‘near’ (i.e. in the tens of metres), ‘intermediate’ (i.e. in the hundreds of metres), or ‘far’ (i.e. in the thousands of metres). It is important to note that the Popper et al. (2014) criteria for disturbance due to noise are qualitative rather than quantitative, due to a lack of agreed quantitative behavioural response thresholds (e.g. as set out for injury above). Consequently, a source of noise of a particular type (e.g. piling) would be predicted to result in the same potential impact, no matter the level of noise produced or the propagation characteristics. The behavioural criteria for piling operations are summarised in Table 5.13   Open ▸ for the relevant Annex II diadromous fish hearing groups and indicate a high to moderate risk of behavioural effects in the near and intermediate fields (i.e. up to hundreds of metres) and a low risk of behavioural effects in the far field (i.e. thousands of metres). As noted above, these criteria were developed for piling in general, with no consideration of piling characteristics, propagation, site specific considerations etc. 

Table 5.13: Potential Risk for the Onset of Behavioural Effects in Relevant Annex II Diadromous Fish from Piling (Popper et al., 2014)

178. Additional studies have examined the behavioural effects of the sound pressure component of impulsive noise (including piling operations) on a range of fish species. For example, Mueller-Blenkle et al. (2010) recorded behavioural responses of cod Gadus morhua and sole Solea solea to sounds similar to those produced during marine piling, with variation noticed across specimens (i.e. depending on the age, sex, condition etc. of the fish, as well as the possible effects of confinement in cages on the overall stress levels in the fish). This study concluded that it was not possible to find a clear relationship between the level of exposure and the extent of the behavioural response, although an observable behavioural response was reported at 140 dB to 161 dB re 1 μPa SPLpk for cod and 144 dB to 156 dB re 1 μPa SPLpk for sole (Mueller-Blenkle et al., 2010). Regardless, these thresholds should not be interpreted as the level at which an avoidance reaction will be elicited, as the study was not able to show this.
179. Further, a study by Pearson et al. (1992) examined the effects of geophysical survey noise on caged rockfish Sebastes spp. and observed a startle or “C-turn response” at peak pressure levels beginning around 200 dB re 1 μPa. This response was less common with the larger fish. Studies by McCauley et al. (2000) exposed various fish species in large cages to seismic airgun noise and assessed behaviour, physiological and pathological changes. The study observed that:

• a general fish behavioural response was to move to the bottom of the cage during periods of high level exposure (greater than rms levels of around 156 dB to 161 dB re 1 μPa; approximately equivalent to SPLpk levels of around 168 dB to 173 dB re 1 μPa);
• a greater startle response was seen in small fish to the above levels;
• a return to normal behavioural patterns was noticed some 14 to 30 minutes after airgun operations ceased;
• no significant physiological stress increases attributed to air gun exposure; and
• some preliminary evidence of damage to the hair cells was noticed when exposed to the highest levels, although it was determined that such damage would only likely occur at short range from the source (McCauley et al., 2000).

180. Post construction monitoring at the Beatrice Offshore Wind Farm concluded that there were no evidence of adverse effects on sandeel (Ammodytidae) and cod populations between pre and post construction levels over a six year period (Beatrice Offshore Wind Farm Limited, 2021a, 2021b). Based on these studies, it can therefore be assumed that noise impacts associated with installation of an offshore wind development are temporary and that fish communities (specifically cod and sandeel in this case) show a high degree of recoverability following construction.
181. With specific reference to diadromous fish, Harding et al. (2016) failed to produce physiological or behavioural responses in Atlantic salmon when subjected to noise similar to piling. However, the noise levels tested were estimated at <160 dB re 1µPa (rms), below the level at which injury or behavioural disturbance would be expected for Atlantic salmon. Nedwell et al. (2006) used the slightly less sensitive sea trout as a model for comparison to Atlantic salmon, and found no significant behavioural response from piling activities, with modelling suggesting a similar response in Atlantic salmon and sea trout. Bagočius (2015) reported physical impacts on migrating salmonids exposed to piling noise of 218 dB re 1μPa2s (SEL), although at these high noise levels, it would be expected that avoidance reactions would occur, to avoid injury.
182. Noting that there are no published or agreed thresholds for behavioural effects on fish from piling operations, a risk based approach has been undertaken using published literature on the behavioural responses of fish to underwater noise (paragraphs 181 to 180). Based on these studies, modelling has been presented using the 160 dB (SPLpk) noise contour to assess behavioural responses in fish species in general and, for the purposes of this report in Atlantic salmon and sea lamprey ( Figure 5.2   Open ▸ and Figure 5.3   Open ▸ ). It is unlikely that species will experience behavioural disturbance beyond this noise contour, based on the described studies which demonstrated behavioural responses (including avoidance) at levels above this threshold. It’s likely that 160 dB re 1 μPa (SPLpk) is over conservative, given that Atlantic salmon and sea lamprey are at the lower end of the sensitivity spectrum (i.e. hearing groups 1 and 2). The 160 dB (SPLpk) contour is presented on Figure 5.2   Open ▸ and Figure 5.3   Open ▸ for the maximum north and south piling locations using the maximum hammer energy of 4,400 kJ (noting all other hammer energies will result in smaller contours). The extent of the 160 dB (SPLpk) contour should be noted, particularly in terms it’s considerable distance offshore, and its relatively small area of effect in terms of the availability of habitat in the North Sea. While the 150 dB (SPLpk) does extend closer to the shore, particularly for the northern location ( Figure 5.2   Open ▸ ), behavioural disturbance is highly unlikely at this level (based on the studies outlined above). 
183. In addition to this site specific noise modelling has considered criteria presented in the Washington State Department of Transport (WSDOT) Biological Assessment Preparation for Transport Projects Advanced Training Manual (WSDOT, 2011) in this assessment for estimating the distances at which behavioural effects may occur due to noise from impulsive piling (as set out see volume 3, appendix 10.1 of the Array EIA Report). The manual suggests an unweighted sound pressure level of 150 dB re 1 μPa (SPLroot mean square (rms)) as the criterion for onset of behavioural effects, based on work by Hastings (2002). Sound pressure levels in excess of 150 dB re 1 μPa (rms) are expected to cause temporary behavioural changes, such as elicitation of a startle response, disruption of feeding, or avoidance of an area. The document notes that levels exceeding this threshold are not expected to cause direct permanent injury but may indirectly affect the individual fish (such as by impairing predator detection). It is important to note that this threshold is for onset of potential effects, and not necessarily an ‘adverse effect’ threshold.
184. The underwater noise modelling (using the WSDOT (2011) 150 dB re 1 μPa (rms) criterion for the onset of behavioural effects) suggests behavioural responses may extend up to 33 km for single pin piling with a hammer energy of 3,000 kJ (representative of the MDS for wind turbine foundation installation).  For a hammer energy of 4,400 kJ (thus representative of the MDS for OSP installation), this range was modelled out to a maximum range of 49 km from piling activity; noting these ranges will be highly conservative for the less sensitive diadromous species considered here. In some cases (e.g. previous offshore wind projects), TTS has been used as a proxy for behavioural disturbance. The maximum TTS values for single piling with hammer energy of 3,000 kJ were 13.20 km ( Table 5.6   Open ▸ ), 26.96 km for a hammer energy of 4,400 kJ for OSP installation ( Table 5.8   Open ▸ ), and 45.10 km for concurrent piling ( Table 5.10   Open ▸ ). These ranges are therefore, of a similar magnitude (i.e. low tens of kilometres) as those ranges reported for the WSDOT criteria summarised above for the two maximum hammer energies and are likely representative of the absolute maximum ranges of behavioural disturbance to diadromous fish species. The 160 dB (SPLpk) contour presented in Figure 5.2   Open ▸ and Figure 5.3   Open ▸ also extends to the low tens of kilometres from the piling location, further strengthening the conclusion that significant behavioural responses (i.e. those that may lead to disruption of migration or barrier effects) are unlikely beyond this range. As described in paragraph 171, the peak metric will remain the same for both single and consecutive piling. As the single piling scenarios presented in Figure 5.2   Open ▸ and Figure 5.3   Open ▸ are based on the peak metric, potential disturbance ranges will not increase the risk of barrier effects under consecutive piling scenarios.
185. Due to the distance between the Array and the coast (approximately 80 km), these behavioural impacts are unlikely to cause barrier effects to diadromous species as they migrate along the east coast of Scotland, due to the relatively limited area around piling events where noise levels are high enough to cause behavioural responses in the context of the wider fish and shellfish ecology study area (as illustrated in Figure 5.2   Open ▸ and Figure 5.3   Open ▸ and extrapolated from the information presented in paragraph 176 et seq.).

Figure 5.2: Modelled 10 dB SPLpk Noise Contours for Piling Hammer Energy of 4,400 kJ at the North Location

Figure 5.3: Modelled 10 dB SPLpk Noise Contours for Piling Hammer Energy of 4,400 kJ at the South Location

                        Summary of underwater noise modelling

186. Sea lamprey and Atlantic salmon close to piling operations may experience injury or mortality. However, diadromous fish species tend to be highly mobile and are unlikely to be particularly reliant on the marine environment within the fish and shellfish ecology study area other than to pass through during migration. Therefore, piling is unlikely to result in significant mortality of Annex II diadromous species. The use of soft start piling procedures (see Table 5.4   Open ▸ ) will allow many individuals in close proximity to piling to flee the ensonified area and will also reduce the overall acoustic energy entering the marine environment, therefore reducing the likelihood of injury and mortality.
187. Atlantic salmon and sea lamprey may experience behavioural effects in response to piling noise, including a startle response, disruption of feeding, or avoidance of an area. As discussed in paragraphs 176 et seq., these would be expected to occur at ranges up to low tens of kilometres, depending on the maximum hammer energies. Due to the distance of the site boundary from the Scottish coast (approximately 80 km), potential behavioural impacts are highly unlikely to cause barrier effects to diadromous species as they migrate along the east coast of Scotland, due to the relatively limited area around piling events where noise levels are high enough to cause behavioural responses (as demonstrated in Figure 5.2   Open ▸ and Figure 5.3   Open ▸ ).

                        Construction phase

                        River Dee SAC

Atlantic salmon

188. As outlined in paragraphs 163 to 175, Atlantic salmon within close proximity to piling operations may experience injury or mortality due to underwater noise from piling or UXO clearance. However, Atlantic salmon are highly mobile, and may only use the fish and shellfish ecology study area to pass through during migration (noting that at-sea behaviour is largely unknown). Therefore, it is unlikely that this impact will result in significant mortality or injury to the Atlantic salmon feature of this SAC. Further, as presented in Table 5.4   Open ▸ , the designed in measure of soft start piling procedures will allow individuals in close proximity to piling to move away from the ensonified area and reduce the total amount of acoustic energy entering the marine environment. In addition, the designed in measure of low order UXO disposal will reduce the noise levels and their potential for injury in the vicinity of UXO clearance operations. Overall, these two designed in measures further reduce the likelihood of injury and mortality.
189. As outlined in paragraphs 176 et seq., underwater noise during piling would result in behavioural responses in the vicinity of the Array, although these may occur out to a range in the low tens of kilometres, and thus not represent a significant barrier to migration to and from the SAC, particularly in terms of the vast availability of habitat in the North Sea and distance between the coast and the site boundary ( Figure 5.2   Open ▸ and Figure 5.3   Open ▸ ). The behavioural disturbance modelling results are also highly precautionary as they were modelled against the maximum hammer energy, which will not realistically occur over the duration of the piling programme. Further, the potential underwater noise impacts will be short term and intermittent in nature during the construction phase (i.e. piling occurring over up to 602 days over eight years). As such, there is negligible risk of disruption to migration.

Freshwater pearl mussel

190. Adult freshwater pearl mussel are confined to freshwater environments, and there is therefore no pathway for direct effects associated with this impact. However, there is potential for indirect impacts on the larval stage of freshwater pearl mussel if Atlantic salmon (their host species) are impacted. As detailed in paragraphs 188 to 189, underwater noise in the construction phase will not lead to significant mortality or injury to Atlantic salmon and is unlikely to result in barriers to migration. Therefore, it can also be concluded that there will be no indirect impact to freshwater pearl mussel.

Conclusion

191. Adverse effects on the qualifying Annex II diadromous fish features of the River Dee SAC which undermine the conservation objectives of the SAC will not occur as a result of underwater noise during construction activities. Potential effects from these activities on the relevant conservation objectives (as presented in paragraphs 69 to 71) are discussed in turn below in Table 5.14   Open ▸ .

Table 5.14: Conclusions Against the Conservation Objectives of the River Dee SAC from Underwater Noise Generated during Piling and UXO Clearance in the Construction Phase of the Array Alone

192. It can be concluded, beyond reasonable scientific doubt, that there is no risk of an adverse effect on the integrity of the River Dee SAC as a result of underwater noise generated during piling and UXO clearance with respect to the construction phase of the Array alone.

                        River South Esk SAC

Atlantic salmon

193. As outlined in paragraphs 163 to 175, Atlantic salmon within close proximity to piling operations may experience injury or mortality due to underwater noise from piling or UXO clearance. However, Atlantic salmon are highly mobile, and may only use the fish and shellfish ecology study area to pass through during migration (noting that at-sea behaviour is largely unknown). Therefore, it is unlikely that this impact will result in significant mortality or injury to the Atlantic salmon feature of this SAC. Further, as presented in Table 5.4   Open ▸ , the designed in measure of soft start piling procedures will allow individuals in close proximity to piling to move away from the ensonified area and reduce the total amount of acoustic energy entering the marine environment. In addition, the designed in measure of low order UXO disposal will reduce the noise levels and their potential for injury in the vicinity of UXO clearance operations. Overall, these two designed in measures further reduce the likelihood of injury and mortality.
194. As outlined in paragraphs 176 et seq., underwater noise during piling would result in behavioural responses in the vicinity of the Array, although these may occur out to a range in the low tens of kilometres, and thus not represent a significant barrier to migration to and from the SAC, particularly in terms of the vast availability of habitat in the North Sea and distance between the coast and the site boundary ( Figure 5.2   Open ▸ and Figure 5.3   Open ▸ ). The behavioural disturbance modelling results are also highly precautionary as they were modelled against the maximum hammer energy, which will not realistically occur over the duration of the piling programme. Further, the potential underwater noise impacts will be short term and intermittent in nature during the construction phase (i.e. piling occurring over up to 602 days over eight years). As such, there is negligible risk of disruption to migration.