13.5 Relevant Information on Annex II Marine Mammals
13.5.1 Sensitivity to Auditory Injury
Elevated underwater noise during piling and vessel activity/other activities
Grey seal and harbour seal
1057 Seals are less dependent on hearing for foraging than cetacean species, but may rely on sound for communication and predator avoidance (e.g. Deecke et al., 2002). Seals detect swimming fish with their vibrissae (Shulte-Pelkum et al., 2007) but, in certain conditions, they may also listen to sounds produced by vocalising fish in order to hunt for prey. Thus, the ecological consequences of a noise induced threshold shift in seals are a reduction in fitness, reproductive output and longevity (Kastelein et al., 2018a). Hastie et al., (2015) reported that, based on calculations of SEL of tagged harbour seals during the construction of the Lincs Offshore Wind Farm (Greater Wash, UK), at least half of the tagged seals would have received sound levels from pile driving that exceeded auditory injury thresholds for pinnipeds (PTS). However, population estimates indicated that the relevant population trend is increasing and therefore, although there are many other ecological factors that will influence the population health, this indicated that predicted levels of PTS did not affect a sufficient number of individuals to cause a decrease in the population trajectory (Hastie et al., 2015). Hastie et al. (2015), however, noted that due to paucity of data on effects of sound on seal hearing, the exposure criteria used are intentionally conservative and therefore predicted numbers of individuals likely to be affected by PTS would also have been highly conservative.
1058 There is some evidence of noise-induced PTS in harbour seals, with the first confirmed report of PTS following a known acoustic exposure event in a marine mammal (Reichmuth et al., 2019). The underwater hearing sensitivity of a trained harbour seal was evaluated before and immediately following exposure to 4.1 kHz tonal fatiguing stimulus, and rather than the expected pattern of TTS onset and growth, an abrupt threshold shift of > 47 dB was observed half an octave above the exposure frequency. While hearing at 4.1 kHz recovered within 48 h, there was a permanent threshold shift of at least 8 dB at 5.8 kHz, and hearing loss was evident for more than ten years.
1059 Despite the uncertainty in the ecological effects of PTS on seals, seals rely on hearing much less than cetaceans and therefore would exhibit some tolerance (i.e. the effect is unlikely to cause a change in either reproduction or survival rates). In addition, it has been proposed that seals may be able to self-mitigate (i.e. reduce their hearing sensitivity in the presence of loud sounds in order to reduce their perceived SPL) (Kastelein et al., 2018a). Although this evidence suggests a lower sensitivity of pinnipeds to PTS, based on uncertainties a precautionary approach has been taken.
1060 The telemetry data confirmed connectivity between Firth of Tay and Eden Estuary SAC, designated for harbour seal, and the Proposed Development marine mammal study area. The population of harbour seal is mostly concentrated within the Firth of Tay and Eden Estuary SAC and Firth of Forth, however the population within the Tay SAC is continuing to decline without indication of recovery within last 20 years (see volume 3, appendix 10.2 of the Offshore EIA Report for more information). Population modelling work conducted for the Firth of Tay and Eden Estuary SAC population has concluded that if this declining trend continues, the population may become extinct within the next 20 years (Hanson et al., 2017). Although it is unknown what is the reason for this decline, this population is deemed sensitive to any additional anthropogenic disturbance, especially during the breeding season (spring and summer). No population trajectory is available for Firth of Forth, although sporadic counts in the area indicate that the decline is localised within the SAC and may not represent the trends in the overall MU population (SCOS, 2020; Sinclair et al., 2020). As outlined in paragraph 1026 et seq., harbour seals are generalist feeders and can forage on variety of species, usually within 50 km from the coast. Individuals may be particularly sensitive to anthropogenic disturbance or changes in prey distribution especially during breeding season.
Harbour porpoise
1062 Scientific understanding of the biological effects of threshold shifts is limited to the results of controlled exposure studies on small numbers of captive animals (reviewed in Finneran 2015) where TTS are experimentally induced (since it is unethical to induce PTS in animals) and thresholds for PTS extrapolated using TTS growth rates.
1063 Studies of auditory injury in relation to a typical piling sequence have suggested that hearing impairment as a result of exposure to piling noise is likely to occur where the source frequencies overlap the range of peak sensitivity for the receptor species rather than across the whole frequency hearing spectrum (Kastelein et al., 2013). Kastelein et al. (2013) demonstrated experimentally that for simulated piling noise (broadband spectrum), harbour porpoise’s hearing around 125 kHz (the key frequency for echolocation) was not affected. Instead, a measurable threshold shift in hearing was induced at frequencies of 4 kHz to 8 kHz, although the magnitude of the hearing shift was relatively small (2.3 dB to 3.6 dB at 4 kHz to 8 kHz) due to the lower received SELs at these frequencies. This was due to most of the energy from the simulated piling occurring in lower frequencies (Kastelein et al., 2013). Subsequently, Kastelein et al. (2017) confirmed sensitivity declined sharply above 125 kHz. The susceptibility of harbour porpoise to threshold shifts was further corroborated in a series of studies measuring temporary shifts in hearing in harbour porpoise at high amplitude frequencies of 0.5 kHz to 88.4 kHz. Here the greatest shift in mean TTS occurred at 0.5 kHz, which is very close to the lower bound of porpoise hearing (Kastelein et al., 2021). Hearing always recovered within 60 minutes after the fatiguing sound stopped.
1064 In addition to the frequency characteristics of the source, the duty cycle of fatiguing sounds is also likely to affect the magnitude of a hearing shift. Kastelein et al. (2014) suggested that hearing may recover to some extent during inter-pulse intervals. Similarly, Finneran (2015) highlighted that whilst a threshold shift can accumulate across multiple exposures, the resulting shift will be less than the shift from a single, continuous exposure with the same total SEL.
1065 There is some evidence of self-mitigation by cetaceans to minimise exposure to sound. The animal can change the orientation of its head so that sound levels reaching the ears are reduced, or it can suppress hearing sensitivity by one or more neurophysiological auditory response control mechanisms in the middle ear, inner ear, and/or central nervous system. Kastelein et al. (2020) highlighted the lack reproducibility of TTS in a harbour porpoise after exposure to repeated airgun sounds and suggested the discrepancies may be due to self-mitigation.
1066 Extrapolating the results from captive bred studies to how animals may respond in the natural environment should, however, be treated with caution as it is not possible to exactly replicate natural environmental conditions. In addition, the small number of test subjects would not account for intraspecific differences (i.e. differences between individuals) or interspecific differences (i.e. extrapolating to other species) in response. However, based on our current understanding, since PTS is a permanent and irreversible hearing impairment it is expected that harbour porpoise is sensitive to this effect as the loss of hearing would affect key life functions (e.g. communication, predator detection, foraging, mating and maternal fitness) and could lead to a change in an animal’s health (if chronic) or vital rates (if acute) (Erbe et al., 2018). Morell et al. (2021) showed the first case of presumptive noise-induced hearing loss, based on inner ear analysis in a free-ranging harbour porpoise. Subject to the limitations of available empirical evidence a potential consequence of a disruption in key life functions is that the health of impacted animals would deteriorate and potentially lead to reduced birth rate in females and mortality of individuals (Costa, 2012).
1067 Given the uncertainty surrounding the effects of PTS on survival and reproduction and the importance of sound for echolocation, foraging and communication in all cetaceans, harbour porpoise, an IEF of international value, is deemed to be of high vulnerability and low recoverability. The sensitivity of the receptor to PTS from elevated underwater noise during piling, vessel activity and other activities is therefore, considered to be high.
Bottlenose dolphin
1068 Individual dolphins experiencing PTS would suffer a biological effect that could impact the animal’s health and vital rates (Erbe et al., 2018). Bottlenose dolphin is classed as high-frequency cetaceans (Southall et al., 2019). There are frequency-specific differences in the onset and growth of a noise-induced threshold shift in relation to the characteristics of the noise source and hearing sensitivity of the receiving species. For example, exposure of two captive bottlenose dolphins to an impulsive noise source between 3 kHz and 80 kHz found that there was increased susceptibility to auditory fatigue between frequencies of 10 to 30 kHz (Finneran and Schlundt, 2013). The SELcum threshold incorporates hearing sensitivities of marine mammals and the magnitude of effects were considerably smaller compared to the very high frequency (e.g. harbour porpoise) and low frequency (e.g. minke whale) species, highlighting that high frequency species are less sensitive to the frequency components of the piling noise signal. The assessment considered the irreversibility of the effects (i.e. as noted for harbour porpoise) and importance of sound for echolocation, foraging and communication in small, toothed cetaceans.
1069 Given the uncertainty surrounding the effects of PTS on survival and reproduction and the importance of sound for echolocation, foraging and communication in all cetaceans, bottlenose dolphin, IEF of international value, is deemed to be of high vulnerability and low recoverability. The sensitivity of the receptor to PTS from elevated underwater noise during piling, vessel activity and other activities is therefore, considered to be high.
Elevated underwater noise as a result of site-investigation surveys
Grey seal, harbour seal, harbour porpoise, bottlenose dolphin
1070 For geotechnical surveys, injury to marine mammals is unlikely to occur beyond a few tens of metres (i.e. up to 60 m for harbour porpoise) and noise from vessels themselves is likely to deter marine mammals beyond this range. The maximum range for PTS from geophysical surveys (SBP) is 360 m. Sills et al. (2020) evaluated TTS onset levels for impulsive noise in seals following exposure to underwater noise from a seismic air gun and found transient shifts in hearing thresholds at 400 Hz were apparent following exposure to four to ten consecutive pulses (SELcum 191 dB – 195 dB re 1 µPa2s; 167 dB – 171 dB re 1 µPa2s with frequency weighting for phocid carnivores in water).
1071 Marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and low recoverability. The sensitivity of the receptor to PTS from elevated underwater noise during site investigation surveys is therefore, considered to be high.
Elevated underwater noise as a result of UXO clearance
Grey seal, harbour seal, harbour porpoise, bottlenose dolphin
1072 The acoustical properties of explosives are characterised by a short shock wave, comprising a sharp rise in pressure followed by an exponential decay with a time constant of a few hundred microseconds (see volume 3, appendix 10.1 of the Offshore EIA Report). The interactions of the shock and acoustic waves create a complex pattern in shallow water, and this was investigated further by Von Benda-Beckmann et al. (2015). As harbour porpoises have high sensitivity to noise, impacts on these species are most often assessed in a scientific literature.
1074 In the same study Von Benda-Beckmann et al. (2015) modelled possible effect ranges for 210 explosions (of up to 1,000 kg charge mass) that had been logged by the Royal Netherland Navy (RNLN) and the Royal Netherlands Meteorological Institute (RNMI) over a two year period (2010 and 2011). Using the empirical measurements of SEL out to 2 km to validate the model (described above in paragraph 1073), the authors found that the effect distances ranged between hundreds of metres to just over 10 km (for charges ranging from 10 kg up to 1,000 kg). Near the surface, where porpoises are known to spend a large proportion of time (e.g. 55% based on Teilmann et al., 2007) the SELs were predicted to be lower with effect distances for the onset of PTS just below 5 km. The authors caveat these results as, whilst the model could provide a reasonable estimate of the SEL within 2 km (since the empirical measurements were made out to this point), estimates above this distance required further validation since the uncorrected model systematically overestimated SEL. Salomons et al. (2021) analysed the sound measurements performed near two detonations of UXO (charge masses of 325 kg and 140 kg). From the weighted SEL values and threshold levels from Southall et al. (2019), a PTS effect distance in the range 2.5 km – 4 km has been derived (Salomons et al., 2021).
1075 By comparing experimental data and model predictions, Salomons et al. (2021) found thar harbour porpoises are at risk of permanent hearing loss at distances of several kilometres from large explosives, (i.e. distance between 2 km and 6 km based on 140 kg and 325 kg charge masses). Following clearance of ground mines in the Baltic Sea in 2019, 24 harbour porpoises were found dead in the period after those clearing events along the coastline (Siebert et al., 2022). The post-mortem examination found that in ten cases the cause of death was associated with a blast injury, however the charge masses of the explosives in this study are unknown (Siebert et al., 2022).
1076 Not much is known about sensitivity of bottlenose dolphin to blasting. However, during a clearance of relatively small explosive (35 kg charge) at an important feeding area for a resident community of bottlenose dolphin in Portugal, acoustic pressure levels in excess of 170 dB e 1µPa were measured. Despite pressure levels being 60 dB higher than ambient noise, no adverse effects were recorded in the behaviour or appearance of resident community (Santos et al., 2010). Nonetheless, other studies reported that external injuries consistent with inner ear damage have been found in dolphins subjected to explosives, with little change in surface animal behaviour near blast areas (Ketten, 1993).
1077 Robinson et al. (2020) described a controlled field experiment and compared the sound produced by high-order detonations with a low-order disposal method (i.e. deflagration). He found that using low order techniques offers a substantial reduction in acoustic output over traditional high-order methods, with the peak SPLpk and SELcum observed being typically > 20 dB lower for the deflagration of the same sized munition (a reduction factor of just over ten in SPLpk and 100 in acoustic energy). The study also reported that the acoustic output depends on the size of the shaped charge, rather than the size of the UXO itself. Considering the above, compared to high-order methods, Robinson et al. (2020) provided the evidence that low order techniques offers the potential for greatly reduced acoustic noise exposure of marine mammals.
1078 The sensitivity of the receptors to the injury from impulsive underwater noise has been described previously for piling and is presented in paragraphs 1057 to 1061.
1079 All marine mammals, which are IEFs of international value, are deemed to be of high vulnerability and low recoverability. The sensitivity of the receptor to PTS from elevated underwater noise during UXCO clearance is therefore, considered to be high.
13.5.2 Sensitivity to Behavioural disturbance
Elevated underwater noise during piling
1080 Studies have shown that acoustic disturbance to marine mammals may lead to the interruption of normal behaviours (such as feeding or breeding) and avoidance, leading to displacement from the area and exclusion from critical habitats (Goold, 1996; Weller et al., 2002; Castellote et al., 2010, 2012). Noise may also cause stress which in turn can lead to a depressed immune function and reduced reproductive success (Anderson et al., 2011; De Soto et al., 2013). The extent to which an animal will be behaviourally affected, however, is very much context-dependent and varies both inter- and intra-specifically. A summary of known behavioural sensitivities of different species to underwater noise from piling at other wind farm sites is provided in paragraph 1088 et seq., noting that the conclusions drawn are subject to the limitations of extrapolating results from one project to another.
Grey seal and harbour seal
1081 Strong disturbance could result in displacement of seals from an area. Whilst mild disturbance has the potential to disturb individuals, this constitutes only slight changes in behaviour, such as changes in swimming speed or direction, and is unlikely to result in population-level effects. Although there are likely to be alternative foraging sites for both harbour seal and grey seal, barrier effects as a result of installation of monopiles could either prevent seals from travelling to forage from haul-out sites or force seals (particularly harbour seal) to travel greater distances than is usual during periods of piling.
1083 Hastie et al. (2021) recently demonstrated that anthropogenic noise can influence foraging decisions in seals and such decisions were consistent with a risk/profit balancing approach. The study measured the relative influence of perceived risk of a sound (silence, pile driving, and a tidal wind turbine) and prey patch quality (low density versus high density), in grey seals in an experimental pool environment. Foraging success was highest under silence, but under tidal wind turbine and pile driving treatments success was similar at the high-density prey patch but significantly reduced under the low-density prey patch. Therefore, avoidance rates were dependent on the quality of the prey patch as well as the perceived risk from the anthropogenic noise.
1087 Grey seal and harbour seals, IEFs of international value, are deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to disturbance as a result of elevated underwater noise during piling is therefore, considered to be medium.
Harbour porpoise
1089 The variance in behavioural responses to increased subsea noise is well documented and is context specific. Factors such as the activity state of the receiving animal, the nature and novelty of the sound (i.e. previous exposure history), and the spatial relation between sound source and receiving animal are important in determining the likelihood of a behavioural response and therefore their sensitivity (Ellison et al., 2012). Empirical evidence from monitoring at offshore wind farms during construction suggests that pile driving is unlikely to lead to 100% avoidance of all individuals exposed, and that there will be a proportional decrease in avoidance at greater distances from the pile driving source (Brandt et al., 2011). This was demonstrated at Horns Rev Offshore Wind Farm, where 100% avoidance occurred in harbour porpoises at up to 4.8 km from the piles, whilst at greater distances (10 km plus) the proportion of animals displaced reduced to < 50% (Brandt et al., 2011). A recent study on piling at the BOWL suggests that harbour porpoise may adapt to increased noise disturbance over the course of the piling phase, thereby showing a degree of tolerance and behavioural adaptation (Graham et al., 2019). This study also demonstrated that the probability of occurrence of harbour porpoise (measured as porpoise positive minutes) increased exponentially moving further away from the noise source. Similarly, at a study of seven offshore wind farms constructed in the German Bight, Brandt et al., (2018) also showed that detections of harbour porpoise declined several hours before the start of pling within the vicinity (up to 2 km) of the construction site and were reduced for about one to two hours post-piling, whilst at the maximum effect distances (from 17 km out to approximately 33 km) avoidance only occurred during the hours of piling. In this study, porpoise detections during piling were found at sound levels exceeding 143 dB re 1µPa2s and at lower received levels - at greater distances from the source - there was little evident decline in porpoise detections (Brandt et al., 2018). These studies demonstrate the dose-response relationship between received noise levels and declines in porpoise detections although noting that the extent to which responses could occur will be context-specific such that, particularly at lower received levels (i.e. 130 dB -140 dB re 1µPa2s), detectable responses may not be apparent from region to region.
1090 A recent article by Southall et al. (2021) introduces a behavioural response severity spectrum, building on earlier work presented in Southall et al. (2007) and the expanding literature in this area. Southall et al. (2021) illustrates the progressive severity of possible responses within three response categories: survival (e.g. resting, navigation, defence), feeding (e.g. search, consumption, energetics), and reproduction (e.g. mating, parenting). For example, at the most severe end of the spectrum (scored 7 to 9), where sensitivity is highest, displacement could occur resulting in movement of animals to areas with an increased risk of predation and/or with sub-optimal feeding grounds. A failure of vocal mechanisms to compensate for noise and interruption of key reproductive behaviour including mating and socialising could occur. In these instances, there would likely be a reduction in an individual’s fitness leading to potential breeding failure and impact on survival rates.
1091 Acknowledging the limitations of the single step-threshold approach for strong disturbance and mild disturbance (i.e. does not account for inter-, or intra-specific variance or context-based variance), harbour porpoise within the area modelled as ‘strong disturbance’ would be most sensitive to behavioural effects and therefore may have a response score of seven or above according to Southall et al. (2021). At the lower end of the behavioural response spectrum, the potential severity of effects reduces. Whilst there may be some detectable responses that could result in effects on the short-term health of animals, these are less likely to impact on an animals’ survival rate. For example, mild disturbance (score four to six) could lead to effects such as changes in swimming speed and direction, minor disruptions in communication, interruptions in foraging, or disruption of parental attendance/nursing behaviour (Southall et al., 2021).
1092 Although harbour porpoise may be able to avoid the disturbed area and forage elsewhere, there may be a potential effect on reproductive success of some individuals. As mentioned previously, it is anticipated that there would be some adaptability to the elevated noise levels from piling and therefore survival rates are not likely to be affected. Due to uncertainties associated with the effects of behavioural disturbance on vital rates of harbour porpoise, the assessment is highly conservative as it assumes the same level of sensitivity for both strong and mild disturbance, noting that for the latter the sensitivity is likely to be lower.
1093 Harbour porpoise, an IEF of international value, is deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to disturbance as a result of elevated underwater noise during piling is therefore, considered to be medium.
Bottlenose dolphin
1095 There is limited information regarding the specific sensitivities of bottlenose dolphin to disturbance from piling noise as most studies have focussed on harbour porpoise. A study of the response of bottlenose dolphin to piling noise during harbour construction works at the Nigg Energy Park in the Cromarty Firth (north-east Scotland) found that there was a measurable (albeit weak) response to impact and vibration piling with animals reducing the amount of time they spent in the vicinity of the construction works (Graham et al., 2017). Another study investigating dolphin detections in the Moray Firth during impact piling at the Moray East and BOWL found surprising results at small temporal scales with an increase in dolphin detections on the southern Moray coast on days with impulsive noise compared to days without (Fernadex-Betelu et al., 2021). Predicted maximum received levels in coastal areas were 128 dB re. 1µPa2s and 141 dB re. 1µPa2s during piling at BOWL and Moray Offshore Renewables Limited (MORL) respectively (Fernadex-Betelu et al., 2021). The authors of this study warn that caution must be exercised in interpreting these results as increased click changes do not necessarily equate to larger groups sizes but may be due to a modification in behaviour (e.g. an increase in vocalisations during piling) (Fernadex-Betelu et al., 2021). The results of this study do, however, suggest that impulsive noise generated during piling at the offshore wind farms did not cause any displacement of bottlenose dolphins from their population range. Notably, the received levels during piling at MORL are higher than those predicted for the outer isopleths (130 dB and 135 dB re. 1µPa2s) that overlap with the CES MU 2 m - 20 m depth contour during piling at the Proposed Development suggesting that disturbance at these lower noise levels is unlikely to lead to displacement effects.
1096 The Southall et al. (2021) severity spectrum applies across all marine mammals and therefore it is expected that, as described for harbour porpoise, strong disturbance in the near field could result in displacement whilst mild disturbance over greater ranges would result in other, less severe behavioural responses.
1097 Bottlenose dolphin may be able to avoid the disturbed area and whilst there may some impacts on reproduction in closer proximity to the source (i.e. within the area of ‘strong disturbance’), these are unlikely to impact on survival rates as some tolerance is expected to build up over the course of the piling. It is anticipated that animals would return to previous activities once the impact had ceased.
1098 Bottlenose dolphin, IEF of international value, is deemed to be of medium vulnerability and high recoverability. The sensitivity the of receptor to disturbance as a result of elevated underwater noise during piling to disturbance is therefore, considered to be medium.
Elevated underwater noise as a result of site-investigation surveys
Grey seal, harbour seal, harbour porpoise, bottlenose dolphin
1099 The transmission frequencies of many commercial sonar systems (approximately 12 kHz – 1800 kHz) overlap with the hearing and vocal ranges of many species (Richardson et al., 1995), and whilst many are high frequency sonar systems with peak frequencies well above marine mammal hearing ranges, it is possible that relatively high levels of sound are also produced as sidebands at lower frequencies (Hayes and Gough, 1992) so may elicit behavioural responses in marine mammals. Fine-scale data from porpoises equipped with high-resolution location and dive loggers when exposed to airgun pulses at ranges of 420 m – 690 m with noise level estimates of 135 dB–147 dB re 1 µPa2s (SEL) show different responses to noise exposure (van Beest, et al., 2018). One individual displayed rapid and directed movements away from the exposure site whilst two individuals used shorter and shallower dives (compared to natural behaviour) immediately after exposure. This noise-induced movement typically lasted for eight hours or less, with an additional 24-hour recovery period until natural behaviour was resumed.
1100 Results from 201 seismic surveys in the UK and adjacent waters demonstrated that cetaceans (including bottlenose dolphin) can be disturbed by seismic exploration (Stone and Tasker, 2006), with small odontocetes showing strongest lateral spatial avoidance, moving out of the area, whilst mysticetes and killer whales showed more localised spatial avoidance, orienting away from the vessel and increasing distance from source but not leaving the area completely.
1101 A study by Sarnocińska et al. (2020) indicated temporary displacement or change in harbour porpoise echolocation behaviour in response to a 3D seismic survey in the North Sea. No general displacement was detected from 15 km away from any seismic activity but decreases in echolocation signals were detected up to 8 km – 12 km from the active airguns. Taking into account findings of other studies (Dyndo et al., 2015; Tougaard et al., 2015) harbour porpoise disturbance ranges due to airgun noise are predicted to be smaller than to pile driving noise at the same energy. The reason for this is because the perceived loudness of the airgun pulses is predicted to be lower than for pile driving noise due to less energy at the higher frequencies where porpoise hearing is better (Sarnocinska et al., 2020). Similarly, Thompson et al. (2013) used passive acoustic monitoring and digital aerial surveys to study changes in the occurrence of harbour porpoises across a 2,000 km2 study area during a commercial two-dimensional seismic survey in the North Sea and found acoustic detections decreased significantly during the survey period in the impact area compared with a control area, but this effect was small in relation to natural variation. Animals were typically detected again at affected sites within a few hours, and the level of response declined through the ten-day survey suggesting exposure led to some tolerance of the activity (Thompson et al., 2013). This study suggested that prolonged seismic survey noise did not lead to broader-scale displacement into suboptimal or higher-risk habitat. Likewise, a ten month study of overt responses to seismic exploration in humpback whales Megaptera novaeangliae, sperm whales Physeter macrocephalus and Atlantic spotted dolphins Stenella frontalis, demonstrated no evidence of prolonged or large-scale displacement of each species from the region during the survey (Weir, 2008).
1102 Hastie et al. (2014) carried out behavioural response tests to two sonar systems (200 kHz and 375 kHz systems) on grey seals at SMRU seal holding facility. Results showed that both systems had significant effects on the seals’ behaviour. Seals spent significantly more time hauled out during the 200 kHz sonar operation and although seals remained swimming during operation of the 375 kHz sonar, they were distributed further from the sonar.
1103 It is expected that, to some extent, marine mammals will be able to adapt their behaviour to reduce impacts on survival and reproduction rates and tolerate elevated levels of underwater noise during site investigation surveys. Marine mammals, which are IEFs of international value, are deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to disturbance from elevated underwater noise during site investigation surveys is therefore considered to be medium.
Elevated underwater noise as a result of vessel activity and other activities
Grey seal, harbour seal, harbour porpoise, bottlenose dolphin
1104 Disturbance levels for marine mammal receptors will be dependent on individual hearing ranges and background noise levels within the vicinity. Sensitivity to vessel noise is most likely related to the marine mammal activity at the time of disturbance (IWC, 2006; Senior et al., 2008).
1105 Cetaceans can both be attracted to, and disturbed by, vessels. For example, resting dolphins are likely to avoid vessels, foraging dolphins will ignore them, and socialising dolphins may approach vessels (Richardson et al., 1995).
1108 There is, however, evidence of habituation to boat traffic and therefore a slight increase from the existing levels of traffic in the vicinity of the Proposed Development may not result in high levels of disturbance. For example, Lusseau et al. (2011) (SNH commissioned report) undertook a modelling study which predicted that increased vessel movements associated with offshore wind development in the Moray Firth did not have an adverse effect on the local population of bottlenose dolphin, although it did note that foraging may be disrupted by disturbance from vessels.
13.5.3 Sensitivity to TTS
Elevated underwater noise as a result of UXO clearance
Grey seal and harbour seal
1112 A study measuring recovery rates of harbour seal following exposure to a sound source of 193 dB re 1 μPa2s (SELcum) over 360 minutes found that recovery from TTS to the pre-exposure baseline was estimated to be complete within 72 minutes following exposure (Kastelein et al., 2018a). These results are similar to recovery rates found in SEAMARCO (2011), which showed that for small TTS values, recovery in seals was very fast (around 30 minutes) and the higher the hearing threshold shift, the longer the recovery. Kastelein et al. (2019a) also demonstrated recovery was rapid, with hearing recovered fully within two hours. Therefore, in most cases, reduced hearing for such a short time probably has little effect on the total foraging period of a seal. If hearing is impaired for longer periods (hours or days) the impact is likely to be ecologically significant (SEAMARCO, 2011). The results indicate that harbour seal (and therefore grey seal, using harbour seal as a proxy) are less vulnerable to TTS than harbour porpoise for the noise bands tested. In addition, it is 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 both reproduction and survival rates and would be able to return to previous behavioural states or activities once the impacts had ceased.
Harbour porpoise
1113 Explosions during UXO clearance activities and associated underwater noise have the potential to produce behavioural disturbance, however there are no agreed thresholds for the onset of a behavioural response generated as a result of explosion. Given different nature of the sound, using noise levels and probability of a response to pile driving would not be appropriate. Southall et al. (2007) suggests 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 used in other assessments where the impacts of UXO clearance on marine mammals have been investigated. TTS is a temporary and reversible hearing impairment and therefore, it is anticipated that any animals experiencing this shift in hearing would recover after they are no longer exposed to elevated noise levels (i.e. they may have moved beyond the injury zone or piling has ceased). The implication of animals experiencing TTS, leading to potential displacement, is not fully understood, but it is likely that aversive responses to anthropogenic noise could temporarily affect life functions as described for PTS. However, due to the reversible nature of TTS, this 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 on the degree of shift in hearing experienced. A study measuring recovery rates of harbour porpoise following exposure to sound source of 75 dB re 1 μPa (SEL) over 120 minutes found that recovery to the pre-exposure threshold was estimated to be complete within 48 minutes following exposure (the higher the hearing threshold shift, the longer the recovery) (SEAMARCO, 2011).
1114 Finneran et al. (2000) investigated the behavioural and auditory responses of two captive bottlenose dolphins 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 p-p (peak-peak) was observed. Behavioural shifts, such as delaying approach to the test station and avoiding the ‘start’ station, were recorded at 196 dB and 209 dB re 1 µPa p-p for the two dolphins and continued at higher levels. There are several caveats to this study (discussed in Nowacek et al. (2007)), (i.e. the signals used in this study were distant and the study measured masked-hearing signals). The animals used in the experiment were also trained and rewarded for tolerating high levels of noise and subsequently, it can be anticipated that behavioural disruption would likely be observed at lower levels in other contexts.
1115 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) (Kastelein et al., 2021). In a series of studies measuring TTS occurrence in harbour porpoise at a range of frequencies typical of high amplitude anthropogenic sounds (0.5 kHz to 88.4 kHz) the greatest shift in mean TTS occurred at 0.5 kHz, which is very close to the lower bound of porpoise hearing (Kastelein et al., 2021). Hearing always recovered within 60 minutes after the fatiguing sound stopped. Scientific understanding of the biological effects of TTS is limited to the results of controlled exposure studies on small numbers of captive animals (reviewed in Finneran, 2015). 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
1117 All marine mammals, which are IEFs of international value are deemed to be of medium vulnerability and high recoverability. The sensitivity of the receptor to TTS is therefore, considered to be low.