5.3. Disturbance to Marine Mammals

  1. Beyond the area in which auditory injury may occur, effects on marine mammal behaviour are an important measure of potential impact. Non-trivial disturbance may occur when there is a risk of animals incurring sustained or chronic disruption of behaviour or when animals are displaced from an area, with subsequent redistribution being significantly different from that occurring due to natural variation.
  2. To consider the possibility of disturbance resulting from the Array, it is necessary to consider:
  • whether or not a noise can be detected/heard by an animal above background noise levels or level of acclimatisation above background levels;
  • the likelihood that the noise could cause non-trivial disturbance;
  • the likelihood that the sensitive animals will be exposed to that noise; and
  • whether the number of animals exposed are likely to be significant at the population level.
  1. Assessing these impacts is however a very difficult task due to the complex and variable nature of noise propagation, the variability of documented animal responses to similar levels of noise, and the availability of population estimates, and regional density estimates for all marine mammal species. Behavioural responses are widely recognised as being highly variable and context specific (Southall et al., 2007; 2019; 2021). Assessing the severity of such potential impacts and development of probability-based response functions continues to be an area of ongoing scientific research interest (Graham et al., 2019; Harris et al., 2018; Southall et al., 2021).
  2. Southall et al. (2007) recommended that at the time the only feasible way to assess whether a specific noise could cause disturbance is to compare the circumstances of the situation with empirical studies. Joint Nature Conservation Committee (JNCC) guidance in the United Kingdom (UK) (JNCC, 2010) indicates that a score of five or more on the Southall et al. (2007) behavioural response severity scale could be significant. The more severe the response on the scale, the lower the amount of time that the animals will tolerate it before there could be adverse consequences to life functions, which would constitute a disturbance. The severity scale was revised in Southall et al. (2021), which included splitting severity assessment methods on captive studies from assessments on field studies. Behavioural responses related to field studies included impacts to survival, reproduction, and foraging.
  3. Southall et al. (2007 and 2021) both present a summary of observed behavioural responses for various mammal groups exposed to different types of noise: continuous (non-pulsed) or impulsive (single or multiple pulsed).
  4. Disturbance to marine mammals is discussed in more detail in volume 2, chapter 10.

5.3.1. Continuous (Non-Pulsed, Non-Impulsive) Noise

  1. For non-pulsed noise (e.g. installation of pile foundations using drilling, vessels etc.), the lowest sound pressure level at which a score of five or more on the Southall et al. (2007) behavioural response severity scale occurs for low frequency cetaceans is 90 dB to 100 dB re 1 μPa (rms). However, this relates to a study involving only migrating gray whales Eschrichtius robustus. A study for minke whale showed a response score of three at a received level of 100 dB to 110 dB re 1 μPa (rms), with no higher severity score encountered for this species. For mid frequency cetaceans, a response score of eight was encountered at a received level of 90 dB to 100 dB re 1 μPa (rms), but this was for one mammal (a sperm whale Physeter macrocephalus) and might not be applicable for the species likely to be encountered in the vicinity of the Array. For Atlantic white-beaked dolphin, a response score of three was encountered for received levels of 110 dB to 120 dB re 1 μPa (rms), with no higher severity score encountered. For high frequency cetaceans such as bottlenose dolphins a number of individual responses with a response score of six are noted ranging from 80 dB re 1 μPa (rms) and upwards. There is a significant increase in the number of mammals responding at a response score of six once the received sound pressure level is greater than 140 dB re 1 μPa (rms).
  2. It is worth noting that the above sound pressure levels are based on the rms sound pressure level metric, which was historically often reported in such studies. More recent studies often use other metrics such as the SEL and care must be taken not to directly compare noise levels quoted using different parameters (refer to section 3 for a discussion of these different metrics).
  3. The National Marine Fisheries Service (NMFS) (2005) guidance sets the marine mammal Level B harassment threshold (analogous to disturbance) for continuous noise at 120  dB re 1 μPa (rms). This threshold is based on studies by Malme et al. (1984) which investigate the effects of noise from the offshore petroleum industry on migrating gray whale behaviour offshore Alaska. This value sits approximately mid-way between the range of values identified in Southall et al. (2007) for continuous noise but is lower than the value at which the majority of marine mammals responded at a response score of six (i.e. once the received rms sound pressure level is greater than 140 dB re 1 μPa). Considering the paucity and high level variation of data relating to onset of behavioural effects due to continuous noise, any ranges predicted using this number are likely to be probabilistic and potentially over precautionary.
  4. It is worth nothing that the distinction between impulsive and non-impulsive noise was removed from Southall et al. (2021) as “some source types, such as airguns, may produce impulsive noises near the source and non-impulsive noises at greater ranges”. However, Southall et al. (2021) does not present thresholds for assessing disturbance, therefore the thresholds discussed in section 5.3.1 have been adopted.

5.3.2. Impulsive (Pulsed) Noise

  1. Southall et al. (2007) presents a summary of observed behavioural responses due to multiple pulsed noise, although the data is primarily based on responses to seismic exploration activities (rather than for piling). Although these datasets contain much relevant data for LF cetaceans, there is less data for MF or HF cetaceans within the document. Low frequency cetaceans, other than bowhead whales (Balaena mysticetus), were typically observed to respond significantly at 140 dB to 160 dB re 1 μPa (rms). Behavioural changes at these levels during multiple pulses may have included visible startle response, extended cessation or modification of vocal behaviour, brief cessation of reproductive behaviour or brief/minor separation of females and dependent offspring. The data available for MF cetaceans indicate that some significant response was observed at a SPL of 120 dB to 130 dB re 1 μPa (rms), although the majority of cetaceans in this category did not display behaviours of this severity until exposed to a level of 170 dB to 180 dB re 1 μPa (rms). Furthermore, other MF cetaceans within the same study were observed to have no behavioural response even when exposed to a level of 170 dB to 180 dB re 1 μPa (rms).
  2. More recently, Graham et al. (2019) describes empirical evidence from piling at the Beatrice Offshore Wind Farm (Moray Firth, Scotland) used to derive a dose-response curve for harbour porpoise[7]. The unweighted single pulse SEL contours were plotted in 5 dB increments and applied the dose-response curve to estimate the number of animals that would be disturbed by piling within each stepped contour. The study shows a 100% probability of disturbance at an (unweighted) SEL of 180 dB re 1 μPa2s, 50% at 155 dB re 1 μPa2s and dropping to approximately 0% at an SEL of 120 dB re 1 μPa2s. This approach to understanding the behavioural effects from piling has been applied at other UK offshore wind farms (for example Seagreen Alpha/Bravo Environmental Statement Chapter 10 Marine Mammals (Seagreen Wind Energy, 2018), Hornsea Three Environmental Statement Volume 2 Chapter 4 Marine mammals (Orsted, 2020) and Awel y Môr Environmental Statement Volume 2, Chapter 7: Marine mammals (RWE, 2022)). Similar stepped/probability-based threshold criteria have been used on other studies such as for assessing the response of marine mammals to geophysical activities (e.g. Southall et al., 2017). The data were subsequently used to develop a dose-response curve. The assessment of behavioural response and disturbance is presented in volume 2, chapter 10.
  3. Southall et al. (2007) suggested that there was a general paucity of data relating to the effects of noise on pinnipeds in particular. One study using ringed Pusa hispida, bearded Erignathus barbatus and spotted Phoca largha seals (Harris et al., 2001) found onset of a significant response at a received sound pressure level of 160 dB to 170 dB re 1 μPa (rms), although larger numbers of animals showed no response at noise levels of up to 180 dB re 1 μPa (rms). It is only at much higher sound pressure levels in the range of 190 dB to 200 dB re 1 μPa (rms) that significant numbers of seals were found to exhibit a significant response. For comparison, for non-pulsed noise one study elicited a significant response on a single harbour seal at a received level of 100 dB to 110 dB re 1 μPa (rms), although other studies found no response or non-significant reactions occurred at much higher received levels of up to 140 dB re 1 μPa (rms).
  4. In more recent studies, and following a similar method to Graham et al. (2019), a telemetry study undertaken by Russell et al. (2016) investigating the behaviour of tagged harbour seals during pile driving at the Lincs Wind Farm in the Wash found that there was a proportional response at different received noise levels. Dividing the study area into a 5 km x 5 km grid, the authors modelled SELss levels and matched these to corresponding densities of harbour seals in the same grids during periods of non-piling versus piling to show change in usage. The study found that there was a significant decrease during piling activities at predicted received SEL levels of between 142 dB re 1 µPa2s and 151 dB re 1 µPa2s.
  5. Southall et al. (2007) also noted that due to the uncertainty over whether HF cetaceans may perceive certain noises and due to the paucity of data, it was not possible to present any data on responses of HF cetaceans. However, Lucke et al. (2009) showed a single harbour porpoise consistently showed aversive behavioural reactions to pulsed noise at received SPL above 174 dB re 1 μPa (peak-to-peak) or a SEL of 145 dB re 1 μPa2s, equivalent to an estimated[8]  sound pressure level of 166 dB re 1 μPa rms.
  6. There is much intra-category and perhaps intra-species variability in behavioural response. As such, a conservative approach should be taken to ensure that the most sensitive marine mammals remain protected.
  7. The High Energy Seismic Survey (HESS) workshop on the effects of seismic (i.e. pulsed) noise on marine mammals (HESS, 1997) concluded that mild behavioural disturbance would most likely occur at rms noise levels greater than 140 dB re 1 μPa (rms). This workshop drew on studies by Richardson (1995) but recognised that there was some degree of variability in reactions between different studies and mammal groups. Consequently, for the purposes of this study, a precautionary level of 140 dB re 1 μPa (rms) is used to indicate the onset of low-level marine mammal disturbance effects for all mammal groups for impulsive noise.
  8. The approach to be employed for the Array is therefore to plot unweighted single pulse SEL contours in 5 dB increments and apply the appropriate dose-response curve to estimate the number of animals that would be disturbed by noise from the piling within each stepped contour. For cetaceans, the dose-response curve will be applied from the Beatrice data (Graham et al., 2019) (refer to Figure 5.2   Open ▸ ), whilst for pinnipeds the dose-response curve will be applied using Whyte et al. (2020). Although the Whyte paper derives more recent response curves, these are only proposed for pinnipeds and hence the need to also include data from older sources for other key species.

Figure 5.2:
The Probability of a Harbour Porpoise Response (24 hrs) in Relation to the Partial Contribution of Unweighted Received Single-Pulse SEL for the First Location Piled (Purple Line), the Middle Location (Green Line) and the Final Location Piled (Blue Line). Reproduced with Permission from Graham et al. (2019)

Figure 5.2: The Probability of a Harbour Porpoise Response (24 hrs) in Relation to the Partial Contribution of Unweighted Received Single-Pulse SEL for the First Location Piled (Purple Line), the Middle Location (Green Line) and the Final Location Piled (Blue Line). Reproduced with Permission from Graham et al. (2019)

 

  1. The dose-response approach is a widely accepted approach to assessing potential behavioural effects of noise from piling and has been applied at other recent UK offshore wind farms (e.g. Seagreen Alpha/Bravo, Awel y Môr, Hornsea Three and Hornsea Four).
  2. For impulsive noise sources other than piling (e.g. UXO clearance, some geotechnical and geophysical surveys), this assessment adopts the NMFS (2005) Level B harassment threshold of 160 dB re 1 μPa (rms) for impulsive noise. Level B harassment is defined by NMFS (2005) as having the potential to disturb a marine mammal or marine mammal stock in the wild by causing disruption of behavioural patterns, including, but not limited to, migration, breathing, nursing, breeding, feeding, or sheltering but which does not have the potential to injure a marine mammal or marine mammal stock in the wild. This is similar to the JNCC (2010) description of non-trivial disturbance and has therefore been used as the basis for onset of behavioural change in the assessment.
  3. For assessing the severity of behavioural response, the distinction between impulsive and non-impulsive noise was removed from Southall et al. (2021) as “some source types, such as airguns, may produce impulsive noises near the source and non-impulsive noises at greater ranges”. Southall et al. (2021) instead assigns categories to various sources based on the operational characteristics and applies revised severity assessments to selected studies in each category. For example, Table 7 within that paper details a number of observational studies of marine mammals and their responses to piling, with an indication of severity of response and in some cases a received level. However, Southall et al. (2021) does not present thresholds for assessing disturbance, therefore the thresholds discussed in Table 5.3   Open ▸ have been adopted for this study. The assessment of disturbance and behavioural response is presented in full in volume 2, chapter 10.
  4. A recent position statement from Natural Resources Wales (NRW, 2023) presents a number of disturbance criteria specifically for assessing the impacts on harbour porpoise. This document recommends as a first instance using the 143 dB re 1 µPa2s SEL­ss contour as an indicator of disturbance from pile driving, as proposed by Tougaard (2021). This value is based on measurements undertaken to inform the changes to guidelines from the Danish Energy Agency, and the contour was seen to extend to 20 km to 30 km from the piling site.
  5. It is important to understand that exposure to noise levels in excess of the behavioural change threshold stated above does not necessarily imply that the noise will result in significant disturbance. As noted previously, it is also necessary to assess the likelihood that the sensitive receptors will be exposed to that noise and whether the numbers exposed are likely to be significant at the population level.

 

Table 5.3:
Disturbance Criteria for Marine Mammals Used in this Technical Report

Table 5.3: Disturbance Criteria for Marine Mammals Used in this Technical Report

 

  1. There is, however, a considerable degree of uncertainty and variability in the onset of disturbance and therefore any disturbance ranges should be treated as potentially over precautionary. Another important consideration is that the majority of noise produced by project activities, with the exception of operational wind turbine noise, will be either temporary or transitory, as opposed to permanent and fixed. These important considerations are not taken into account in the noise modelling but will be assessed in the relevant marine ecology topic chapters.