1. Introduction

  1. This Subsea Noise Technical Report presents the results of a desktop study undertaken by Seiche Ltd. considering the potential effects of underwater noise on the marine environment from construction of the Berwick Bank Wind Farm (hereafter referred to as the ‘Proposed Development’).
  2. The location of the Proposed Development in the North Sea, in the outer Firth of Forth and Tay, is illustrated in Figure 1.1   Open ▸ . The planned activities at this site fall into four categories of pre-construction, construction, operation and maintenance, and decommissioning based events. Within each of these four working categories different underwater noise sources are identified. These noise sources are both continuous and intermittent in characteristics.
  3. Sound is readily transmitted into the underwater environment and there is potential for the sound emissions from the survey to adversely affect marine mammals and fish. At close ranges from the noise source with high noise levels permanent or temporary hearing damage may occur to marine species, while at a very close range gross physical trauma is possible. At long ranges the introduction of any additional noise could potentially cause short-term behavioural changes, for example to the ability of species to communicate and to determine the presence of predators, food, underwater features, and obstructions. This report provides an overview of the potential effects due to underwater noise from the proposed survey on the surrounding marine environment. 
  4. The primary purpose of this underwater noise study is to predict the likely range for the onset of potential injury (i.e. permanent threshold shifts (PTS) in hearing) and behavioural effects on different marine fauna when exposed to the different anthropogenic noises that occur during different phases of the Proposed Development. The results from this underwater noise appraisal have been used to inform the following volume 2 chapters of the Environmental Impact Assessment (EIA) Report in order to determine the potential impact of underwater noise on marine life:
  • volume 2, chapter 8: Benthic Subtidal and Intertidal Ecology;
  • volume 2, chapter 9: Fish and Shellfish Ecology; and
  • volume 2, chapter 10: Marine Mammals.
    1. Consequently, the sensitivity of species, magnitude of potential impact and significance of effect from underwater noise associated with the development are addressed within the relevant chapters.

1.2. Conversion Factors

  1. A comprehensive study evaluating the evidence and justification for different conversion factors has been undertaken following advice received during the Marine Mammal Road Map pre-application consultation process (see volume 3, appendix 10.1, annex A). From the conversion factors evaluated, a variable conversion factor (β) has been used in the underwater noise assessment ranging from β = 4% at the start of piling to β = 0.5% at the end of piling when the pile is almost fully embedded in the seabed.
  2. This scenario has been chosen as it was considered to represent the best balance of realism and precaution in conversion factor, particularly compared to a conversion factor of 10% reducing to 1% which was considered over-precautionary and therefore misrepresentative of the potential kinetic energy converted to sound energy. A 1% constant conversion factor was considered less representative compared to 4% reducing to 0.5% for a partially submersible hammer as would be used for the Proposed Development. Note, however, to adopt a precautionary assessment and to mitigate for uncertainties in the true value of the conversion factor the marine mammal EIA took forward the predicted ranges from either the 4% reducing conversion factor or 1% constant conversion factor, whichever led to the greatest ranges using the relevant noise thresholds for injury and disturbance. Volume 3, appendix 10.5 provides quantitative outcomes (impact ranges and number affected) of underwater noise modelling for the range of conversion factors modelled.

Figure 1.1:
The Proposed Location of Berwick Bank Wind Farm in the North Sea

Figure 1.1: The Proposed Location of Berwick Bank Wind Farm in the North Sea

2. Study Area

  1. The modelled area is approximately rectangular and covers the Proposed Development array area and export cable corridor ( Figure 1.1   Open ▸ ) and an area extending to about 60 km from the boundaries north, east, and south and the Firth of Forth estuary to the west. The site covers the Firth of Forth Banks Complex Nature Conservation Marine Protected Area (ncMPA). Bathymetry data used for modelling purposes was obtained from the General Bathymetric Chart of the Oceans (GEBCO) and showed the water depth (lowest astronomical tide (LAT)) within the Proposed Development array area to range between 35 m and 70 m deep. Within the modelling area, the water depth is typically shallower than 80 m, with some limited regions to the south being 110 m deep, and to the north being 140 m deep.

Figure 2.1:
Subsea Noise Study Area

Figure 2.1: Subsea Noise Study Area

3. Acoustic Concepts and Terminology

  1. Sound travels through the water as vibrations of the fluid particles in a series of pressure waves. These waves comprise a series of alternating compressions (positive pressure) and rarefactions (negative pressure). Because sound consists of variations in pressure, the unit for measuring sound is usually referenced to a unit of pressure, the Pascal (Pa). The decibel (dB) scale is used to conveniently communicate the large range of acoustic pressures encountered, with a known pressure amplitude chosen as a reference value (i.e. 0 dB). In the case of underwater sound, the reference value (Pref) is taken as 1 μPa, whereas the airborne sound is usually referenced to a pressure of 20 μPa. To convert from a sound pressure level referenced to 20 μPa to one referenced to 1 μPa, a factor of 20 log (20/1) (i.e. 26 dB has to be added to the former quantity). Thus 60 dB re 20 μPa is the same as 86 dB re 1 μPa, although differences in sound speeds and different densities mean that the decibel level difference in sound intensity is much more than the 26 dB when converting pressure from air to water. All underwater sound pressure levels in this report are quantified in dB re 1 μPa.
  2. There are several descriptors used to characterise a sound wave. The difference between the lowest pressure variation (rarefaction) and the highest-pressure variation (compression) is called the peak to peak (or pk-pk) sound pressure level. The difference between the highest variation (either positive or negative) and the mean pressure is called the peak pressure level. Lastly, the root mean square (rms) sound pressure level is used as a description of the average amplitude of the variations in pressure over a specific time window. Decibel values reported should always be quoted along with the Pref value employed during calculations. For example, the measured Sound Pressure Level (SPLrms) value of a pulse may be reported as 100 dB re 1 µPa. These descriptions are shown graphically in Figure 3.1   Open ▸ .

Figure 3.1:
Graphical Representation of Acoustic Wave Descriptors

Figure 3.1: Graphical Representation of Acoustic Wave Descriptors

  1. The SPLrms is defined as:

  1. The magnitude of the rms sound pressure level for an impulsive sound (such as that from a seismic source array) will depend upon the integration time, T, used for the calculation (Madsen, 2005). It has become customary to utilise the T90 time period for calculating and reporting rms sound pressure levels. This is the interval over which the cumulative energy curve rises from 5% to 95% of the total energy and therefore contains 90% of the sound energy.
  2. Another useful measure of sound used in underwater acoustics is the Sound Exposure Level (SEL). This descriptor is used as a measure of the total sound energy of an event or a number of events (e.g. over the course of a day) and is normalised to one second. This allows the total acoustic energy contained in events lasting a different amount of time to be compared on a like for like basis[1]. The SEL is defined as:

  1. The frequency, or pitch, of the sound is the rate at which the acoustic oscillations occur in the medium (air/water) and is measured in cycles per second, or Hertz (Hz). When sound is measured in a way which approximates to how a human would perceive it using an A-weighting filter on a sound level meter, the resulting level is described in values of dBA. However, the hearing faculty of marine mammals is not the same as humans, with marine mammals hearing over a wider range of frequencies and with a different sensitivity. It is therefore important to understand how an animal’s hearing varies over its entire frequency range to assess the effects of anthropogenic sound on marine mammals. Consequently, use can be made of frequency weighting scales (M-weighting) to determine the level of the sound in comparison with the auditory response of the animal concerned. A comparison between the typical hearing response curves for fish, humans and marine mammals is shown in Figure 3.2   Open ▸ [2].
  2. Other relevant acoustic terminology and their definitions used in the report are detailed in paragraphs 16 to Error! Reference source not found..
  3. Third octave bands - The broadband acoustic power (i.e. containing all the possible frequencies) emitted by a sound source, measured/modelled at a location within the survey region is generally split into and reported in a series of frequency bands. In marine acoustics, the spectrum is generally reported in standard one-third octave band frequencies, where an octave represents a doubling in sound frequency.
  4. Source level (SL) - The source level is the sound pressure level of an equivalent and infinitesimally small version of the source (known as point source) at a hypothetical distance of 1 m from it. The source level may be combined with the transmission loss (TL) associated with the environment to obtain the received level (RL) in the far field of the source. The far field distance is chosen so that the behaviour of the distributed source can be approximated to that of a point source. Source levels do not indicate the real sound pressure level at 1 m.
  5. TL at a frequency of interest is defined as the loss of acoustic energy as the signal propagates from a hypothetical (point) source location to the chosen receiver location. The TL is dependent on water depth, source depth, receiver depth, frequency, geology, and environmental conditions. The TL values are generally evaluated using an acoustic propagation model (various numerical methods exist) accounting for the above dependencies.

Figure 3.2:
Comparison Between Hearing Thresholds of Different Animals

Figure 3.2: Comparison Between Hearing Thresholds of Different Animals


  1. The RL is the sound level of the acoustic signal recorded (or modelled) at a given location, that corresponds to the acoustic pressure/energy generated by a known active sound source. This considers the acoustic output of a source and is modified by propagation effects. This RL value is strongly dependant on the source, environmental properties, geological properties and measurement location/depth. The RL is reported in dB either in rms or peak-to-peak SPL, and SEL metrics, within the relevant one-third octave band frequencies. The RL is related to the SL as:

                                     RL = SL – TL                                                   

where TL is the transmission loss of the acoustic energy within the survey region.

  1. The directional dependence of the source signature and the variation of TL with azimuthal direction α (which is strongly dependent on bathymetry) are generally combined and interpolated to report a 2-D plot of the RL around the chosen source point up to a chosen distance.

4. Acoustic Assessment Criteria

4.1. Introduction

  1. Underwater noise has the potential to affect marine life in different ways depending on its noise level and characteristics. Richardson et al. (1995) defined four zones of noise influence which vary with distance from the source and level. These are:
  • The zone of audibility: this is the area within which the animal can detect the sound. Audibility itself does not implicitly mean that the sound will affect the marine mammal.
  • The zone of masking: this is defined as the area within which noise can interfere with the detection of other sounds such as communication or echolocation clicks. This zone is very hard to estimate due to a paucity of data relating to how marine mammals detect sound in relation to masking levels (for example, humans can hear tones well below the numeric value of the overall noise level).
  • The zone of responsiveness: this is defined as the area within which the animal responds either behaviourally or physiologically. The zone of responsiveness is usually smaller than the zone of audibility because, as stated previously, audibility does not necessarily evoke a reaction.
  • The zone of injury/hearing loss: this is the area where the sound level is high enough to cause tissue damage in the ear. This can be classified as either temporary threshold shift (TTS) or PTS. At even closer ranges, and for very high intensity sound sources (e.g. underwater explosions), physical trauma or even death are possible.
    1. For this study, it is the zones of injury and disturbance (i.e. responsiveness) that are of concern (there is insufficient scientific evidence to properly evaluate masking). To determine the potential spatial range of injury and disturbance, a review has been undertaken of available evidence, including international guidance and scientific literature. The following sections summarise the relevant thresholds for onset of effects and describe the evidence base used to derive them.

4.2. Injury (Physiological Damage) to Mammals

  1. Sound propagation models can be constructed to allow the received noise level at different distances from the source to be calculated. To determine the consequence of these received levels on any marine mammals which might experience such noise emissions, it is necessary to relate the levels to known or estimated potential impact thresholds. The injury criteria proposed by Southall et al. (2019) are based on a combination of linear (i.e. un-weighted) peak pressure levels and mammal hearing weighted SEL. The hearing weighting function is designed to represent the bandwidth for each group within which acoustic exposures can have auditory effects. The categories include:
  • Low Frequency (LF) cetaceans (i.e. marine mammal species such as baleen whales (e.g. minke whale Balaenoptera acutorostrataI));
  • High Frequency (HF) cetaceans (i.e. marine mammal species such as dolphins, toothed whales, beaked whales and bottlenose whales (e.g. bottlenose dolphin Tursiops truncates and white-beaked dolphin Lagenorhynchus albirostris));
  • Very High Frequency (VHF) cetaceans (i.e. marine mammal species such as true porpoises, river dolphins and pygmy/dwarf sperm whales and some oceanic dolphins, generally with auditory centre frequencies above 100 kHz) (e.g. harbour porpoise Phocoena phocoena));
  • Phocid Carnivores in Water (PCW) (i.e. true seals (e.g. harbour seal Phoca vitulina and grey seal Halichoreus grypus));
  • hearing in air is considered separately in the group Phocid Carnivores in Air (PCA); and
  • Other Marine Carnivores in Water (OCW): including otariid pinnipeds (e.g. sea lions and fur seals), sea otters and polar bears; air hearing considered separately in the group Other Marine Carnivores in Air (OCA).
    1. These weightings have therefore been used in this study and are shown in Figure 4.1   Open ▸ .

Figure 4.1:
Hearing Weighting Functions for Pinnipeds and Cetaceans (Southall et al., 2019)

Figure 4.1: Hearing Weighting Functions for Pinnipeds and Cetaceans (Southall et al., 2019)


  1. Injury criteria are proposed in Southall et al. (2019) are for two different types of sound as follows:
  • impulsive sounds which are typically transient, brief (less than one second), broadband, and consist of high peak sound pressure with rapid rise time and rapid decay (ANSI, 1986 and 2005; NIOSH, 1998). This category includes sound sources such as seismic surveys, impact piling and underwater explosions; and
  • non-impulsive sounds which can be broadband, narrowband or tonal, brief or prolonged, continuous or intermittent and typically do not have a high peak sound pressure with rapid rise/decay time that impulsive sounds do (ANSI, 1995; NIOSH, 1998). This category includes sound sources such as continuous running machinery, sonar, and vessels.
    1. The criteria for impulsive and non-impulsive sound have been adopted for this study given the nature of the sound source used during construction activities. The relevant criteria proposed by Southall et al. (2019) are as summarised in Table 4.1   Open ▸ and Table 4.2   Open ▸ .
    2. These updated marine mammal injury criteria were published in March 2019 (Southall et al., 2019). The paper utilised the same hearing weighting curves and thresholds as presented in the preceding regulations document NMFS (2018) (and prior to that Southall et al. (2007)) with the main difference being the naming of the hearing groups and introduction of additional thresholds for animals not covered by NMFS (2018). A comparison between the two naming conventions is shown in Table 4.3   Open ▸ .
    3. For avoidance of doubt, the naming convention used in this report is based upon those set out in Southall et al. (2019). Consequently, this assessment utilises criteria which are applicable to both NMFS (2018) and Southall et al. (2019).


Table 4.1:
Summary of PTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)

Table 4.1: Summary of PTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)


Table 4.2:
Summary of TTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)

Table 4.2: Summary of TTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)


Table 4.3:
Comparison of Hearing Group Names between NMFS (2018) and Southall et al. (2019)

Table 4.3: Comparison of Hearing Group Names between NMFS (2018) and Southall et al. (2019)


4.3. Disturbance to Marine Mammals

  1. Beyond the area in which injury may occur, the effect on marine mammal behaviour is the most important measure of potential impact. Significant (i.e. 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 significant disturbance resulting from the Proposed Development, it is therefore necessary to consider the likelihood that the sound could cause non-trivial disturbance, the likelihood that the sensitive receptors will be exposed to that sound and whether the number of animals exposed are likely to be significant at the population level. Assessing this is however a very difficult task due to the complex and variable nature of sound propagation, the variability of documented animal responses to similar levels of sound, and the availability of population estimates, and regional density estimates for all marine mammal species.
  3. Southall et al. (2007) recommended that the only currently feasible way to assess whether a specific sound could cause disturbance is to compare the circumstances of the situation with empirical studies. Joint Nature Conservation Committee (JNCC) guidance in the 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 significant adverse effects on life functions, which would constitute a disturbance.
  4. Southall et al. (2007) 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).

4.3.2.    Continuous (Non-Pulsed, Non-Impulsive) Sound

  1. For non-pulsed sound (e.g. drilled piles, vessels etc.), the lowest sound pressure level at which a score of five or more occurs for low frequency cetaceans is 90 dB to 100 dB re 1 μPa (rms). However, this relates to a study involving migrating grey whales. A study for minke whales 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 Proposed Development. For Atlantic white-beaked dolphin Lagenorhynchus albirostris, a response score of three was encountered for received levels of 110 to 120 dB re 1 μPa (rms), with no higher severity score encountered. For high frequency cetaceans such as bottlenose dolphins Tursiops truncatus, 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. The NMFS (2005) guidance sets the marine mammal level B harassment threshold for continuous noise at 120 dB re 1 μPa (rms). This value sits approximately mid-way between the range of values identified in Southall et al. (2007) for continuous sound but is lower than the value at which the majority of 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 sound, it is recommended that any ranges predicted using this number are viewed as probabilistic and potentially over precautionary.

4.3.3.    Impulsive (Pulsed) Sound

  1. Southall et al. (2007) presents a summary of observed behavioural responses due to multiple pulsed sound, although the data are primarily based on responses to seismic exploration activities (rather than for piling). Although these datasets contain much relevant data for LF cetaceans, there are no strong data for MF or HF cetaceans. Low frequency cetaceans, other than bow-head whales, were typically observed to respond significantly at a received level of 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 1μPa (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 1μPa (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 1μPa (rms).
  2. A more recent study is described in Graham et al. (2017). Empirical evidence from piling at the Beatrice Offshore Wind Farm (Moray Firth, Scotland) was used to derive a dose-response curve for harbour porpoise. 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 (un-weighted) 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 is an accepted approach to understanding the behavioural effects from piling and has been applied at other UK offshore wind farms (for example Seagreen and Hornsea Three).
  3. According to Southall et al. (2007) there is a general paucity of data relating to the effects of sound 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 non-pulsed sound, 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). No data are available for higher noise levels and the low number of animals observed in the various studies means that it is difficult to make any firm conclusions from these studies.
  4. Southall et al. (2007) also notes that, due to the uncertainty over whether HF cetaceans may perceive certain sounds and due to 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 sound 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[3] rms sound pressure level of 166 dB re 1 μPa.
  5. Clearly, 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.
  6. The High Energy Seismic Survey (HESS) workshop on the effects of seismic (i.e. pulsed) sound on marine mammals (HESS, 1997) concluded that mild behavioural disturbance would most likely occur at rms sound 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 sound.
  7. This assessment adopts a conservative approach and uses the NMFS (2005) Level B harassment threshold of 160 dB re 1 μPa (rms) for impulsive sound, excluding piling which is assessed based on SEL in volume 2, chapter 10. 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 this assessment.
  8. It is important to understand that exposure to sound levels in excess of the behavioural change threshold stated above does not necessarily imply that the sound 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 sound and whether the numbers exposed are likely to be significant at the population level.


Table 4.4:
Disturbance Criteria for Marine Mammals Used in this Study

Table 4.4: Disturbance Criteria for Marine Mammals Used in this Study


4.4. Fish

  1. Adult fish not in the immediate vicinity of the noise generating activity are generally able to vacate the area and avoid physical injury. However, larvae and eggs are not highly mobile and are therefore more likely to incur injuries from the sound energy in the immediate vicinity of the sound source, including damage to their hearing, kidneys, hearts and swim bladders. Such effects are unlikely to happen outside of the immediate vicinity of even the highest energy sound sources.
  2. For fish, the most relevant criteria for injury are considered to be those contained in the recent Sound Exposure Guidelines for Fishes and Sea Turtles (Popper et al., 2014). Popper et al. (2014) guidelines do not group by species but instead broadly group fish into the following categories based on their anatomy and the available information on hearing of other fish species with comparable anatomies:
  • Group 1: fishes with no swim bladder or other gas chamber (e.g. elasmobranchs, flatfishes and lampreys). These species are less susceptible to barotrauma and are only sensitive to particle motion, not sound pressure. Basking shark Cetorhinus maximus, which does not have a swim bladder, falls into this hearing group.
  • Group 2: fishes with swim bladders but the swim bladder does not play a role in hearing (e.g. salmonids). These species are susceptible to barotrauma, although hearing only involves particle motion, not sound pressure.
  • 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.
  • 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 spp. and shads (Alosinae)). 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.
  • Sea turtles: There is limited information on auditory criteria for sea turtles and the effect of impulsive noise is therefore inferred from documented effects to other vertebrates. Bone conducted hearing is the most likely mechanism for auditory reception in sea turtles and, since high frequencies are attenuated by bone, the range of hearing are limited to low frequencies only. For leatherback turtle Dermochelys coracea the hearing range has been recorded as between 50 Hz and 1,200 Hz with maximum sensitivity between 100 Hz and 400 Hz; and
  • Fish eggs and larvae: separated due to greater vulnerability and reduced mobility. Very few peer-reviewed studies report on the response of eggs and larvae to anthropogenic sound.
    1. The guidelines set out criteria for injury due to different sources of noise. Those relevant to the Proposed Development are considered to be those for injury due to impulsive piling sources only, as non-impulsive sources were not considered to be a key potential impact and therefore were screened out of the guidance[4]. The criteria include a range of indices including SEL, rms and peak SPLs. Where insufficient data exist to determine a quantitative guideline value, the risk is 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). It should be noted that these qualitative criteria cannot differentiate between exposures to different noise levels and therefore all sources of noise, no matter how noisy, would theoretically elicit the same assessment result. However, because the qualitative risks are generally qualified as “low”, with the exception of a moderate risk at “near” range (i.e. within tens of metres) for some types of animal and impairment effects, this is not considered to be a significant issue with respect to determining the potential effect of noise on fish.
    2. The injury criteria used in this noise assessment for impulsive piling are given in Table 4.5   Open ▸ . In the table, both peak and SEL criteria are unweighted. Physiological effects relating to injury criteria are described below (Popper et al., 2014; Popper and Hawkins, 2016):
  • Mortality and potential mortal injury: either immediate mortality or tissue and/or physiological damage that is sufficiently severe (e.g. a barotrauma) that death occurs sometime later due to decreased fitness. Mortality has a direct effect upon animal populations, especially if it affects individuals close to maturity.
  • Recoverable injury: Tissue and other physical damage or physiological effects, that are recoverable but which may place animals at lower levels of fitness, may render them more open to predation, impaired feeding and growth, or lack of breeding success, until recovery takes place.
  • TTS: Short term changes in hearing sensitivity may, or may not, reduce fitness and survival. Impairment of hearing may affect the ability of animals to capture prey and avoid predators, and also cause deterioration in communication between individuals; affecting growth, survival, and reproductive success. After termination of a sound that causes TTS, normal hearing ability returns over a period that is variable, depending on many factors, including the intensity and duration of sound exposure.


Table 4.5:
Criteria for Onset of Injury to Fish and Sea Turtles due to Impulsive Piling (Popper et al., 2014)

Table 4.5: Criteria for Onset of Injury to Fish and Sea Turtles due to Impulsive Piling (Popper et al., 2014)


  1. The criteria used in this noise assessment for non-impulsive piling and other continuous noise sources, such as vessels, are given in Table 4.6   Open ▸ . The only numerical criteria for these sources are for recoverable injury and TTS for Groups 3 and 4 Fish.


Table 4.6:
Criteria for Onset of Injury to Fish and Sea Turtles due to Non-Impulsive Sound (Popper et al., 2014)

Table 4.6: Criteria for Onset of Injury to Fish and Sea Turtles due to Non-Impulsive Sound (Popper et al., 2014)


  1. The criteria used in this noise assessment for non-impulsive piling and other continuous noise sources, such as vessels, are given in Table 4.7   Open ▸ .


Table 4.7:
Criteria for Injury to Fish due to Explosives (Popper et al., 2014)

Table 4.7: Criteria for Injury to Fish due to Explosives (Popper et al., 2014)


  1. It should be noted that there are no thresholds in Popper et al. (2014) in relation to noise from high frequency sonar (>10 kHz). This is because the hearing range of fish species falls well below the frequency range of high frequency sonar systems. Consequently, the effects of noise from high frequency sonar surveys on fish has not been conducted as part of this study, due to the frequency of the source being beyond the range of hearing and also due to the lack of any suitable thresholds.
  2. Behavioural reaction of fish to sound has been found to vary between species based on their hearing sensitivity. Typically, fish sense sound via particle motion in the inner ear which is detected from sound-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 sound) can be detected by fish without swim bladders[5].
  3. Highly sensitive species such as herring have elaborate specialisations of their auditory apparatus, known as an otic bulla – a gas filled sphere, connected to the swim bladder, which enhances hearing ability. The gas filled swim bladder in species such as cod and salmon may be involved in their hearing capabilities, so although there is no direct link to the inner ear, these species are able to detect lower sound frequencies and as such are considered to be of medium sensitivity to noise. Flat fish and elasmobranchs have no swim bladders and as such are considered to be relatively less sensitive to sound pressure.
  4. The most recent criteria for disturbance are considered to be those contained in Popper et al. (2014) which set out criteria for disturbance due to different sources of noise. The risk of behavioural effects is 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 4.8   Open ▸ .


Table 4.8:
Criteria for Onset of Behavioural Effects in Fish and Sea Turtles for Impulsive and Non-Impulsive Sound (Popper et al., 2014)

Table 4.8: Criteria for Onset of Behavioural Effects in Fish and Sea Turtles for Impulsive and Non-Impulsive Sound (Popper et al., 2014)


  1. It is important to note that the Popper et al. (2014) criteria for disturbance due to sound are qualitative rather than quantitative. Consequently, a source of noise of a particular type (e.g. piling) would result in the same predicted potential impact, no matter the level of noise produced or the propagation characteristics.
  2. Therefore, the criteria presented in the Washington State Department of Transport Biological Assessment Preparation for Transport Projects Advanced Training Manual (WSDOT, 2011) are also used in this assessment for predicting the extent of behavioural effects due to impulsive piling. The manual suggests an un-weighted sound pressure level of 150 dB re 1 μPa (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.