4. Baseline

  1. Background or “ambient” underwater noise is created by several natural sources, such as rain, breaking waves, wind at the surface, seismic noise, biological noise, and thermal noise. Anthropogenic noises related to the Array activities can be either impulsive (pulsed) such as impact piling, or non-impulsive (continuous) such as ship engines, and the magnitude of the potential impact on marine species will depend heavily on these characteristics. Biological noise sources include marine mammals (using noise to communicate, build up an image of their environment and detect prey and predators) as well as certain fish and shrimp. Anthropogenic noise sources of noise in the marine environment include fishing boats, ships (non-impulsive), marine construction, seismic surveys, and leisure activities (all could be either impulsive or non-impulsive), all of which add to ambient background noise. Other anthropogenic noise within the vicinity of the Array will arise primarily from shipping, the offshore oil and gas industry, subsea geophysical and geotechnical surveys, and the offshore renewables industry.
  2. Historically, research relating to both physiological effects and behavioural disturbance of noise on marine receptors has typically been based on determining the absolute noise level for the onset of that effect (whether presented as a single onset threshold or a dose-response/probabilistic function). Consequently, the available numerical criteria for assessing the effects of noise on marine mammals, fish, and shellfish, tend to be based on the absolute noise level criteria, rather than the difference between the baseline noise level and the noise being assessed (Southall et al., 2019).
  3. Baseline or background noise levels vary significantly depending on multiple factors, such as seasonal variations and different sea states. Lack of long term measurements/noise data is a widely recognised gap in knowledge in relation to general noisescape and potential effects of human activities on marine species. Understanding the baseline noise level could therefore be valuable in enabling future studies to assess long term effects related to continuous noise levels over time in addition to activity specific effects such as masking, i.e. interfering with useful noises such as predator or prey activity. However, the value of establishing the precise baseline noise level is limited in relation to the current assessment methods due to the lack of available evidence-based studies on the effects of noise relative to background levels on marine receptors.

 

5. Acoustic Assessment Criteria

  1. Section 5 describes the background and criteria on which the assessment has been based. Agreement on the methodology was agreed with NatureScot via the Array EIA Scoping Report (volume 3, appendix 6.1) and post-Scoping consultation via email on 05 December 2023. Consultation relevant to underwater noise is included in volume 3, appendix 5.1 and responses to consultation have been addressed in volume 2, chapter 10.

5.1. Introduction

  1. Underwater noise has the potential to affect marine species 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 noise. Audibility itself does not implicitly mean that the noise 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 noises 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 noise in relation to masking levels[6] (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 noise level is high enough to cause tissue damage in the ear. This can be classified as either a Temporary Threshold Shift (TTS) or PTS. At even closer ranges, and for very high intensity noise sources (e.g. underwater explosions), physical trauma or even death are possible.
  1. For the study contained within this technical report, it is the zones of injury and disturbance (i.e. responsiveness) that are of interest (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.

5.2. Injury (Physiological Damage) to Mammals

  1. Noise propagation models can be constructed to allow the received noise level at different distances from the source to be calculated. To determine the potential 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 auditory injury (PTS/TTS) threshold criteria proposed by Southall et al. (2019) are based on a combination of unweighted peak pressure levels and mammal hearing weighted SEL. The hearing weighting function is designed to represent the frequency characteristics (bandwidth and noise level) for each group within which acoustic signals can be perceived and therefore assumed have auditory effects. The categories include:
  • Low Frequency (LF) cetaceans: marine mammal species such as baleen whales (e.g. minke whale Balaenoptera acutorostrata).
  • High Frequency (HF) cetaceans: 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: 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): 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).
  • 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 weighting functions have therefore been used in this study and are shown in Figure 5.1   Open ▸ .

Figure 5.1:
Hearing Weighting Functions for Pinnipeds and Cetaceans (Southall et al., 2019)

Figure 5.1: Hearing Weighting Functions for Pinnipeds and Cetaceans (Southall et al., 2019)

 

  1. Auditory injury criteria proposed in Southall et al. (2019) are for two different types of noise as follows:
  • Impulsive noises 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 noise sources such as seismic surveys, impact piling and underwater explosions.
  • Non-impulsive noises 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 noises do (ANSI, 1995; NIOSH, 1998). This category includes noise sources such as continuous running machinery, sonar, and vessels.
  1. The criteria for impulsive and non-impulsive noise have been adopted for this study given the nature of the variety of noise source used during the various activities. The relevant criteria proposed by Southall et al. (2019) are as summarised in Table 5.1   Open ▸ and Table 5.2   Open ▸ .

 

Table 5.1:
Summary of PTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)

Table 5.1: Summary of PTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)

 

Table 5.2:
Summary of TTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)

Table 5.2: Summary of TTS Onset Acoustic Thresholds (Southall et al., 2019; Tables 6 and 7)