242. NPL provide a review of the potential effects of noise on marine mammals (Appendix 9.1). The potential impacts of underwater noise on marine mammals can be summarised as lethal, physical injury, auditory injury, behavioural disturbance and masking, a brief description of each is provided below.
243. Very close to the source, the high peak pressure sound levels have the potential to cause death, or severe injury leading to death. High exposure levels from underwater sound sources can also cause hearing impairment; taking the form of a temporary loss in hearing sensitivity (Temporary Threshold Shift or TTS), or a permanent loss of hearing sensitivity (Permanent Threshold Shift or PTS). The potential for injury is not just related to the level of the underwater sound and its frequency relative to the hearing bandwidth of the animal, but is also influenced by the duration of exposure. Marine mammals may exhibit varying intensities of behavioural response at lower noise levels. The response can vary due to exposure level, the hearing sensitivity of the individual, context, previous exposure history or habitation, motivation and ambient noise levels (e.g. Southall et al. 2007).
244. The United States Marine Mammal Criteria Group of the National Marine Fisheries Service (NMFS) have proposed the 'M-weighting' model (Southall et al. 2007), as part of the Marine Mammal Noise Exposure Criteria. The marine mammal noise exposure criteria were developed through consensus of an expert committee and peer-reviewed. The criteria have found acceptance internationally and are now being recommended in the UK for use in environmental impact assessments, although these are currently being revised by National Oceanic and Atmospheric Administration (NOAA 2013).
245. In Southall et al. (2007), the injury criteria consider both SEL and SPL, where the SPL is considered for a peak level, and is not subjected to a weighted response. In terms of behavioural criteria, two general approaches are considered, both of which are described by Southall et al. (2007); the single pulse behavioural disturbance criteria and behavioural response severity scaling for multiple pulses. In this assessment the single pulse behavioural disturbance criterion is considered for the purpose of estimating ranges where a strong aversive response might occur (assumed to represent strong (near 100%) avoidance as it is based on the onset of TTS). However, using these criteria does not account for the potential disturbance associated with the duration of the noise producing activity. The behavioural response severity scaling for multiple pulses (Southall et al. 2007) is used as an indicator of ranges where behavioural changes and some level of reduction in animal abundance may be expected (possible avoidance) over the duration of the noise impact. The scaling was developed to delineate those behaviours that are relatively
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 67
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