EIA Quality Mark: Developing best practice in noise impact assessment
Jennifer Smith reviews noise assessment guidance against international approaches to discuss best practice development in this arena
The UK is party to several international conventions that aim to minimise impacts from anthropogenic noise across Europe and around the world. These include the Convention on Migratory Species (CMS), the Agreement on the Conservation of Cetaceans in the Black Sea Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS), and the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBAMS). The conventions compel party countries to develop environmental management and mitigation measures as a means of achieving this goal.
Several regulatory bodies have responded by developing guidance on how to assess noise-related impacts on marine mammals, which have been identified as particularly sensitive to anthropogenic noise because of the vital role hearing plays in the behavioural ecology of those species. While the need to quantify potential impacts to marine mammals from underwater noise is unanimous, the methods employed to assess such impacts differ between countries. Traditionally, guidance has focused on minimising risks of injury to marine mammals through the development of quantitative noise exposure reference values, called ‘acoustic injury thresholds’. However, internationally standardised threshold values are currently unavailable, and guidance remains inconsistent between countries.
The primary driver for discrepancies between assessment guidance is the fact that acoustic injury thresholds are inferred from in situ measurements of temporary change to hearing ability, termed ‘temporary threshold shift’ (TTS). Direct measurements of auditory injury, or ‘permanent threshold shift’ (PTS), are impossible to obtain without causing injury to the subject. In turn, the development of acoustic injury metrics depends upon physiological research on marine mammal hearing – not always easy to acquire – and the prediction of when temporary hearing change becomes permanent and injurious. In the face of such complexities, countries have varied in adapting their guidance with the mounting evidence on species-specific auditory response.
Marine mammal acoustic injury metrics have been proposed by numerous sources during the past two decades (NMFS, 1995; HESS, 1999; Southall et al., 2007; Lucke et al., 2008; JNCC et al., 2010; Scottish Government, 2014). Although care has been taken to update the mitigation measures provided in the UK guidance on minimising acoustic injury to marine mammals (JNCC, 2010; JNCC, 2017), the metrics used to assess potentially injurious activities have gone unchanged since 2010 (Southall et al., 2007; Lucke et al., 2008; SNCBs, 2010; Marine Scotland, 2014).
The National Oceanic and Atmospheric Administration (NOAA), which regulates noise impacts to marine mammals in US waters, has undertaken several revisions to its guidance in response to new research on marine mammal hearing (NMFS, 1995; NOAA, 2016; NOAA, 2018). The most recent of these revisions (NOAA, 2018) reflects new measurements of TTS across a broad range of species (Finneran, 2015; Finneran, 2016). This research has been used to identify more conservative injury thresholds than those proposed in the current UK guidance (SNCBs, 2010; Marine Scotland, 2014) based on older data from Southall et al. (2007) and Lucke et al. (2008). Moreover, the NOAA (2018) guidance considers instantaneous peak sound pressure and cumulative sound exposure levels (24 hours) for impulsive and non-impulsive sounds based on the varied hearing profiles of cetaceans, instead of the M-weighted hearing profiles currently used in the UK guidance (JNCC, 2010).
The revised injury threshold metrics from the NOAA (2018) guidance have been incorporated into Xodus’ noise propagation modelling, which we use to identify impact ranges associated with noise-generating activities. Incorporation of the more conservative TTS values from the NOAA (2018) revisions into our noise impact assessments ensures we are providing best practice methods for the identification of potential noise-induced injury to marine mammals. This approach is supported by the CMS, which confirms the use of the NOAA acoustic guidelines as a benchmark for noise impact assessment criteria (Prideaux, 2017).
In future, best practice in noise impact assessments may consider the onset of TTS as a metric for auditory injury. New evidence suggests that prolonged TTS can degrade the neural transmission between hair cells and nerves in the inner ear, causing permanent hearing loss (Kujawa and Liberman, 2009; Kujawa and Liberman, 2015). Germany has already incorporated this knowledge into its impact assessment guidance to protect harbour porpoises (Prideaux, 2017). In addition to revising its criteria in response to published data, future improvements to UK noise impact assessment guidance should consider the use of TTS metrics as injury criteria to provide more robust protection for the many and varied marine mammal species its waters host.
Jennifer Smith is a graduate environmental consultant in the EIA Team at Xodus Group, Aberdeen
Image credit | iStock
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