“Establishing noise exposure criteria for marine mammals has proven to be a difficult and contentious issue”.
Why do we need noise exposure policy?
A significant source of impact on marine organisms is sound from loud, impulsive underwater noise activities. Marine mammals are considered to be particularly sensitive and policy makers must put regulations in place to mitigate impact. However, establishing noise exposure criteria for marine mammals, including limits above which undesired negative effects are expected to occur has proven difficult.
There are many sources of sound and these are typically generalised to account for wide ranges of sound sources and environmental conditions. Sound can be split into a series of frequency bands and not all frequencies are perceived equally well. One of the main recommendations for setting thresholds has been to take the hearing abilities of a species of interest into account.
Auditory frequency weighting
Data availability to generate marine mammal species specific weighting functions is rare. Several weighting functions have been proposed and there remains controversy over which functions to use. Using incorrect weighting may lead to greater impact on animals or unnecessarily tight restrictions on activities.
The October 2017 paper “Why is auditory frequency weighting so important in regulation of underwater noise?” by Jakob Tougaard and Michael Dahne, highlights the importance of selecting the right frequency weighting. Assessing the risk of Temporary Threshold Shift (TTS) in porpoises exposed to pile driving noise, they report that time until TTS depended strongly on choice of weighting function: with results 2 orders of magnitude larger for an audio-weighted TTS criterion relative to an unweighted criterion.
Regulation of underwater noise
Due to the importance of weighting functions in the regulation of underwater noise Tougaard and Dahne strongly suggest continued experimental effort targeting this weighting for marine mammals and different noise sources. And they encourage flexibility by regulators and the legislation they produce to allow incorporation of new results as soon as they can be validated.
Importance to us
Here at SMRU Consulting we provide specialist input to the Environmental Impact Assessments (EIA) process as well as expert review of developer (EIAs). We strive to use the best science available to support our innovative, robust and environmentally sound solutions for clients active in the marine environment. To do that we need to stay up to date with the literature. We hope you will join us on our journey to keep current and read along with us.