Abstract
This report investigates the differences between two sets of marine mammal noise exposure criteria in the prediction of permanent auditory injury in marine mammals: the Southall criteria, published in 2007 and the NOAA criteria (also known as the NMFS criteria), published in 2016. Both have been widely adopted in the UK and beyond, with the NOAA criteria considered as the most up-to-date scientific approach. This has created a need among regulators and their advisors for scientific advice on the quantitative differences resulting from the application of each set of criteria, and the reasons underpinning those differences in order to understand these differences in legacy assessments consented under Southall and now being reassessed under NOAA.
Since there are differences between the criteria in both the cumulative noise exposure thresholds and the auditory weightings (which account for frequency sensitivity in marine mammal hearing), the interplay between these two factors is important.
Cefas has undertaken an analysis and modelling exercise which applied both criteria to representative noise assessment scenarios for UK waters. The aims were:
(i) investigate which set of criteria are more precautionary for each marine mammal hearing group;
(ii) investigate whether there are consistent differences in the effect ranges and effect areas predicted for permanent auditory injury, known as Permanent Threshold Shift (PTS).
The noise assessment scenarios consisted of four typical noise sources: percussive pile driving, seismic airguns, explosions, and vibratory pile driving, which were each modelled in representative environments.
- In general, the NOAA criteria were found to be consistently more precautionary than Southall criteria for low frequency cetaceans (baleen whales) and for highfrequency cetaceans (including the harbour porpoise);
- For phocid pinnipeds (including grey and harbour seals) NOAA criteria are the
same as the Southall criteria for the peak Sound Pressure Level (SPL) criterion,
and consistently less precautionary for the impulsive and non-impulsive
cumulative Sound Exposure Level (SEL) criteria; - For mid-frequency cetaceans (including dolphin species) NOAA criteria were
either equivalent or less precautionary, except in the case of vibratory pile driving,
a result which is not expected to apply to other non-impulsive low-frequency
sources, such as drilling, shipping, or dredging.
In addition to reporting quantitative differences between the criteria in the scenarios assessed, the differences in effect ranges and effect areas predicted by the criteria were analysed. The resulting figures included in this report provide guidance on the likely effect ranges and effect areas that would be predicted under one set of criteria based on the predictions for the other criteria (according to the scenarios which were modelled). These results can assist developers, regulators, and their advisors in interpreting differences in predictions between the criteria and in anticipating the likely effect of applying a different set of criteria based on existing predictions.