Potential audibility of three Acoustic Harassment Devices (AHDs) to marine mammals in Scotland, UK

We model acoustic characteristics of three Acoustic Harassment Devices (AHDs), deployed from a fully-operational salmonid fish farm, located in the Sound of Mull, Scotland (UK), using empirical seabed and water column measurements at the same location. Ranges at which AHDs are potentially audible to five marine mammal species present within the survey region were: harbour porpoise (Phocoena phocoena), bottlenose dolphin (Tursiops truncatus), killer whale (Orcinus orca), grey seal (Halichoerus grypus), and common seal (Phoca vitulina).
Within the Sound of Mull, all three AHDs could be heard throughout the water column, and for two models of AHD, a behavioural disturbance level of between 140 dB – 180 dB was observed at 1.3 km. Habitat displacement is a cause for concern, particularly if a number of fish farms within a small area all deploy AHDs simultaneously. This can create a confusing sound field of varying intensity, which has potential to deter harbour porpoises from sections of their habitat.
If positioned effectively AHD devices have potential to deter all five marine mammal species from industrial operations such as aquaculture facilities. Source levels, propagation and transmission loss measurements were highly variable and should be considered as site specific, meaning new estimates should be made for each situation.