Potential Audibility of Three Acoustic Harassment Devices (AHDs) to Marine Mammals in Scotland, UK

Abstract

The modelled acoustic characteristics of three Acoustic Harassment Devices (AHDs) deployed from a fully operational salmonid fish farm, located in the Sound of Mull, Scotland (UK) are presented, using empirical seabed and water column measurements at the same location. In the Beaufort Sea state 0, the depth range of 10--50 m is the maximum range at which AHDs are potentially audible to five marine mammal species. The species present within this survey region are: the harbour porpoise, Phocoena phocoena (99.1 km), the killer whale, Orcinus orca (110 km), the bottlenose dolphin, Tursiops truncatus (89.6 km), the common seal, Phoca vitulina (88 km), and the grey seal, Halichoerus grypus (69 km). Consequently, within the Sound of Mull, all three AHDs could be heard throughout the water column by all species. For two models of AHDs, a behavioural disturbance level of between 140 dB-180 dB is observed at 1.3 km. Habitat displacement is a cause for concern, particularly if several 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, AHDs have the 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.