Prediction of marine mammal auditory-impact risk from Acoustic Deterrent Devices used in Scottish aquaculture

Acoustic Deterrent Devices (ADDs) are used worldwide to deter pinnipeds from predating fish-aquaculture facilities. Desk-based noise-propagation modelling of six commercial ADD models, and a ‘fictional’ ADD was performed, the latter involving alternating source level, frequency, duty cycle, noise-exposure duration, and number of ADDs active simultaneously. Potential auditory impacts on marine mammals were explored using the Southall et al. (2019) criteria. Depending on operational characteristics, real ADDs were predicted to cause Temporary Threshold Shift (TTS) to Very High Frequency (VHF) cetaceans at ranges of 4–31 km, and a single fictional device operating at the highest outputs tested was predicted to cause TTS to VHF cetaceans at up to 32 km. Cumulative effects of 23 real fish-farm ADDs produced noise across large swathes of the Inner-Hebrides. The single variable causing greatest reduction in potential impact to marine mammals from fictional ADDs was SL.