Exploring influences of lower-frequency Acoustic Deterrent Devices (ADDs) on harbour porpoise in Scottish coastal waters

Description

Acoustic Deterrent Devices (ADDs) are widely used to deter pinniped depredation from finfish farms by emitting loud, aversive sounds, which may also impact non-target species such as harbour porpoise (Phocoena phocoena). One potential method of reducing ADD noise impacts on species such as porpoises involves lowering output frequencies from typical ranges of 10-20 kHz downto <2 kHz, where porpoises’ hearing sensitivity is poorer compared to pinnipeds. We compared responses of wild harbour porpoises to experimental playbacks of an artificial ADD-like sinusoidal tonal burst signal (RMS source level 154-170 dB re 1 μPa-m) transmitted at two frequency ranges (8-18 kHz [HF] and 1-2 kHz [LF]). Signals were transmitted from a salmon farm in Bloody Bay (Sound of Mull, Scotland, UK). An array of 22 C-POD click train detectors was deployed to 5 km from the source and farm. Signal playbacks varied randomly between HF- and LF signals and a silent control, and occurred over 33 days (08/09-11/10/2016). During this period, 138 experimental playback bouts occurred, including 53 HF-signals, 38 LF-signals and 47 silent controls. C-POD data were used to infer porpoise presence and analysed using nonparametric Kruskal-Wallis tests and GAM-GEE models to evaluate effects of different signals and environmental covariates on porpoise detection probabilities. At most C-PODs, porpoise detection rates were significantly lower during both HF- and LF-signal playbacks relative to silent controls. GAM-GEE modelling identified “ADD-like signal presence” as an important factor determining porpoise detection probabilities at distances up to 800 – 1000 m from source. At greater distances, other covariates (e.g. day-night, ebb-flood and spring-neap cycles) dominated. Based on these results, widespread application of lower-frequency ADDs with signal characteristics similar to those tested would, by themselves, be unlikely to significantly reduce risk of acoustic impacts on harbour porpoises in Scottish waters, when compared to conventional ADD signals.

Period	10 Dec 2019
Event title	World Marine Mammal Conference
Event type	Conference
Location	Barcelona, Spain
Degree of Recognition	International