Source Levels of an Acoustic Harassment Device System on an Operational Scottish Salmonid Farm

Acoustic Harassment Devices (AHDs) are used worldwide to deter pinnipeds from predating fish-aquaculture facilities; however, effects on non-target species are of concern. This study focused on the newly developed, Research & Development (R&D) OTAQ Aquaculture SealFence AHD system, tested at a fully operational salmonid farm in Scotland, located within a Special Area of Conservation. The primary aim was to estimate the Source Levels (SLs) of the R&D AHD system in real field conditions and assess its signal propagation. Field measurements revealed that AHD signals were detectable up to 4.2 km. The estimated SLs ranged from 123.14 to 134.52 dB re 1μPa RMS @ 1 m. High variation in SPLs was observed at close distances from the AHD, likely due to signal directionality, reflections from fish cages and farm structures, and multipath interference from the water surface and seabed. These processes can produce constructive and destructive interference, resulting in pronounced variability in the measured AHD signal at short ranges. Background noise measurements suggest that introduction of anthropogenic noise into the marine environment, combined with other sources, should always be considered. This study provides valuable empirical data on the acoustic output of a new AHD system, highlighting its potential to minimise noise pollution compared to existing devices; however, further research is needed to evaluate its efficacy in deterring seals and its impact on non-target species.