Long-term underwater passive acoustic monitoring of the soundscape in an Atlantic salmon sea cage

Abstract Understanding the underwater acoustic environment in Atlantic salmon ( Salmo salar ) farming is essential for evaluating potential impacts on fish welfare. This study presents a long-term characterisation of soundscapes in an open sea cage, distinguishing between baseline normal conditions, delousing and net cleaning operations. A hydrophone was deployed inside an open sea cage, with passive acoustic recordings collected from July through November 2023. Operational activities were tracked using operational logs, AIS data for the nearby area and time-lapse imagery of the farm site. A total of 66 days were categorised as ‘normal’ operation days, 24 as days with net cleaning activities and 11 days included delousing activities at the farm. Results show that net cleaning consistently produced higher sound pressure levels (SPLs) for frequencies over 200 Hz, compared to normal operations and delousing. Spectral analysis revealed distinct acoustic signatures for each activity, with net cleaning showing the highest variability and intensity. The 95th percentile for the broadband signal of 20 Hz to 12.8 kHz for days where the monitored cage was cleaned was 150 dB re 1 μPa. These findings suggest that net cleaning may pose a greater risk of inducing acoustic stress in farmed salmon compared to other routine operations. However, further research is needed to determine whether salmon habituate to these sounds or experience cumulative stress over time. Passive acoustic monitoring offers a valuable tool for assessing the impact of aquaculture practices on the rearing environment of Atlantic salmon.