Characterisation of the underwater soundscape at Norwegian salmon farms

The average fish farm has become larger and more mechanised over recent decades, employing an array of machines (workboats, wellboats, pumps, compressors) that generate noise, potentially reducing the health, welfare and productivity of cultured fish or wild animals in the vicinity of the farm. We measured underwater sound pressure levels (SPLs) at 10 Atlantic salmon ( Salmo salar ) aquaculture sites in Norway, including 4 open sea-cage sites, 2 closed sea-cage sites, and 4 onshore sites. The soundscape (amplitude, frequency and variability of sound pressure waves) varied considerably, not only between sites, but also over time and space within sites, and generally peaked at frequencies within the sensitive range of salmonids (∼20–500 Hz). Continuous root-mean-square SPLs under typical farming conditions were higher within tanks and closed sea-cages than open sea cages, especially at low frequencies (mean 1-s SPL: 112–129 vs. 98–105 dB re 1 μPa over 20–100 Hz), but were relatively predictable compared to open sea-cages, where diurnal patterns and visits by workboats and wellboats exposed fish to fluctuating sound levels (range 1-s SPL: 83–157 dB re 1 μPa over 20–100 Hz). SPLs >140–150 dB re 1 μPa over 20–1000 Hz were rare at all sites, although impulsive SPLs with >173 dB re 1 μPa were observed on two occasions. More work is needed to understand long-term effects of ambient noise levels and the predictability of sounds on health and welfare outcomes for farmed fish. • Salmon farming systems are dominated by low frequency sounds audible to fishes. • Onshore systems and closed sea-cages are typically louder than open sea-cages. • Open sea-cages have a variable soundscape strongly influenced by vessel activity. • Sound pressure levels were generally not sufficient to cause hearing loss or injury.