Effects of intensity and predictability of the environmental soundscape in Atlantic salmon

Commercial aquaculture in both land and sea facilities is associated with an increase of anthropomorphic noise pollution, which may impose a chronic stress challenge for captive fish. The nature of the sound exposure in both time and intensity may affect salmonids in different ways. There is however a lack of knowledge on how predictable and unpredictable sounds affect farmed salmon during the production cycle. We compared fish exposed to predictable, unpredictable or control (low intensity) sound conditions during the freshwater stage and exposed all groups to unpredictable sound during the seawater phase. We assessed their behavioral responses, the physiological (cortisol) and neuroendocrine (brain monoamines) stress response, and the expression of target genes in the brain. We found that exposing farmed salmon that experienced the quietest and most predictable environment during the freshwater phase were the least reactive to unpredictable sounds in the sea phase. Notably, when subjected to a novel confinement stress test, both control and unpredictable groups responded with an increase in plasma cortisol and serotonergic activity in all studied brain areas. However, the fish from the predictable group generally showed a more blunted response in brain serotonergic activity to stress. Importantly, a blunted serotonergic response accompanied by an increase in cortisol, has been associated with dysregulation of the neuroendocrine system in response to chronic stress. Our results suggest that providing salmon with a low sound environment during the freshwater phase will result in a more resilient fish when encountering a typical sea-cage soundscape (i.e. characterized by unpredictability).