Abstract
Salmon may sense and respond to a range of environmental variables within sea-cages, including light, temperature, salinity, dissolved oxygen, water currents and certain chemical treatments used during production. Environments within sea-cages are typically highly variable in both space and time, with the greatest variation occurring with depth. Preferred swimming depths and densities of salmon are the result of active trade-offs among environmental influences and an array of internal motivational factors such as feed and perceived threats. When preferred levels of multiple environmental cues exist at different depths, behavioural responses to temperature, light, the entry of feed, oxygen levels or the presence of treatment chemicals may dominate and override behavioural responses to all other drivers and determine swimming depths. Behavioural trade-offs in response to environmental drivers typically result in schooling at specific depths within sea-cages at densities 1.5 to 5 times their stocked density, and up to 20 times in extreme cases. Understanding the spatial and temporal variability of key environmental variables within sea-cages and how salmon respond to them may enable modifications to sea-cage environments to improve welfare outcomes, feeding regimes, artificial light management strategies and the efficacy of sea-lice treatments.