Individual variation in swimming depth and growth in Atlantic salmon (Salmo salar L.) subjected to submergence in sea-cages

Large individual variation exists in the coping repertoire of Atlantic salmon (Salmo salar) in response to environmental changes in sea-cages. We compared the growth and behaviour of individual salmon within and between submerged (no surface access) and standard cages (with surface access serving as controls). Using high resolution data storage tags (DSTs) and measures of individual growth rates, we determined if behavioural outcomes differed among individuals. Two commercial-scale experiments involving different challenges were conducted; one with 0.5 kg salmon (n = 3800), submerged below 4 m for 22 days with 24 h continuous light, and the other with 4 kg salmon (n = 2300) submerged below 10 m for 42 days with natural light. Shallow, short submergences with constant underwater illumination resulted in a diurnal swimming depth pattern similar to control fish, although with slightly less variation among individual swimming depths at night and more variation during the day. Individuals with highest growth rates tended to swim deeper at night at end of the period of submergence. Salmon exposed to longer, deeper and dark submergence conditions displayed more irregular diurnal swimming patterns. Large variations in coping strategies were evident, likely as a result of differing levels of negative buoyancy among fish, possibly caused by deflated swim bladders. Submerged individuals with high growth rates swam either with a large amplitude diurnal cycle, or deeper during the day compared to fish with lower growth rates. Short-term, illuminated and shallow submergence resulted in low variation in swimming depth compared to the control fish, indicating that it did not compromise welfare. However, the more challenging deep, dark and long-term submergence with large differences in individual coping styles and shifts in diurnal swimming patterns compared to control fish, suggest compromised welfare for individual fish that coped poorly. Submerged farming can be a new method of culturing salmon only if the negative buoyancy caused by gas leakage from the swim bladder is avoided.

► Irregular diurnal swimming was seen for salmon in deep and dark submerged conditions.
► Shallow submergences with artificial light did not change the diurnal swimming.
► Fish with high growth changed depth level more often than slower growing individuals.