Thermo- and photoregulatory swimming behaviour of caged Atlantic salmon: Implications for photoperiod management and fish welfare

This study was used to investigate where groups of caged Atlantic salmon (Salmo salar L.) reside in relation to photoperiod management in a thermally stratified environment. Underwater lamps were deployed at depths of either 1, 5 or 10 m for two weeks in the winter, spring and summer seasons and swimming depth and schooling densities were recorded by means of echointegration. Changes in swimming depth strongly indicated thermo- and photoregulatory behaviour, sometimes resulting in a behavioural trade-off between a preference for the highest temperature available and attraction to the brighter parts of the cage. This behaviour also strongly influenced the space use in the cages and schooling densities of up to 10 times the stocking density (2–9 kg m− 3) was observed. Differences in swimming depths were observed between lamp deployment depths during every season (P < 0.05). The salmon preferred to swim in the highest temperature available in daytime while this was modulated by lamp depth if light intensity and temperature maximum deviated at night. A calculated preference index indicated that thermoregulation was most distinct during the summer, when the available temperature range was widest (7–14 °C). The results suggest that farmers should develop site-specific light management to secure efficient photoperiod treatment and ensure that the behavioural needs characteristic of caged salmon is met.