Effect of environmental factors on swimming depth preferences of Atlantic salmon (Salmo salar L.) and temporal and spatial variations in oxygen levels in sea cages at a fjord site

Behavioural indicators of welfare in farmed fish

“…In fish tanks and cages, there will usually be areas that are more favourable than the others and restricting these volumes will result in a trade-off between packing density (number of fish per cubic metre, Juell and Fosseidengen 2004) and environmental conditions. Especially in sea cages, there can be large environmental gradients, with highly variable temperature, salinity, oxygen, current speed and light conditions (Juell 1995, Juell and Fosseidengen 2004, Johansson et al 2006. Fish are attracted to the most optimal areas and avoid the unfavourable ones as a result of both attractions to a part of the rearing volume with preferred environmental conditions and avoidance/repulsion from areas with poor environmental conditions.…”

“…Fish are attracted to the most optimal areas and avoid the unfavourable ones as a result of both attractions to a part of the rearing volume with preferred environmental conditions and avoidance/repulsion from areas with poor environmental conditions. Such conditions cause a trade-off between packing density and the benefits of being in the relatively best areas, and at high stocking densities, the less competitive fish will be forced to stay in suboptimal areas (Juell 1995, Johansson et al 2006). The behaviour of the fish can also have an effect on environmental conditions, and the high local concentrations of fish can cause reduced water flow and increased oxygen consumption leading to areas with hypoxia.…”

Atlantic salmon Salmo salar instantaneously follow vertical light movements in sea cages

“…Frenzl et al 2014), or environmental conditions. Temperature is known to override light attraction in Atlantic sal mon (Oppedal et al 2007), but it may not have been the driver for the behaviour of the surface-positioned subgroup in this study as warmer depths optimising salmon thermoregulation (Johansson et al 2006) were in the lower cage sections. It is possible that the surface fish were infected by salmon lice (0.85 lice fish −1 detected before the start of our study) and selected shallow lowsalinity areas to self-treat against these freshwatersusceptible parasites (Gjelland et al 2014).…”

“…For instance, free-living infective stages of the parasitic sal mon louse Lepeophtheirus salmonis concentrate at different depths depending on vertical salinity gradients (Jones & Beamish 2011). Also, sal mon periodically aggregating in a narrow depth range at extreme densities (Oppedal et al 2011) can lower dissolved oxygen levels as they respire (Johansson et al 2006). These adverse conditions may be mitigated by manipulating salmon depth in sea cages over short time scales.…”

“…For instance, light attractiveness fades in temperature-stratified environments when optimal temperatures for salmon thermoregulation (~17°C; Johansson et al 2006) occur outside illuminated depths (Oppedal et al 2007). Attraction also diminishes if lights are of lower intensity (Stien et al 2014).…”