| Summary: | This thesis aims to describe behavioural responses and welfare parameters for the physostome Atlantic salmon and physoclist Atlantic cod when out of neutral buoyancy at different degrees and periods of time. The background for this approach is to improve the culture conditions by fully submergence of the farming installations, as the water below 10 m depth often is more stable in terms of environmental factors such as temperature and current in addition to the lack of waves, which again opens for alternative sites in more exposed oceanic areas. Salmon were submerged in large-scale fully submersible cages at depths between 4 to 15 m for 22 days (Paper I) and 10 to 25 m for 42 days (Paper II) under different light conditions and at various times of year. The behaviour of individual salmon in a school under submerged conditions was studied to reveal whether a range of coping abilities among individuals exists during the new and more challenging conditions (Paper III). Atlantic cod in an experimental submersible cage were raised from five different starting depths (between 30 and 8 m) and lowered from surface position to 10 m, 20 m and 30 m to test a protocol for safe lifting and lowering steps. Based on the behavioural responses, safe acclimation times before the next vertical step at high and low sea temperatures were identified (Paper IV). In Papers I and II, the general patterns of swimming depth and schooling density were studied at group level using echo-sounders in addition to swimming speed and swimming angle based on instantaneous observations with underwater cameras. Welfare parameters were defined as weight gain, feed intake, feed utilisation and fin and vertebral condition during the experimental period. The behaviour of individual salmon was studied by monitoring the swimming depth and body temperature using data storage tags implanted in randomly selected fish. Whether the development of diel vertical migration (DVM) activity during feeding was linked with individual growth rates was analysed to study ...
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