Effects of different stocking densities on physiological parameters and growth of adult Atlantic salmon ( Salmo salar L.) reared in circular tanks

Adult Atlantic salmon, Salmo salar, at initial mean size of 1.75 kg, were stocked at three different fish densities, 35–45, 65–85 and 100–125 kg/m 3, for 101 and 143 days in circular tanks under controlled environmental conditions including adequate flow-through of oxygenated water. Primary, secondary and tertiary indicators of stress in the fish were measured during the exposure period. As a primary indicator of stress plasma cortisol level was measured. This was found not to be affected at high, compared to low, fish density at the end of the exposure period. As secondary indicators of stress, several haematological and plasma metabolic and osmoregulatory parameters were measured. Although sometimes significantly different, these were found to lie within the normal range for salmonids, also for the group at high density. As a tertiary indicator of stress growth, as increases in weight and length with time, was measured. No significant differences were found between the groups, and the condition factor improved similarly during the exposure period. The results indicate that the fish did not experience a high-level state of chronic stress, not even at the high density. Social and size hierarchies were apparently not established at increased densities. The upper density limit for Atlantic salmon post-smolts, in suitable land-based rearing systems, appears to be higher than the highest one investigated here. Further studies are needed.