Effects of stress and functional feeds on health and performance of Atlantic salmon

This thesis investigated whether microalgae-enriched functional feeds can improve the health and growth performance of Atlantic salmon reared in recirculating aquaculture systems (RAS) and allow the fish to cope with stress more effectively. In the first trial, salmon smolts were fed diets supplemented with various microalgae for eight weeks in brackish water followed by two weeks in seawater. The results showed that feeding 8% Chlorella vulgaris can improve feed utilisation and modulate the immune response of Atlantic salmon. Transferring the salmon from brackish water to seawater induced an acute stress response and led to a greatly reduced immune gene-expression, but only limited effects of the dietary treatments were detected. The subsequent trials tested feeding different inclusion levels and administration methods of Chlorella vulgaris and Schizochytrium limacinum for eight weeks to post-smolts followed by an acute oxidative stress induced by peracetic acid (PAA) treatment. Feeding S. limacinum improved feed conversion and protein retention and modulated bacterial beta diversity of the digesta. PAA treatment induced a pronounced stress and immune response but feeding 14% C. vulgaris once per week attenuated the immune responses in the gill. A fourth trial involved artificially elevating plasma cortisol levels in post-smolt Atlantic salmon to model chronic stress and understand potential interactions when feeding chronically stressed fish a functional feed. Cortisol treatment reduced growth and the expression of antiviral genes in the heart, whereas feeding S. limacinum had only a minor effect on the physiological responses induced by cortisol. Overall, this work demonstrates that microalgae-enriched feeds have the potential to mitigate stress and promote health in RAS-reared Atlantic salmon, although the effectiveness depends on the type of stressor and the specific algal species used.

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