| Summary: | An optimal diet for Atlantic salmon (Salmo salar) should promote a healthy fish that is robust to changes in its environmental conditions and can withstand the handling it will encounter under farming conditions, all while promoting good and rapid growth. The plant ingredients commonly used in aquafeeds do not have an ideal FA composition for salmon. In particular, they are lacking the n-3 fatty acids (FA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential nutrients for salmon. Further, they are rich in the n-s6 FA linoleic acid (LA) and the n-9 FA oleic acid (OA), which are not common in the natural diet of salmon. The exact requirement of EPA and DHA for Atlantic salmon is, however, still not known. There are also indications that a higher inclusion of dietary n-6 FA can increase the requirement for EPA and DHA. Many previous trials investigating these nutrients have been short-term, land-based trials where the fish have been shielded from difficult situations. Though such trials can define minimum requirements, the practical requirements need to be determined in a challenging environment. The focus of this thesis has been to investigate the effect of dietary EPA, DHA and n-6 FA on the robustness of Atlantic salmon exposed to challenging environmental conditions. It has been suspected that dietary n-6 can affect the requirement of EPA and DHA, and that it may have an effect on the response to chronic and acute stress. To investigate this, a feeding experiment was conducted with three diets containing equal absolute amounts of n-3 FA and increasing n-6 FA (n-6/n-3 ratios of 1, 2 and 6), as well as a final diet with double absolute n-3 FA content and an n-6/n-3 ratio of 1. This allowed for a separation between effects of ratio and absolute amounts on tissue FA levels. These diets were used in a 13-week growth trial and a 4-week stress trial. In the stress trial, half the fish were exposed to a repeated stressor (hypoxia) three times weekly, while the other half were undisturbed ...
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