Early performance, stress- and disease-sensitivity in rainbow trout fry (Oncorhynchus mykiss) after total dietary replacement of fish oil with rapeseed oil. Effects of EPA and DHA supplementation

Different vegetable oils have been investigated as potential substitutes of fish oil in aquaculture feed and several of them have proven to be successful in terms of fish growth rates, survival rates, biometric indices or feeding efficiency, even when used as the only oils in the feed. However, final fish product composition (fish whole body or fish fillet) usually reflects feed composition, and fish products from fish fed with only vegetable oils usually show a deficiency in long chain polyunsaturated fatty acids (PUFAs). The welfare and health consequences for the fish of these alterations of the fatty acid profile are not well known. In the current study, three experimental diets using supercritically defatted fish meal as well as rapeseed oil as a 100% replacement of fish oil, and differing in essential fatty acid (eicosapentaneoid acid - EPA, docosahexaenoic acid - DHA) supplementation, were tested in rainbow trout, Oncorhynchus mykiss. The growth performance, feed conversion ratio and digestive enzyme profiles were evaluated. Further, stress-and infection-challenge experiments were performed to assess the robustness of the fish against stress and disease. The trout fry fed the experimental diets showed a different fatty acid profile than fish fed a commercial diet, with lower EPA and DHA levels but higher linolenic acid (18:3n-3) and oleic acid (18:1n-9) levels. No significant effects were found in growth performance, feed conversion ratio or digestive enzymatic activity among diets. The fish stress responsiveness (brain and plasma stress marker response) to acute and repeated stressors was in general also similar irrespective of the diet. Infection trials, however, showed a differential ability of the fish to survive upon a controlled infection with the bacterium Flavobacterium psychrophilum. Fish fed with the experimental diets displayed higher mortalities than fish fed a commercial trout fry diet. The current results highlight the need of further research on nutritional requirements to optimize the disease resilience in farmed fish. These results also demonstrate that the adaptation of the fish to new formulated feed (in this case associated to plant oil ingredients) should not be assessed solely based on the growth performance of the fish. Other aquaculture relevant welfare indicators, as disease or stress resilience, should be part of a more complete assessment of feed adequacy for fish farming purposes.