Altered fatty acid compositions in atlantic salmon (Salmo salar) fed diets containing linseed and rapeseed oils can be partially restored by a subsequent fish oil finishing diet

Atlantic salmon postsmolts were fed a control diet or one of 9 experimental diets containing various blends of two vegetable oils, linseed (LO) and rapeseed oil (RO), and fish oil (FO) in a triangular trial design, for 50 wk. After sampling, fish previously fed 100% FO, LO and RO were switched to a diet containing 100% FO for a further 20 wk. Fatty acid compositions of flesh total lipid were linearly correlated with dietary fatty acid compositions (r = 0.99-1.00, P < 0.0001). Inclusion of vegetable oil at 33% of total oil significantly reduced the concentrations of the highly unsaturated fatty acids, eicosapentaenoate [20:5(n-3)] and docosahexaenoate [22:6(n-3)], to approximately 70 and 75%, respectively, of the values in fish fed 100% FO. When vegetable oil was included at 100% of total dietary lipid, the concentrations of 20:5(n-3) and 22:6(n-3) were significantly reduced to approximately 30 and 36%, respectively, of the values in fish fed FO. Transfer of fish previously fed 100% vegetable oil to a 100% FO diet for 20 wk restored the concentrations of 20:5(n-3) and 22:6(n-3) to approximately 80% of the value in fish fed 100% FO for 70 wk, although the values were still significantly lower. However, in fish previously fed either 100% LO or RO, concentrations of 18:2(n-6) remained approximately 50% higher than in fish fed 100% FO. This study suggests that RO and LO can be used successfully to culture salmon through the seawater phase of their growth cycle; this will result in reductions in flesh 20:5(n-3) and 22:6(n-3) concentrations that can be partially restored by feeding a diet containing only marine FO for a period before harvest.