| Summary: | Vegetable oils (VO) are globally accepted alternatives for fish oil (FO) in aquafeeds. The lack of n-3 long-chain (≥C20) polyunsaturated fatty acids (n-3 LC-PUFA) in VO is a major constraint. Echium oil (EO), rich in stearidonic acid (SDA), has the potential to increase endogeneous n-3 LC-PUFA biosynthesis. We tested whether feeding Atlantic salmon an EO-based diet in both freshwater and seawater would increase n-3 LC-PUFA levels by comparing the fatty acid (FA) profiles in liver and white muscle to fish fed FO and rapeseed oil (RO)-based diets. The gene expression of n-3 LC-PUFA biosynthetic enzymes was measured to demonstrate the underlying mechanism of n-3 LC-PUFA biosynthesis. After prolonged feeding with EO diet from freshwater to seawater phases, EO fish had higher n-3 LC-PUFA levels in both liver and white muscle compared to RO fish. However, FO fish had the highest n-3 LC-PUFA levels in examined tissues. Δ6 Desaturase gene expression in liver and white muscle was up-regulated in RO fish only, liver Δ5 desaturase gene expression was reduced in seawater and liver FA elongase gene expression was regulated by an interaction between dietary oil and environment. This study showed that feeding Atlantic salmon from parr to smolt using an SDA enriched diet increases n-3 LC-PUFA biosynthesis in liver and white muscle through increased supply of the n-3 LC-PUFA precursor SDA. The down regulation of Δ5 desaturase gene expression in the liver of seawater fish may explain environmental differences in n-3 LC-PUFA biosynthesis
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