The omega-3 fatty acids e icosapentaenoic a cid (EPA) and d ocosahexaenoic acid (DHA) are essential nutrients for farmed fish and for their human consumers. F ish oil is one of the main sources of these long-chain polyunsaturated fatty acids; however, due to an increasing demand for EPA and DHA and with a limited supply from wild fisheries, there is a need for alternative sources that are sustainable and cost effective for aquaculture. Here we investigated replace ment of fish oil with a high-DHA, low-EPA oil extracted from single-celled thraustochytrids . This provided a unique opportunity to determine how these dietary fatty acids independently influence lipid metabolism and physiological pathways in salmonids.
A 16-week feeding trial was conducted with Atlantic salmon fed diets with a complete or partial replacement of fish oil with microbial oil. There was no significant difference in growth performance among the dietary treatment s but we observed differences in lipid composition and gene expression. We investigated total lipid class and fatty acid composition in liver and muscle tissues using thin-layer chromatography and gas chromatography with flame ionization detection and mass spectrometric detection. Our results showed no significant differences in total lipids and lipid class concentrations among the dietary treatments for both tissues; however, significant difference were observed in proportions of omega-3 and omega-6 total lipid fatty acids and phospholipid fatty acids. In addition, there were differences in the phytosterol composition and in triacylglycerol and phospholipid molecular species. These results correlated with hepatic lipid metabolism biomarkers, although some levels were the same with high dietary DHA (high microbial oil) and high EPA (fish oil ) indicating successful replacement of fish oil with microbial oil.