Effect of dietary substitution of fish oil with flaxseed or sunflower oil on muscle fatty acid composition in juvenile steelhead trout (Oncorhynchus mykiss) reared at varying temperatures

The effect of changing temperature on muscle fatty acid composition was examined in juvenile steelhead trout (~120g) with complete substitution of fish oil (herring oil; HE) with either flax seed oil (FLX; also known as linseed oil) or sunflower oil (SF). The temperature was increased from ~10.0°C to 18.0°C following seasonal temperature changes in ~2°C incremental steps with plateaus. Dietary lipid analysis showed the HE diet was rich in marine lipids, docosahexaenoic acid (DHA, 22:6ω3), eicosapentaenoic acid (EPA, 20:5ω3) and docosapentaenoic acid (DPA, 22:5ω3). In contrast FLX was rich in linolenic acid (ALA, 18:3ω3) and SF in linoleic acid (LA, 18:2ω6). Both diet and temperature had no significant effect on growth but significant effects were observed on muscle fatty acid composition. Proportions of marine fatty acids in HE fed fish were significantly higher than in both SF and FLX fish at the end (41, 31 and 31% respectively), with significantly lower levels of terrestrial plant fatty acids compared to the beginning of the experiment. Substitution of SF and FLX had a direct influence on muscle EPA:AA and ω6:ω3 ratios which could be detrimental to the health of the fish, as well as to the quality of the end product for human consumption. •Effects of dietary fish oil substitution with flax or sunflower oil were examined.•Temperature increased from 10 to 18°C following seasonal changes, in 2°C steps.•The major temperature affect was a significant decrease in 18:1ω9 and 18:2ω6.•Growth was similar to dietary controls but there were major differences in PUFA.•Full substitution with seed oil adversely affected muscle EPA:AA and ω6:ω3 ratios.