Substituting fish oil with canola oil in the diet of triploid rainbow trout: Effects on growth, health, and fatty acid metabolism

This study evaluated the effects of replacing dietary fish oil with canola-derived lipids, either partially (75%, CO75) or fully (CO100), on growth, digestive function, antioxidant capacity, and gene expression in triploid Oncorhynchus mykiss. A fish-oil control (FO; 100% fish oil) was included. Fish (n = 1200; initial body weight 208 g) were randomly allocated to 12 net cages (4 replicates per diet) and fed for 12 weeks. Growth indicators (specific growth rate, viscerosomatic index, and hepatosomatic index) did not differ among treatments ( P > 0.05), whereas condition factor and relative intestinal length increased with greater canola inclusion ( P < 0.05). FO fish showed higher protease and amylase activities in stomach and intestine ( P < 0.05), while CO75 fish exhibited the highest digestive activity in pyloric caeca and elevated lipase activity across digestive organs ( P < 0.05). Plasma glucose, cholesterol, and non-esterified fatty acids decreased under canola-based diets ( P < 0.05). Antioxidant status indicated higher malondialdehyde and lower total antioxidant capacity in CO75 ( P < 0.05); hepatic sod expression peaked and cat decreased in FO fish ( P < 0.05), with nrf2 and gpx remaining unchanged (P > 0.05). Immune genes ifn , il‑10rl , il‑12p35a2 , and il‑12b2 showed highest transcription in FO fish, whereas il‑12p35b was upregulated in CO75 ( P < 0.05). Lipogenic genes ( acc , fas , Δ5 , Δ6 , elovl2 , elovl5 ) were most active in FO‑fed trout, while lipid transport/oxidation genes ( lpl , cd36 , apob , apoe , mtp , ldlr , hslα , cpt1 , aco , lrp‑1 , srbi , srebp1 ) increased with canola replacement ( P < 0.05). Villus height was unaffected ( P > 0.05), but muscular thickness increased and microvilli shortened in CO groups ( P < 0.05). Overall, canola-based substitution altered digestion, antioxidant responses, and lipid metabolism without compromising growth, plasma oxidative status altered in CO75, as indicated by higher MDA and lower T‑AOC ( P < 0.05). • Canola oil substituted for fish oil in triploid rainbow trout diets at 75% and 100% levels. • Growth performance and health were not significantly affected by the substitution. • Fatty acid compositions were significantly altered by the substitution. • The n-3/n-6 PUFA ratio was higher in fish fed the 75% canola oil diet. • Canola oil shows potential as a substitute for fish oil in trout diets.