Performance, feed utilization, and hepatic metabolic response of weaned juvenile Atlantic bluefin tuna (Thunnus thynnus L.): effects of dietary lipid level and sourcePerformance, feed utilization, and hepatic metabolic response of weaned juvenile Atlantic bluefin tuna (Thunnus thynnus L.): effects of dietary lipid level and source

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English. t Two trials were performed using extrudeddiets as on-growing feeds for weaned Atlantic bluefintuna (Thunnus thynnus; ABT) to establish adequatedietary levels of both lipid and omega-3 long-chainpolyunsaturated fatty acids (LC-PUFAs), and impactson lipid metabolism via liver gene expression. In trial A,ABT were fed with either a commercial feed(Magokoro®; MGK) as a reference diet or two experimental feeds differing in lipid levels (15 or 20%) usingkrill oil (KO) as the single lipid source in order toestimate suitable lipid content. Fish fed MGK displayedthe highest growth, followed by 15KO, and therefore adietary lipid content of 15% was considered preferableto 20% at this stage. In trial B, fish were fed MGK,15KO, or a feed containing 15% lipid with a blend ofKO and rapeseed oil (RO) (1:1, v/v; 15KORO). Fish fed15KO and 15KORO showed no difference in weightgain, specific growth rate, and fork length. Increasingdietary lipid level or including vegetable oil, RO, in thefeeds did not increase liver lipid content. Liver fatty acidcompositions largely reflected dietary profilesconfirming very limited endogenous LC-PUFA biosynthesis. Liver of ABT fed 15KO and 20KO displayed thehighest contents of docosahexaenoic acid (DHA). Thehepatic expression of genes encoding enzymes and transcription factors involved in lipid and fatty acid metabolism, as well as genes encoding antioxidant enzymes,showed that many of these genes were regulated bydietary lipid and LC-PUFA content. Results suggestedthat ABT juveniles can be on-grown on inert dry feedsthat support good fish growth and the accumulation ofDHA.