The present experiment evaluated the effects of fat (Kilka fish oil) levels (0, 5, 10, 15 and 20%) in the diet (35% constant protein) on performance of rainbow trout. The experiment was performed in a completely randomized design, with 4 replicates, 200 grower fish with an average weight of 200±11 gr, each. They were randomly divided into 20 groups of 10 fish per group reared in separate pens. Fish were acclimated to the experimental conditions for 10 days before treatments began and then the experiment was performed over a period of 8 weeks. Measured criteria were weight gain (WG), specific growth rate (SGR), feed conversion ratio (FCR), condition factor (CF) and flesh chemical composition (fat, protein, ash), and fat stored in liver. Results showed that the diet with 15 percent fat content had significantly (P<0.05) better WG and FCR than other diets, but no significant differences were observed for SGR and CF. Diets with 15 and 20 percent fat contents caused the highest fat content in meat and liver and showed significant differences (P<0.05) with the meat and liver fat content in fish fed fat-free diets. Diet with 15 percent fat content caused significantly (P<0.05) lowest protein content in meat. No significant differences were observed in the ash content of the flesh. From the results of the present study, the level of 15 percent Kilka fish oil is recommended for the rainbow trout diet.

Keywords: Rainbow trout, Fat, Weight gain, Feed conversion ratio, Flesh chemical compositon

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