Fish Oil Replacement by a Blend of Vegetable Oils in Diets for Juvenile Tench (Tinca tinca Linnaeus, 1758): Effects on Growth Performance and Whole-Body Composition

SIMPLE SUMMARY: Fish oil (FO) can be totally replaced by a blend of vegetable oils (30% linseed oil, 20% corn oil, and 50% olive oil) in diets for juvenile tench (Tinca tinca Linnaeus, 1758) without negative effects on survival rate and growth performance. Diets affected the composition of juveniles so that lipid content was significantly lower in animals fed diets without FO. Moreover, the content of linolenic acid (18:3n-3) increased as FO substitution did, being significantly higher with respect to control diet (without vegetable oils) from 40% or higher FO replacement diets. No differences in saturated, monounsaturated, and polyunsaturated fatty acids were found in the fish whole-body. Nutritional indices can be considered within optimal values for healthy foods. ABSTRACT: Among freshwater species, tench (Tinca tinca Linnaeus, 1758) is considered as a promising species for the diversification of aquaculture, but the intensification of techniques is necessary to promote and consolidate its culture. Adequate feeding in early growth phases is essential to face further grow-out. Fish oil (FO) is the main source of lipids in fish diets, but its production is unsustainable, and thus, alternative oils should be considered. A 90-day experiment was performed to evaluate the effects of partial and total replacement of cod liver oil (FO) by a blend of vegetable oils (VO) in juvenile tench. Six isonitrogenous and isolipidic diets with different levels of a VO blend containing 30% linseed oil, 20% corn oil, and 50% olive oil were tested: 0% (control), 20%VO, 40%VO, 60%VO, 80%VO, and 100%VO. With all diets, survival was 100%, and there were not differences in growth performance (total length (TL); weight (W); specific growth rate (SGR); feed conversion ratio (FCR); and biomass gain (BG)). Compared to the control group, whole-body lipid content decreased significantly in the 100%VO group. No differences in total saturated (SFA), monounsaturated (MUFA), and polyunsaturated (PUFA) fatty acids were found in the whole-body. The content of linolenic acid (ALA) in the whole-body increased as FO substitution did, being significantly higher with respect to control diet from 40% FO replacement diets. Nutritional indices, such as ΣPUFA/ΣSFA and Σn-6/Σn-3 ratios, tended to increase with increasing VO content, whereas the EPA + DHA showed an opposite trend. A total replacement of FO by the blend of VO did not affect the growth performance and fatty acid profile of juvenile tench. Further research on the effects of VO diet on nutritional quality in tench reared to commercial size should be performed.