A nutritional study was conducted to evaluate the influence of dietary fish oil (FO) sparing by using a mixture of vegetal oils (VO) on rainbow trout (Oncorhynchus mykiss) juveniles (30.0 g). A basal diet was supplemented with the experimental oils at a 10% level, including FO (FO diet), a blend of VO (sunflower, soybean and rapeseed oils with a 1:1:1 ratio), and a mixture (1:1 ratio) of FO and the blends of VO (MIX). In a raceway system, 450 fish were stocked in nine rectangular concrete tanks (50 fish/ tank−1). Three experimental diets, with three replicates each, were offered to fish up to visual satiation for eight weeks. Growth performance did not change among groups (P>0.05). The highest and lowest proportions of n-3 polyunsaturated fatty acids (PUFA), n-3 long chain-PUFA, especially docosahexaenoic acid, and n-3/n-6 PUFA ratio were in the whole body of FO and the VO groups, respectively (P<0.05). Serum catalase and glutathione peroxidase activities in VO group were higher than others. Serum superoxide dismutase activity in fish fed FO and VO diets was higher than MIX group (P<0.05). VO group had higher serum lysozyme, alternative complement pathway, and total immunoglobulin levels than other experimental groups. MIX group had highest serum triglyceride, cholesterol, and low-density lipoproteins. Serum high-density lipoproteins, aspartate aminotransferase, and alanine aminotransferase levels in VO treatment were in the lowest level compared to others (P<0.05). The highest relative transcription levels of fatty acid synthase, delta-6 fatty acyl desaturase, and apolipoprotein b-100 genes were in the liver of fish fed the MIX diet. The relative transcription level of the fatty acid-binding protein1 gene did not change (P>0.05). In summary, the results of this study revealed that dietary FO sparing with VO did not adversely affect the growth and health indices of rainbow trout; nevertheless, it had adverse effects on its nutritional values by reducing whole-body n-3 LC-PUFA content.
