The dietary effect of sodium butyrate, Butirex®C4 (NaBT), on some physiological and immunological responses, as well as carcass and fillet quality were studied in rainbow trout, Oncorhynchus mykiss (initial body weight, 16.63±0.10 g; mean ± SE) fed two-level fishmeal (FM) diets. Six experimental diets were formulated to include two levels of FM (15% and 30%) and supplemented with incremental levels of NaBT (0%, 0.25%, and 0.5%). An improvement in growth performance was obtained in fish fed diet containing 0.5% NaBT+30% FM. Dietary NaBT inclusion significantly increased the digestive enzyme activity of fish compared to those without a NaBT diet. The highest immune parameters for mucus and serum were observed in fish fed with 0.25% NaBT+30% FM and 0% NaBT+30% FM diets, respectively. Fish fed diets containing 0.5% NaBT+15% FM and 0.25% NaBT+30% FM had significantly higher antioxidant enzymes activity (superoxide dismutase (SOD) and glutathione peroxidase activity (GPx)) in comparison to other groups. At selected NaBT levels, fish fed with FM (15% and 30%) showed higher bactericidal activity in comparison to group fed diet without NaBT supplementation. Based on the results of the intestine microflora, the highest gastrointestinal tract (GIT) total count and GIT lactic acid count were observed in groups included with 0% NaBT+15% FM and 0.5% NaBT+30% FM. Fish fed diets containing 0.5% NaBT+15% and 30% FM showed improvement in fillet quality in comparison to other groups. In conclusion, dietary NaBT supplementation, especially at the inclusion level of 0.5%, can improve physiological and immunological responses and reduce the adverse effects of the diets with the lower FM in rainbow trout.
