One step toward aquaculture sustainability of a carnivorous species: Fish meal replacement with barley protein concentrate plus wheat gluten meal in Caspian brown trout (Salmo trutta caspius)

•Supplementing barley protein concentrate (BPC) up to 330 g/kg to diets did not negatively affect growth performance.•Amino acid and fatty acid profiles were similar in control and 50BPC treatments.•There was a positive correlation between growth rate and digestive enzyme activities. The present study evaluated the effect of replacing fish meal (FM) with barley protein concentrate (BPC) plus wheat gluten meal (WGM) on growth performance, carcass composition, digestibility, digestive enzyme activities, amino acid, and fatty acids profile of Caspian brown trout as a slow-growing and carnivorous fish species. Five experimental diets including Control (0 g/kg BPC), 25BPC (165 g/kg BPC), 50BPC (330 g/kg BPC), 75BPC (495 g/kg BPC), and 100BPC (660 g/kg BPC) were formulated. Also, WGM was added to diets to make them isonitrogenous. A total of 300 fish (13.53 ± 1.1 g) were farmed in three replicates per treatment for eight weeks. Results showed that FM could be replaced by BPC up to 50 % without any negative effect on growth performance (330 g/kg FM, 330 g/kg BPC). Fish was fed with dietary Control, 25BPC, and 50BPC digested protein and energy significantly better than those fed other diets. Accordingly, there were negative linear relations between BCP level in diets and trypsin, pepsin, and aminopeptidase activities (P < 0.01). There was no significant difference in the C20:5n3, C22:6n3, and long-chain polyunsaturated fatty acids (LC-PUFA) levels (g/kg) between individuals fed dietary Control (82.6, 148.5, 278, respectively) and 50BPC (67.0, 132.5, 244.7, respectively). Finally, pepsin and aminopeptidase had strong positive relations with weight gain, specific growth rate, the apparent digestibility coefficient of protein, lysine, and methionine values. Therefore, we suggest formulating diets with BPC up to 330 g/kg plus 40 g /kg WGM in Caspian brown trout diets based on unseen negative effects on investigated factors.