Effects of dietary inorganic salts supplementation on growth performance, bone mineral deposition, intestinal morphology and immune response of turbot juveniles (Scophthalmus maximus L.) in fermented soybean meal‐based diets

The composition of mineral in fermented soybean meal (FSBM) was far from that of fish meal (FM). The lack of minerals might be one of the factors restricting the replacement of FM by FSBM. Thus, a 10‐week feeding experiment was conducted to evaluate the effects of mixed inorganic salts (IS) supplementation in FSBM‐based diets on growth, bone mineral deposition, intestinal morphology and immune response of turbot juveniles (Scophthalmus maximus L.). The whole FM diet was used as the FM group. Three isonitrogenous (544 g/kg crude protein) and isolipidic (108 g/kg crude lipid) diets were formulated. Three graded levels of IS were supplemented in FSBM‐based diet (the control group) at 0, 1 fold and 2 folds according to the difference in composition of minerals from major ingredients. To be more specific: FM (0), FSBM (0), FSBM/IS‐1 (Ca(H₂PO₄)₂: 13.10 g/kg, CaCl₂: 11.00 g/kg, NaCl: 9.55 g/kg, FeSO₄·7H₂O: 0.92 g/kg, Na₂SeO₃: 0.56 mg/Kg, ZnSO₄·H₂O: 5.60 mg/Kg); FSBM/IS‐2 (Ca(H₂PO₄)₂: 26.58 g/kg, CaCl₂: 21.87 g/kg, NaCl: 19.11 g/kg, FeSO₄·7H₂O: 1.84 g/kg, Na₂SeO₃: 1.12 mg/Kg, ZnSO₄·H₂O: 13.66 mg/Kg). Each diet was randomly allocated to quadruplicate groups of 30 turbot juveniles. Results showed that no significant difference in survival rate was observed among dietary treatments (p > .05). Fish fed the diet with 2 folds IS had significantly higher specific growth rate than the control group and 1 fold IS (p < .05). Feed efficiency of fish fed the diet with 2 folds IS was significantly enhanced compared with the control group and 1 fold IS (p < .05). Bone deposition of calcium, phosphorus, zinc, iron and selenium was significantly increased with increasing dietary IS while sodium was significantly decreased (p < .05). The activities in intestinal trypsin, amylase and lipase significantly increased with increasing dietary IS (p < .05). Besides, the activities of hepatic total antioxidant, total superoxide dismutase and catalase were significantly increased with increasing dietary IS (p < .05). The activities in plasma lysozyme, glutamic pyruvic transaminase, glutamic oxalacetic transaminase and γ‐glutamyl transferase were significantly decreased with increasing dietary IS (p < .05). Results of the present study indicated that 2 folds IS could partially relieve the growth retardation caused by high proportion of dietary FSBM, improve bone mineralization and enhance immune responses.