Effects of rapeseed meal replacing fishmeal on growth, body composition, amino acid digestion and transport, lipid metabolism, and immunity of black carp (Mylopharyngodon piceus)

To assess the feasibility of replacing dietary fishmeal (FM) with rapeseed meal (RSM), a 120-day feeding trial was conducted on juvenile black carp (Mylopharyngodon piceus) with an initial body weight of 5.49 ± 0.02 g. Six isonitrogenous and isolipidic diets were formulated by substituting FM with 0 %, 10 %, 20 %, 30 %, 40 %, and 50 % RSM. The results showed that the 20 % replacement level did not significantly affect growth performance, feed conversion ratio, protease activity, muscle amino acid composition, and crude protein digestibility in black carp. The expressions of amino acid transporter-related genes in the intestine and muscle were downregulated with a significant linear pattern, but none of them showed significant differences at the 20 % replacement level (except for slc6a19 in the muscle). In the 40 % and 50 % replacement levels, the aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities in the serum, and the triglycerides (TG) content in the liver were significantly increased. Dietary RSM promoted lipid synthesis and reduced lipolysis, leading to abnormal lipid deposition, which impaired the health of the liver. In the serum and intestine, excessive dietary RSM reduced the activity of alkaline phosphatase (AKP), acid phosphatase (ACP), lysozyme (LZM), catalase (CAT), and superoxide dismutase (SOD) activity, and downregulated the expression of antioxidant- and immune-related genes, which led to reduced immunity in black carp. In summary, excessive FM replacement by RSM can significantly negatively affect growth, nutrient metabolism, and immunity in black carp. Based on growth performance and feed utilization, RSM can replace up to 20 % of FM in the black carp diet.