Influence of dietary phosphorus levels on growth, body composition, metabolic response and antioxidant capacity of juvenile snakehead ( C hanna argus × C hanna maculata )

A study was conducted to estimate the optimum requirement of dietary phosphorus (P) for Channa argus × Channa maculata . Effects of dietary P levels on the tissue composition, serum biochemical parameters and antioxidant status were also examined. Five practical diets were formulated to contain graded levels (4.8 g kg −1 , 6.4 g kg −1 , 7.9 g kg −1 , 9.4 g kg −1 and 11.0 g kg −1 ) of available P from dietary ingredients and monocalcium phosphate ( MCP ). Each diet was randomly assigned to triplicate groups of 30 juvenile fish (initial body weight, 20.50 ± 0.53 g) for 8 weeks. The results showed that the specific growth rate ( SGR ) and weight gain ( WG ) were all significantly improved by dietary P up to 9.4 g kg −1 ( P  <   0.05) and then levelled off beyond this level. Broken‐line analysis showed maximum weight gain ( WG ) was obtained at dietary available P concentrations of 9.6 g kg −1 . With the increase in dietary P level, protein efficiency rate ( PER ) increased significantly and reached a plateau, while the feed conversion ratio ( FCR ), the mesenteric lipid somatic index ( MSI ) and the whole‐body lipid content significantly reduced ( P  <   0.05). Dietary P levels also affected the mineralization (ash and P) of whole body, vertebrae and scale ( P  <   0.05). Quadratic analysis based on P contents in whole body, vertebrae, scale and ash content in vertebra indicated that the available P requirements were 10.4, 9.8, 10.0 and 10.3 g kg −1 , respectively. However, no differences were found in the whole‐body moisture, crude protein, serum calcium (Ca) contents or Ca/P value, as well as the viscerosomatic index ( VSI ) and hepatosomatic index ( HSI ) among all the treatments ( P  >   0.05). Triglyceride ( TG ), total cholesterol ( TC ), high‐density lipoprotein cholesterol ( HDL ‐C) and low‐density lipoprotein cholesterol ( LDL ‐C) decreased significantly, while serum P content, HDL ‐C/ TC and HDL ‐C/ LDL ‐C value increased significantly with dietary available P levels ( P  <   0.05). No significant changes in superoxide dismutase activity and malondialdehyde ( MDA ) content were observed ( P  >   0.05), but serum catalase ( CAT ) and glutathione peroxidase ( GP x) activities and the ratio of CAT / SOD and GP x/ SOD increased significantly with increasing dietary P levels ( P  <   0.05). In conclusion, the optimal P requirement of juvenile snakehead in practical feed was 9.6 g kg −1 . Signs of P deficiency were characterized by poor growth, slightly reduced mineralization and the antioxidant capacity and an increase in body lipid content.