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Summary To investigate the effects of different carbohydrate (CHO) levels in the diet of Wuchang bream Megalobrama amblycephala, the fish were randomly divided into six treatment groups. Five groups were fed 19, 25, 31, 38 and 47% CHO, respectively, for 8 weeks, and a control group was fed a diet with no CHO. Growth performance and feed utilization were significantly (P < 0.05) affected by the dietary carbohydrate level. Maximum weight gain and specific growth rate occurred at the 31% dietary carbohydrate level. Compared to the control, the 31% CHO group had a significantly increased serum total protein content, respiratory burst activity of leucocytes, serum complement 3 (C3) levels, serum lysozyme activity, serum alkaline phosphatase (AKP) activity and hepatic total antioxidative capacity (T‐AOC), as well as a decrease in serum glutamic‐oxaloacetic transaminase (GOT) activity. Compared to the control, the 47% CHO group had significantly increased serum GOT activity, and a tendency toward an increase in serum cortisol content and a decrease in serum lysozyme activity, hepatic superoxide dismutase (SOD) activity and T‐AOC. The relative level of hepatic heat shock protein 70 (HSP70) mRNA in fish fed the 38% CHO diet was significantly higher than those of fish fed the 19, 25 and 31% CHO diets, respectively (P < 0.05). After challenge with Aeromonas hydrophila, fish fed the 47% CHO had the significantly lowest post‐challenge survival, and fish fed the 31% CHO had the significantly highest post‐challenge survival (P < 0.05). The results of this study suggest that ingestion of excessive dietary CHO can impact the non‐specific immune responses, decrease the hepatic antioxidant abilities, and thus affect the health status of M. amblycephala.

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A 9-week feeding trial was conducted to investigate the dietary methionine requirement of juvenile Megalobrama amblycephala at a constant dietary cystine level. Six semipurified diets were formulated to contain graded dietary methionine levels from 3.9 to 15.4 g kg À1 in about 2.5 g kg À1 increments in the presence of 2.2 g kg À1 cystine. Results showed that specific growth rate (SGR) and protein efficiency ratio (PER) significantly increased with increasing dietary methionine levels from 3.9 to 12.4 g kg À1 and thereafter kept stable. Maximum protein productive value (PPV), nitrogen retention efficiency (NRE) and liver weight were observed in 8.5 g methionine kg À1 diet. Protein contents in whole fish body were positively correlated with dietary methionine level, while lipid contents were negatively correlated with it. Morphological index and hepatic glutamate-pyruvate transaminase (GPT) activities were independent of dietary methionine levels. However, dietary methionine supplementation significantly improved haematological parameters, plasma methionine and total essential amino acid contents and hepatic glutamate-oxaloacetate transaminase (GOT) activities. Analysis of dose response using broken-line regression on the basis of SGR and PPV versus dietary methionine level estimated the optimum dietary methionine requirements of juvenile M. amblycephala to be between 8.5 and 8.4 g kg À1 of diet (25.0 and 24.7 g kg À1 of protein) in the presence of 2.2 g kg À1 cystine, respectively. Hence, the corresponding total sulphur amino acids requirements of this species were calculated to be 10.7 and 10.6 g kg À1 of diet (31.5 and 31.2 g kg À1 of dietary protein).