Effects of Dietary Methionine Levels on the Growth Performance of Juvenile Gibel Carp (Carassius auratus gibelio)

Methionine is usually the first limiting amino acid in many fish diets, especially those containing high levels of plant protein sources, such as soybean meal, peanut meal, and cottonseed meal. In the present study, an 8-week growth trial was conducted to evaluate the effects of dietary methionine (Met) levels on growth performance, nutrients retentions of gibel carp (Carassius auratus gibelio) with the initial weight of 2.60 g based on practical diets. Six isonitrogenous (crude protein content 37.22%) and isoenergetic (gross energy 17.61KJ/kg) diets were formulated with different levels of DL-methionine, i.e., 0, 0.08%, 0.16%, 0.24%, 0.32%, 0.40%. The determined Met levels were 0.57%, 0.64%, 0.74%, 0.81%, 0.88% and 0.95% and then defined as DLM0 (basal diet), DLM1, DLM2, DLM3, DLM4, and DLM5, respectively. Fifty fish were batch weighted and put into each tank and each treatment had triplicate tanks. All the experimental fish were stocked in a flow-through system, fed four times a day (9:00, 12:00, 15:00, 18:00). The water temperature fluctuated from 26 to 28C and dissolved oxygen was maintained at ~7.0 mg/L. The results indicated that specific growth rate (SGR) and protein productive value (PPV) significantly increased with the DL-Methionine supplemented diets (P0.05), indicating that gibel carp could utilize crystallized DL-Met. Maximum final body weight (FBW, 11.15 g), highest specific growth rate (2.60%/d), and protein productive value (37.02%) were observed in fish fed with diet containing methionine at 0.951% level (DLM5). There were no significantly differences on morphometric parameters and whole body nutrients compositions except crude protein and ash (P0.05). Whole body crude protein of fish fed with Met supplemented diets were higher than that of the control group, while ash content of the control group was significantly lower than those of all other groups (P0.05). The methionine productive value (MPV) was reduced with increasing levels of methionine up to 0.876% of diet (DLM4) (P0.05), and then increased in the DLM5 group. While other essential amino acid productive values increased with the higher methionine levels in diets, a third-order polynomial regression analysis of specific growth rate indicated that the growth performance of juvenile gibel carp might be improved if the methionine levels enhanced subsequently. Broken-line model of MPV indicated that optimum Met level in the diet of juvenile gibel carp was 0.89% of diet (dry matter basis), or 2.17% in terms of dietary protein with cystine level at 0.42%.