Dietary l-methionine requirement of juvenile grouper Epinephelus coioides at a constant dietary cystine level

The present experiment was conducted to determine the methionine requirement and the effect of dietary methionine levels on growth, feed utilization, body composition, haematological and morphometrical parameters in juvenile grouper. Diet 1 with fish meal as the sole protein source was used as a reference. Diets 2-7 were formulated to contain 48% crude protein and supplemented with graded concentrations of l-methionine, resulting in six levels of dietary methionine (0.55-1.81% of dry matter) at a constant dietary cystine level of 0.26%. Fish (initial weight of 13.25 ± 0.19 g, mean ± S.D.) were reared in twenty-one 500-l circular fibreglass tanks provided with flow-through seawater at 28-30.5 °C and salinity of 24-28 mg/l for 8 weeks. Fish fed the reference diet had significantly higher weight gain (WG), specific growth rate (SGR), feed utilization efficiency, protein and lipid contents of whole body, plasma protein and cholesterol concentrations (P < 0.05). Among other treatments fed the diets containing CAAs, increasing dietary methionine up to 1.34% increased growth performance and feed utilization, whole-body protein and lipid contents, condition factor (CF), viscerosomatic index (VSI) and intraperitoneal fat ratio (IPR), beyond which they remained nearly unchanged. Plasma protein concentration showed no significant differences for fish fed the diet containing more than 1.11% methionine. Triacylglycerol increased with increasing dietary methionine levels, but showed a relatively lower value for fish fed the 1.59% methionine diet. Plasma glucose could not be related to dietary treatments. Cholesterol was the highest for fish fed the 1.34% methionine diet but showed no significant difference among other treatments. Fish fed higher-methionine diets exhibited relatively higher essential amino acid (EAA) contents in muscle than fish fed the lower methionine diet (P < 0.05). Broken-line regression analysis of WG against dietary methionine level indicated that optimal dietary methionine requirement for juvenile grouper was 1.31% of the diet (corresponding to 2.73% of dietary protein on a dry weight basis).