Optimal dietary methionine requirement for Juvenile Cobia (Rachycentron canadum)

An 8-week feeding experiment was conducted to quantify the dietary methionine requirement of juvenile cobia (initial weight of 11.61 ± 0.16 g, mean ± SD) reared in indoor flow-through and aerated aquaria. Six amino acid test diets (44% CP and 16% lipid), using fish meal and wheat gluten as intact protein sources supplemented with crystalline amino acids, were formulated to contain graded levels of methionine (0.61%, 0.83%, 1.05%, 1.30%, 1.48% and 1.68% of dry diet, respectively) at a constant dietary cystine level of 0.67%. Each diet was randomly assigned to three aquaria. Growth performance and feed utilization were significantly influenced by the dietary methionine levels (P < 0.05). Maximum weight gain, specific growth rate and protein efficiency ratio occurred at 1.05% dietary methionine (P < 0.05). The hepatosomatic index, viscerosomatic index and condition factor were not significantly affected by dietary methionine levels (P > 0.05). There were no significant differences in protein, lipid, ash and moisture contents in whole body or muscle, also red blood cell count and plasma protein content for fish fed graded methionine levels of diets; however, hemoglobin, hematocrit, leukocyte, plasma glucose and triacylglycerol concentrations showed significant differences among these treatments (P < 0.05). Methionine concentration in plasma significantly increased with an increase of dietary methionine (P < 0.05). Quadratic regression analysis of specific growth rate against dietary methionine levels indicated that the optimal dietary methionine requirement for maximum growth and feed utilization of juvenile cobia is 1.19% of dry diet in the presence of 0.67% cystine (corresponding to 2.64% of dietary protein on a dry weight basis).