| Abstract: | An 8‐week feeding trial was conducted to determine the effects of dietary methionine level on juvenile black sea bream Sparus macrocephalus. Fish (initial body weight: 14.21 ± 0.24 g) were reared in eighteen 350‐L indoors flow‐through circular fibreglass tanks (20 fish per tank). Isoenergetic and isonitrogenous diets contained six levels of L‐methionine ranging from 7.5 to 23.5 g kg−1 of dry diet in 3.0 g kg−1 increments at a constant dietary cystine level of 3.1 g kg−1. Growth performance and feed utilization were significantly influenced by dietary methionine levels (P < 0.05). Maximum weight gain (WG), specific growth rate (SGR), feed efficiency ratio, protein efficiency ratio and protein productive value (PPV) occurred at 17.2 g methionine kg−1 diet, beyond which they showed declining tendency. Protein contents in whole fish body and dorsal muscle were positively correlated with dietary methionine level, while muscle lipid content was negatively correlated with it. Apparent digestibility coefficients (ADCs) of dietary nutrients were significantly affected by dietary treatments except for ADCs of crude lipid. Fish fed the grade level of methionine demonstrated a significant improvement in whole‐body methionine content, total essential amino acids (∑EAA), total non‐essential amino acids (∑NEAAs) and ∑EAA/∑NEAA ratio (P < 0.05). Regarding serum characteristics, significant differences were observed in total cholesterol, glucose and free methionine concentration (P > 0.05), while total protein level and triacylglycerol concentration kept relatively constant among treatments (P < 0.05). Analysis of dose response with second‐order polynomial regression on the basis of either SGR or PPV, the optimum dietary methionine requirements of juvenile black sea bream were estimated to be 17.1 g kg−1 of diet (45.0 g kg−1 methionine of protein) and 17.2 g kg−1 of diet (45.3 g kg−1 methionine of protein) in the presence of 3.1 g kg−1 cystine, respectively. |
