LAUSR

Abstract A six-week growth trial was conducted to evaluate the optimum methionine (Met) requirement in diets of juvenile hybrid grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂). Seven isoproteic (49.6% of dry matter) and isolipidic (7% of dry matter) experimental diets were formulated to contain graded Met levels. Triplicate groups of 15 fish (average initial body weight: 10.61 ± 0.05 g, means ± S.D.) were fed each experimental diet to apparent satiation by hand twice daily (08:00 h and 16:30 h). Results showed that survival, growth, feed utilization, gut micromorphology, anti-oxidation and immunity of juvenile hybrid grouper were significantly affected by dietary Met levels. Fish fed 0.56% dietary Met had significantly lower survival than fish fed other dietary Met levels. Weight gain % (WG%), feed efficiency (FE), protein efficiency ratio (PER) as well as protein productive value (PPV) were increased as dietary Met level increased, reaching a peak value at 1.89% dietary Met level, and then values of these indices started to reduce when dietary Met level further rose. Fish fed 0.56% dietary Met had the highest daily feed intake (DFI) among all experimental treatments. Gut micromorphology including fold height (hF), fold width (wF), enterocyte height (hE), microvillus height (hMV) in fish were elevated by suitable Met supplementations to diets. Fish fed 1.89% dietary Met had higher expression of growth hormone (GH) in pituitary and growth hormone receptor 1 (GHR1), insulin like growth factor-I (IGF-1), target of rapamycin (TOR), S6 kinase 1 (S6K1) in liver than fish fed other dietary Met levels, and lysozyme (LZM) activity and immunoglobulin M (IgM) concentration in serum had a similar trend of variation as the expression of these genes. Generally, the optimum dietary Met requirement for maximum WG% of hybrid grouper was estimated to be 1.45% of dry matter (corresponding to 2.92% of dietary protein) at 0.69% dietary cystine level. Adequate Met content in diets improved feed utilization, gut micromorphology and immunity of this fish species; however, the low dietary Met level (0.56%) significantly reduced the survival of fish.