Dietary methionine requirement of juvenile Pseudobagrus ussuriensis

An 8-week feeding trial was conducted to determine the optimum dietary methionine (Met) requirement of juvenile Pseudobagrus ussuriensis with an initial average weight of 0.60g reared in indoor flow-through and aerated aquaria. Six isonitrogenous (430g kg(-1) protein) and isolipidic (50g kg(-1) lipid) test diets were formulated to contain graded levels of crystalline L-methionine (4.9, 9.0, 11.8, 14.2, 18.1 and 20.8g kg(-1) dry diets, respectively) at a constant dietary cystine level of 2.5g kg(-1) dry diets. Equal amino acid nitrogen was maintained by replacing methionine with non-essential amino acid mixture. Fish were randomly allotted to 18 aquaria (1.0x0.5x0.8m) with 50 fish to each glass aquarium. Fish were fed twice daily (08:00 and 16:00) to apparent satiation. No significant difference was observed in survival of fish (84.67-91.33%). Specific growth rate (SGR), weight gain (WG), feed conversion ratio (FCR), protein productive value (PPV) and protein efficiency ratio (PER) were significantly affected by different dietary methionine levels (P<0.05). WG, SGR PPV and PER increased, while FCR decreased with increasing dietary methionine level from 4.9 to 11.8g kg(-1) (P<0.05). However, with further increase from 11.8 to 20.8g kg(-1), WG, SGR PPV and PER significantly decreased, FCR increased (P<0.05). The whole body and muscle composition were affected by different dietary methionine levels (P<0.05). Condition factor (CF) increased with increasing dietary methionine levels up to 11.8g kg(-1) (P<0.05) and after 11.8g kg(-1) methionine diet, but not significant, declines were observed (P>0.05). Hepatosomatic index (HSI) of the 4.9, 9.0, 11.8 and 14.2g kg(-1) Met diets was significantly higher than that of fish fed diets 18.1 and 20.8g kg(-1) Met diets (P<0.05). Viscerosomatic index (VSI) of the 4.9, 9.0 and 11.8g kg(-1) Met diets was significantly higher than that of fish fed diets 14.2, 18.1 and 20.8g kg(-1) Met diets (P<0.05). Quadratic regression analysis of WG and PER against dietary methionine levels indicated that the optimal dietary methionine requirement for maximum growth and feed utilization of juvenile Pseudobagrus ussuriensis was 14.3 and 14.1g kg(-1) dry diet (35.3 and 34.8g kg(-1) dietary protein), respectively, in the presence of 2.5g kg(-1) dry diets cystine.