Suitable inclusion levels of white fish meal and two kinds of soybean meal in diets for juvenile largemouth bass(Micropterus salmoides)

Eight isonitrogenous,isoenergetic and isolipidic diets were formulated to evaluate the feasibility of partial replacement of white fish meal(WFM)by dehulled soybean meal(DSM)or enzyme-treated dehulled soybean meal(EDSM),and to determine suitable inclusion levels of white fish meal and two kinds of soybean meal in diets for juvenile largemouth bass(Micropterus salmoides),based on growth performance,feed efficiency,body composition,non-specific immunity and hematological parameters.In four diets(D1-D4),inclusion level ratios of WFM to DSM were 45∶ 5,40∶ 12,35∶ 18 and 30∶ 25,respectively,and in the other four diets(E1-E4),the same inclusion level ratios were kept,only substituting EDSM for DSM.Each diet was randomly assigned to triplicate tanks of 35 fish[average initial weight(5.32±0.05)g]with an indoor temperature-controlled recirculating freshwater system.Fish were fed to apparent satiation twice daily(8:00 and 16:00)over 63 days.During the experimental period,water temperature was maintained at(28±1)℃ and dissolved oxygen content kept nearly saturated.The natural light cycle was adopted.The results suggested that no differences in survival rate and specific growth rate were found among treatments.Feed efficiency and protein efficiency ratio,however,occurred to a significant declining tendency when inclusion levels of DSM were higher than 5%,or EDSM higher than12% in diets.With decreasing in WFM inclusion level and increasing in DSM or EDSM inclusion level in diets,apparent digestibility coefficients of both protein and lipid were found to be higher.Meanwhile,protein and moisture concentrations in whole body,muscle and viscera of tested fish decreased significantly,but lipid concentrations became higher.With above changes in dietary WFM and DSM or EDSM levels,hepatosomatic index and liver glycogen content occurred to be lower.Serum lysozyme activity and serum protein content became lower althought serum complement activity(CH50)showed no significant difference among the treatments.While no difference in red cell count was found,the blood parameters of haemoglobin and haematocrit were affected significantly and negatively.In conclusion,it is feasible to use 45% WFM and 5% DSM as main protein sources in diet for juvenile largemouth bass,based on a comprehensive judgment of growth rate,survival rate,body composition,feed efficiency and health of tested fish.Inclusion levels of 40% WFM and 12% EDSM can achieve a similar performance,substituting EDSM for DSM in the diet,by which 11% of WFM can be replaced with EDSM.The effectiveness of enzyme treatment in removing the deleterious effects of antinutrients from soybean meal is confirmed in this study.