Partial replacement of fish meal by soybean meal in diets for juvenile Japanese flounder (<I>Paralichthys olivaceus</I>)

Abstract

The utilization potential of soybean meal as an alternative protein source for fish meal in practical diets for the juvenile Japanese flounder (Paralichthys olivaceus) was assessed in the present study. Four isonitrogenous and isolipidic diets were formulated to contain 47% protein and 9% lipid. The animal-plant protein ratios (A/P ratio) in the diets,representing 4∶1(D1),3∶1(D2),2∶1(D3) and 1∶1(D4),respectively,were calculated by adjustment of supplementing proportion of fish meal (FM) and soybean meal (SBM). Two hundred and forty Japanese flounder with an initial average body weight of (13.22± 0.02) g were randomly assigned into twelve 900-liter tanks equipped with a flow-through seawater system. Each diet was randomly allocated to triplicate groups of twenty fish each. The fish were fed the test diets twice a day at a daily feeding rate of 2%-3% body weight for 56 days. At the end of the feeding trial,the fish were weighed by batch for the determination of weight gain rate (WGR),feed conversion ratio (FCR) and protein efficiency ratio (PER),and then five fish from each tank were randomly sampled and pooled by tank for the analysis of proximate composition in the whole body. Another five fish from each tank were randomly selected to collect serum and liver samples for the analysis of blood urea nitrogen (BUN),glutamic-pyruvic transaminase (GPT),glutamic-oxalacetic transaminase (GOT),free fatty acids (FFA),total protein (TP),triglyceride (TG),cholesterol (CHO),high-density lipoprotein cholesterol (HDL-C),and low-density lipoprotein cholesterol (LDL-C). WGR,PER,and SGR of Japanese flounder decreased,while FCR increased with increasing SBM ratio in diets,and significant differences for the parameters were found when SBM ratio in diets increased to upon 24% (P0.05); HSI followed the same pattern as WGR,and the values in D1 group were significantly lower than those of other groups (P0.05). The values of CF were not different among dietary treatments (P0.05). There were no significant differences in whole body moisture,crude protein,and ash content of the fish among dietary treatments,however,crude lipid content decreased as SBM ratio increased,and the values of D3 and D4 groups were significantly lower than that of D1 group. There was a tendency of marginal increased BUN content in serum and TP content in liver of the Japanese flounder with increased SBM ratio in diets,whereas the opposite trend is true for GOT,GPT activity in serum,and FFA conten in liver (P0.05);TP and HDL-C content in serum (P0.05). Serum TG and liver TG content increased significantly while the A/P ratio decreased to below 2∶1 (P0.05); serum CHO,serum LDL -C and liver CHO content of D1 group were significantly lower than those of other groups (P0.05). These results indicate that higher ratio replacement FM by SBM (D3 and D4) in diets could adversely affect the growth and protein and lipid metabolism of Japanese flounder,and the optimal SBM ratio in diets was 16% based on the results of growth performance,body composition,protein and fat metabolism indices,and cost of weight gain in the present study.