| The research of protein sparing effect of dietary lipids has become a hot topic for aquaculture nutritionists. This study aims to study the influences of dietary protein to energy ratios on growth performance, body compositions, feed utilization efficiency, biochemical indices and digestive enzyme activities of juvenile black seabream, and we hope to deduce the optimum dietary protein to energy ratio, based on this study we can determine the suitable protein and lipid level in feed.The black seabream juveniles were randomly allotted to six groups with triplicate of 20 fish (aver wt 15.25±4.24g). There are six experimental diets formulated to provide varying P/E ratios as follows: 64.37 mg/kcal (protein 31.90%, lipid 22.25%), 73.65 mg/kcal (protein 34.51%, lipid 20.05%), 80.50 mg/kcal (protein 37.05%, lipid 17.65%), 87.91 mg/kcal (protein 39.38%, lipid 14.96%), 96.07 mg/kcal (protein 41.93%, lipid 12.42%) and 104.21 mg/kcal (protein 44.16%, lipid 10.09%). Fish were reared in tanks for 8 weeks with the diets aforementioned respectively. All the fish were killed to collect the carcass, liver, muscle, stomach, intestine and plasma samples, and then analyzed the indices relevantly.The growth trial suggested that the increased dietary P/E ratios could improve the growth of juvenile black seabream. Weight gain rate (WGR), specific growth rate (SGR), apparent digestibility of dry matter and crude protein increased with the improving of dietary P/E ratios significantly (P<0.05), while the condition factor (CF) and hepatosomatic index (HSI) were inversely related to the improvement of dietary P/E ratios.Dietary P/E ratios had obvious effects on digestive enzymes. Protease activity was the highest in stomach, which enhanced both in stomach and intestine with the increasing dietary P/E ratios, and it reach to the top level at 96.07 mg/kcal dietary P/E ratios, then decreased with continue enhancing dietary P/E ratios. Lipase activity was the highest in intestine, which decreased with the increasing dietary P/E ratios regardless of stomach, intestine, and liver. The liver amylase activity was the highest in three digestive organs, which increased with the increasing dietary P/E ratios. Black seabream digestive enzymes showed strong compatibility to the diets given.Fish body composition was significantly related to dietary P/E ratios (P<0.05). Moisture, protein in whole body and liver increased with the increasing P/E ratios in diets. Lipid content in whole body and liver decreased with the increasing dietary P/E ratios. Enhancement of dietary P/E ratios had no significant effect on body ash, calcium, phosphorus content and those of liver. Muscle fat decreased obviously with the increasing of dietary P/E ratios, and it showed there were no marked relation between moisture, protein, ash, calcium, phosphorus content in muscle and dietary P/E ratios.Dietary P/E ratios had significant influences on triglycerides (TG), total cholesterol (T-CHO), glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) in plasma (P<0.05). There were negative correlations between GOT, GPT activities and dietary P/E ratios within the range of 80-104 mg/kcal. Both GOT and GPT activities reduced to the lowest levels at 96.07 mg/kcal dietary P/E ratios. The concentrations of TG and T-CHO decreased with the increasing of dietary P/E ratios. No significant correlation was found between glucose concentration and dietary P/E ratios. Diets of higher P/E ratios could lighten the liver burden and reduce the plasma lipids.Generally considering growth performance, feed efficiency and body compositions as the evaluating index, we can conclude that when the water temperature is 28±1℃, the feasible dietary protein to energy ratios for black seabream juvenile (aver wt 15.25g) is 73.65-104.21 mg/kcal, and the optimum ratio is 100.98 mg/kcal based on the best weight gain.
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