EFFECTS OF DIETARY PROTEIN LEVELS ON THE GROWTH PERFORMANCE, ACTIVITY OF NON-SPECIFIC IMMUNITY AND PROTEIN SYNTHESIS CAPACITY OF RHYNCHOCYPRIS LAGOWSKII DYBOWSKI

To evaluate the effects of different dietary protein levels on the growth performance, non-specific immunity and protein metabolism enzymes of Rhynchocypris lagowskii Dybowski, diets with 24.98%, 30.02%, 34.99%, 40.01% and 44.98% protein formulated with fish meal, soybean meal, rapeseed meal and cottonseed meal were fed to Rhynchocypris lagowskii Dybowski (6.98±0.01) g for 8 weeks in this study. After the feeding trial, a national standard method was used for determining muscle nutrients, and kits were used to measure serum alkaline acid enzyme (AKP), acid phosphatase (ACP), superoxide dismutase (SOD) and lysozyme (LZM) activity. The results showed that, with the increase in protein levels, final mean body weight, weight gain rate and specific growth rate firstly increased with increasing protein levels, and then decreased. When the protein levels were 34.99% and 40.01%, the final mean body weight, weight gain rate and specific growth rate were significantly higher than that in the groups with 24.98% level (P<0.05). With increasing protein levels in diets, feed and protein efficiencies ofRhynchocypris lagowskii Dybowski firstly increased, and then decreased. Feed and protein efficiencies in 40.01% groups were significantly higher than those in 24.98% groups (P<0.05), but there was no significant difference among groups with 30.02%, 34.99%, and 40.01% protein levels (P>0.05). According to the polynomial regression analysis, the specific growth rate ofRhynchocypris lagowskii Dybowski was the highest when protein level was at 35.89%, and the feed efficiency was the highest when the protein level was at 36.11%. The content of crude protein in muscles increased first, and then decreased with increasing protein levels in diets. Among them, the crude protein content in muscles of 40.01% group was significantly higher than that in the other four groups (P<0.05). However, the crude fat content in muscles decreased first, and then increased with increasing protein levels. The crude fat content in 40.01% group is significantly lower than that in the 24.98%, 30.02%, and 34.99% groups (P<0.05), but the 44.98% group showed no significant difference from other groups (P>0.05). The activities of alkaline phosphatase (AKP), acid phosphatase (ACP), superoxide dismutase (SOD) and lysozyme (LZM) in hepatopancreas increased first, and then decreased with increasing protein levels. The activities of AKP, ACP, SOD, and LZM in the 40.01% group were significantly higher than those in the other four groups (P<0.05). The RNA and RNA/DNA ratios in muscles increased firstly, and then decreased with increasing protein levels. Among them, the RNA content and RNA/DNA in 40.01% group were significantly higher than those in the other four groups (P<0.05). According to the polynomial regression analysis, the RNA content in white muscle was highest when the protein level was 36.10%, and the RNA/DNA ratio was highest in white muscle when the protein level was 35.91%. Therefore, in the formulated diets, the most suitable protein level is 34.99% to 40.01%.