Protein Requirement Of Juvenile Nile Tilapia(Oreochromis Niloticus) And Hybrid Tilapia(Oreochromis Niloticus×O.aureus)

The study was conducted to determine the effect of dietary protein level on growth performance,feed utilization and immune function in juvenile Nile tilapia and hybrid tilapia (Oreochromis niloticus×O.aureus). Fish meal, soybean meal, cottonseed meal, rapeseed meal, casein and gelatin were used as protein source to formulate five isoenergetic experimental diets (9%of fat content) at various protein levels (25%,30%,35%,40%and45%).(1) The results showed that final body weight, weight gain (WGR),specific growth rate (SGR),feed conversion ration (FCR), survival rate (SR), condition factor, hepatosomatic index (HSI), viscerosomatic index (VSI) were not significantly affected by dietary protein level (P>0.05).Protein efficiency ratio (PER) was significantly decreased with the increase of dietary protein level. Moisture, crude protein and ash contents of whole body were not significantly affected by dietary protein level (P>0.05),but total fat content in whole body was significantly affected by dietary protein level(P<0.05).Whole body total fat content of fish fed25%dietary protein diet was significantly higher than those of other diet treatments. White blood cell count (WBC),red blood cell count (RBC) and haematocrit (HCT) were not significantly affected by dietary protein level (P>0.05),but haemoglobin (HB) was significantly affected by dietary protein level(P<0.05), haemoglobin of fish fed40%dietary protein diet was significantly higher than that of fish fed25%dietary protein diet (P<0.05),Plasma creatinine, total protein, albumin, globulin, alkaline phosphatase, triglycerides and cholesterol levels were not significantly affected by dietary protein level (P>0.05).Hepatic lysozyme (LZM) activity of fish fed45%dietary protein diet was significant higher than that of fish fed25%dietary protein diet. But hepatic superoxide dismutase (SOD) activity of fish was not significantly affected by dietary protein level (P>0.05).Hepatic malondialdehyde (MDA) content of fish fed35%dietary protein diet was significantly higher than those of fish fed the others (P<0.05).The present study indicates that25%(D1) dietary protein level can meet the requirements for normal growth for Oreochromis niloticus, but hepatic lysozyme (LZM) activity of fish fed25%dietary was lower than fish fed high protein diets, which indicate that, hepatic non-specific immune function is poor.(2) The feeding trial results showed that growth performance, feed utilization and survival were not significant affected by dietary protein level, the protein efficiency ratio and protein retention efficiency were significantly decreased with the increase of dietary protein level. Hepatic catalase activity of fish fed34.9%dietary protein diet (D35) was significantly higher than those of the other diet treatments. However, dietary protein level had no significant effects on hepatic lysozyme activity, superoxide dismutase activity and hepatic malondialdehyde content of tilapia. The challenge trial results showed that dietary protein level had on significant effects on cumulative mortality rate at24hours and48hours after infection with S.agalactiae. The results of present study indicate that24.8%(D25) dietary protein level can meet the requirements for normal growth, immune function and disease resistance of hybrid tilapia.