The Effects And Mechanisms Of Dietary Protein-to-energy Ratio On Growth Performance And Disease Resistance In Golden Pompano(Trachinotus Ovatus)

Golden pompano Trachinotus ovatus has become the most important species of marine fish aquaculture in southern China,contributing to enrichment of national diets and improving economic benefits of farming and living standards.At present,feed coefficient of golden pompano culture is high,and the disease outbreak loss is serious,as well as nutrient requirements of the whole feeding process are not completed,which is not conducive to sustainable development of golden pompano culture.The purpose of this study was to investigate the effect and mechanism of dietary protein-to-energy ratio on growth performance and disease resistance of the large-sized golden pompano,to improve the growth performance of golden pompano and feed utilization,and provide scientific basis for exploitation of environmentally-friendly and efficient golden pompano formulated feed.The main research contents,results and conclusions are as follows:Trail 1.The effect of dietary protein-to-energy ratio on the growth,immune and anti-oxidation of golden pompanoWith Danish low temperature fish meal,soybean meal and cottonseed concentrate protein as protein source and fish oil,perilla seed oil and coconut oil as lipid source,nine test diets（protein/lipid（P/L）:35/8,35/13,35/18,40/8,40/13,40/18,45/8,45/13,45/18）of a 2×3 factorial trial were designed to contain three protein levels（35%,40%and 45%）and three lipid levels（8%,13%and 18%）and fed to fish with initial body weight of around 70 g for 50 days.The results showed that 1)The fish growth performance（weight gain rate,WGR,and specific growth rate,SGR）in fish fed diet with CP40/L18（protein/energy,P/E=22.19mg/k J）were significantly higher（P<0.05）than those of other treatments.The Feed coefficient（FC）of the 35/18,40/18,45/13 and 45/18 groups were significantly lower than those of the 35/8 group（P<0.05）.Protein efficiency ratio（PER）of the 35/13,35/18,while no significant difference was found in the other groups（P>0.05）.Hepatosmatic index（HSI）,the viscerosomatic index（VSI）and the intestinalsomatic index（ISI）of the 35/13 group were significantly greater than that of the 40/18,45/8,45/13 and45/18 groups.Condition factor（CF）of the 40/13,40/18 and 45/8 groups were significantly greater than the 35/8 group,and there was no significant difference among the others.The survival rate（SR）of golden pompano was 96.67%-100%,and there was no significant difference among feed groups.2)The crude protein contents of the whole fish in the 40/8,40/18 and 45/13 groups were significantly higher than among feed groups.The crude fat contents of whole fish in the 35/18 group was significantly higher than those of 40/8 and40/13 groups.The crude ash contents of the whole fish in the 40/18 group was significantly higher than those of 35/13,35/18,40/8 and 40/13.Dietary protein-to-energy ratio had no significant effect on the whole body moisture.3)Serum alkaline phosphatase（AKP）activity of 35/8 group was significantly lower than among the others,and total cholesterol（TCHO）,triglycerides（TG）,high density lipoprotein cholesterol（HDLC）and low density lipoprotein cholesterol（LDLC）of 45/18 group had higher contents than those of all other experimental groups.4)LZM,AKP and ACP activities of 35/8 group showed higher activities in hepatopancreas and lower activities in spleen.The 40/18 group showed higher activities in hepatopancreas than spleen of SOD,T-AOC,GST and GSH-PX,and showed lower MDA conents.The high P/E ratio of 45/8,45/13 and 45/18 groups in hepatopancreas showed higher SOD and GSH-PX activities,while spleen showed higher GSH-PX,GST and MDA conents.5)The results of fatty acids profile in hepatopancreas showed that total fatty acid（TFA）contents of the 45/18 group was significantly higher than those of the feed fat level of 8%groups.The fatty acid（SFA）content was not significantly different in the40/18,45/18 and 35/18 groups,but 40/18 and 45/18 groups showed significantly higher than those of other experimental groups.The polyunsaturated fatty acids（PUFA）,n-3polyunsaturated fatty acids（n-3 PUFA）,n-6 polyunsaturated fatty acids（n-6 PUFA）and n3/n6 of 45/8 group had the highest contents among the experimental groups,and the monounsaturated fatty acid（MUFA）content was the lowest.6)The results of hepatopancreatic metabolism-related enzyme activities showed that 40/18 group had higher activities of rapamycin target protein C1（TORC1）,protein S6 kinase（S6K）,eukaryotic translation initiation factor 4E binding protein 1（4EBP1）,adenylate-activated protein kinase alpha（AMPKa）,and peroxisome proliferator-activated receptor alpha（PPARa）,carnitine palmitoyl transferase 1（CPT1）,fatty acid binding protein（FABP）,sterol regulatory element binding protein-1C（SREBP-1C）,and conents of adenosine triphosphate（ATP）,and had lower conents of adenosine monophosphate（AMP）and fatty acid synthase（FAS）activities.7)The results of the expression levels of hepatopancreatic metabolism-related m RNA showed that expression of PPARα,FAS,phosphoenolpyruvate carboxykinase（PEPCK）,and phosphofructokinase（PFK）m RNA of 35/8 groups were significantly higher than thosein other groups.The m RNA expression of CPT1 and FABP of 35/8 group were significantly lower than those of other groups.The hexokinase（HK）m RNA expression of 35/18 and 40/8 groups were significantly higher than those of other experimental groups.The m RNA expression of orexin（ORE）of 35/18,40/13,40/18,45/8and 45/18 were significantly higher than those of other experimental groups.Trail 2.The effect of dietary P/E ratio on Vibrio harveyi resistance in golden pompanoAfter the above-mentioned feeding experiment finished,the golden pompanos were acclimated in the circulatory water system for 7 days,then randomly stocked 30 fish in 3replicates tanks in non-circulating aquaculture water system（volume 125 L）indoor for the challenge experiment.Each fish was intraperitoneally injected with 0.2 ml 4.23×10⁷CFU·ml^-1Vibrio harveyi,and the number of dead fish was recorded every 12 h for 48 h.The results indicated that 1)the SR of 35/8 and 35/13 groups were significantly lower than those of the other experimental groups（P<0.05）.In the Two-way ANOVA analysis,the protein level significantly affected the SR of golden pompano（P<0.05）,and the protein level was positively correlated with SR.The fat level had not significant influence on SR（P>0.05）,while dietary protein and fat had an interaction on SR（P<0.05）.2)Serum AST activity after the challenge was significantly greater than that in before,which showed that the hepatopancreatic cell structure of golden pompano was greatly damaged after the injection of Vibrio harveyi.In summary,under the experimental conditions,diet containing 40%feed protein and18%fat,corresponding to a protein to energy ratio of 18.99 mg·k J^-1,was recommended for optimal growth,immunity and antioxidant capacityand,and disease resistance to Vibrio harveyi in golden pompano with initial body weight of about 70 g.It has higher body protein content,higher activities of antioxidation in hepatopancreas and spleen,strong process of protein synthesis,lipid synthesis and glycolysis in hepatopancreas,and strong disease resistance to Vibrio harveyi.