Preliminary Study On The Application Of Compound Acidifiers Supplemented In The Diets Of Juvenile American Eel(Anguilla Rostrata)

In recent years,there are more and more reports about compound acidifiers used as a feed additive supplemented in diets to improve the growth and health status of many fish species,but there is little research on the application of compound acidifiers in the eel diets.Therefore,in this study,compound acidifiers supplemented in diets were conducted to evaluate the supplementation effects on juvenile American eel(Anguilla rostrata)by the changes in growth performance,serum biochemical parameters,and intestinal health,which might provide the basement for the application of compound acidifiers in eel diets.Trial Ⅰ Effects of different levels of dietary compound acidifiers supplementation on growth,serum biochemical parameters,and intestinal health of juvenile American eelsSix hundred juvenile American eels with an average body weight of 11.00±0.03 g per fish were randomly divided into five treatment groups.The five groups were fed commercial diets(CAs0 group)and the commercial diets supplemented with compound acidifiers levels of 2.0 g/kg(CAs2 group),3.0 g/kg(CAs3 group),4.0 g/kg(CAs4 group),and 5.0 g/kg(CAs5 group),each treatment group had four replicates with 30 fish in each replicate.The trial period was 12 weeks.Results:Compared with CAs0 group,the weight gain rate,specific growth rate,feeding rate(except for CAs2 group),feed intake,protein efficiency rate,and hepatosomatic index were significantly increased(P<0.05)in the CAs supplemental groups,and the feed conversion ratio was significantly decreased(P<0.05).The viscerosomatic index and survival rate were similar among all the groups(P>0.05).The eels in CAs4 group had the best growth performance.There were no significant differences in moisture,crude protein,crude lipid and ash of the whole fish in different CAs treatment groups(P>0.05).The whole fish calcium contents were increased in the eels in CAs3,CAs4,and CAs5 groups(P<0.05),and the higher contents of whole fish phosphorus were found in the eels of CAs3 and CAs4 group(P<0.05).The whole fish calcium and phosphorus accumulation quantity and accumulation rate were significantly increased by CAs supplementation(P<0.05).The values of calcium and phosphorus accumulation parameters were the highest in the eels of CAs4 group.Compared with CAs0 group,levels of serum urea nitrogen(only CAs4 group),triglyceride,total cholesterol,low density lipoprotein cholesterol and D-lactate,and activities of glutamic-oxalacetic transaminase,glutamic-pyruvic transaminase,and diamine oxidase were significantly decreased in the CAs supplementation group(P<0.05),the value of those parameters was the lowest in the CAs4 group.Levels of serum albumin(except for CAs5 group),high density lipoprotein cholesterol(except for CAs5 group),immunoglobulin M,complement 3,calcium(except for CAs5 group),and phosphorus(except for CAs2 group),and activities of acid phosphatase(only CAs4 group),alkaline phosphatase(only CAs4 and CAs5 group)and lysozyme were significantly increased(P<0.05),the value of those parameters(except for high density lipoprotein cholesterol)was the highest in the CAs4 group.Compared with CAs0 group,the activities of intestinal protease and lipase were significantly increased in the CAs supplementation groups(P<0.05),and the activities of amylase were similar in the different treatment groups(P>0.05).Levels of intestinal total antioxidant capacity,reduced glutathione,and activities of glutathione peroxidase and superoxide dismutase were significantly increased in the CAs3 and CAs4 groups(P<0.05),the increased activity of catalase and decreased level of malondialdehyde were found in the CAs supplemental groups(P<0.05).The villi length and muscular thickness of the intestine(except for CAs2 group)were significantly increased in the CAs supplementation groups(P<0.05),the intestinal microvilli density tended to be increased and then decreased with the increasing level of compound acidifiers supplementation,and the CAs4 group had the highest density of the intestinal microvilli.Overall,the best intestinal health status was found in the eels of CAs4 group.Compared with CAs0 group,indices of Chao1,ACE,Shannon,and PD＿whole＿tree were decreased in the CAs4 group.At the phylum level,the relative abundances of Tenericutes,Proteobacteria,Bacteroidetes,Firmicutes,Actinobacteriota,and Acidobacteria were decreased,and the relative abundances of Spirochaetae and Fusobacteriota were increased.At the genus level,the relative abundances of Bacillus,Terrimonas,Cloacibacterium,Microbacterium,Lysobacter,Sutterella,and Chthoniobacter were significantly decreased in CAs0 group(P<0.05)and the relative abundance of Odoribacter was significantly increased in the CAs4 group(P<0.05).Trial Ⅱ Application effect of compound acidifiers in the diets of American eels cultured under recirculating intensive pondsAfter the eels grading,six intensive ponds culturing American eels(surface area 30 m~2 with0.6 m water depth,initial weight 87.52±0.83 kg/pond)with a similar body size of 240 P(The number of"P"is a technical term for the eel farming industry,namely the number of fish/kg)were randomly divided into two groups(CAs0 group and CAs 4 group),with three ponds in each group.The CAs0 group was fed commercial diets,and the CAs4 group was fed the same commercial diets supplemented with 4 g/kg compound acidifiers.The trial period was 56 d.Results:Compared with CAs0 group,weight gain rate,specific growth rate,and feed intake in CAs4group were significantly increased(P<0.05),while feed conversion ratio,protein efficiency rate,and survival rate were not significantly affected(P>0.05).The average yield per pond in the CAs4group increased by 18.08%,the average gross income increased by 18.10%and the cost of producing American eels per ton decreased by 0.64%.In conclusion,the appropriate level of compound acidifiers supplemented in the diets could significantly improve the growth,regulate serum biochemical parameters beneficially,increase calcium and phosphorus accumulation in whole fish,and enhance the intestinal health of juvenile American eels.The 4 g/kg compound acidifiers supplemented in the diets could increase the yield,decrease the cost of feed and increase the economic benefits of American eels cultured under recirculating intensive ponds.Excessive supplementation of compound acidifiers was detrimental to the growth and intestinal health of juvenile American eels.It was recommended that the compound acidifiers supplementation level in diets of juvenile American eels might be 4 g/kg.