Influence Of Increased Dietary Non-Protein Energy Source On Growth,Metabolism And Immunity Of Barramundi(Lates Calcarifer)Juveniles

This study evaluated the effects of different dietary sources of non-protein energy on growth parameters,feed utilization,body composition,digestive performance,liver metabolism,antioxidant status and immune response of barramundi（Lates calcarifer）juveniles.According to the general nutritional requirements of barramundi,the control group（C）used the diet formula according to crude protein 475 g kg^-1,crude lipid 100 g kg^-1,and nitrogen-free extract 180 g kg^-1.In the experimental group,fish oil andα-starch were used as the source of lipid and carbohydrate,respectively.The crude lipid in the lipid group（HL）increased to 160 g kg^-1,and the nitrogen-free extract in carbohydrate group（HC）increased to 220 g kg^-1.feeding trial was conducted and the experiment lasted 56 days.Results from the present study will improve our understanding on the usage of lipids and carbohydrates in fish feed and to evaluate the availability of non-protein energy sources in feeding formula for barramundi juveniles from different perspectives.The results may provide theoretical basis for optimizing the formula feed for barramundi juveniles.This study mainly includes four parts presented as follows:1.Influence of dietary non-protein energy sources on growth parameters and digestive performances in barramundi juvenilesResults from the present study indicate that the specific growth rate of fish was not significantly affected by the experimental diets（P>0.05）.While weight gain and feed intake were significantly affected by the experiment diets,and the highest values were observed in HL（P<0.05）.In terms of body composition,HL had significantly higher whole body and muscle lipid than the other two groups（P<0.05）,and whole body protein of HL and HC were significantly lower than C（P<0.05）.In terms of somatic parameters,HL’s hepatosomatic index（HSI）was significantly higher than HC（P<0.05）,and its intraperitoneal fat ratio（IPF）was significantly higher than that of the other two groups（P<0.05）.The morphological structure of the foregut groups was different,which was mainly reflected in the number of goblet cells and the mucosal height.The number of goblet cells and the height of the mucosa in the HL were significantly higher than those in the other two groups.In C and HC group,the number of HC goblet cells and the height of mucosal were significantly higher than that of C（P<0.05）.There was no significant difference in muscularis thickness between the three groups（P>0.05）.In stomach,there was no significant difference in pepsin activity between groups（P<0.05）.In foregut,the lipase activity in HL was significantly higher than that in other two groups（P<0.05）,and there was no significant difference in amylase activity（P>0.05）.This study indicates that increasing the proportion of lipid in the dietary increased the food intake of barramundi juveniles,and the phenomenon of abdominal fat accumulation occurred,which induced the improvement of lipid digestion ability;higher proportions of carbohydrate in the diet don’t affect their growth and digestive performance.2.Influence of dietary non-protein energy sources on metabolism in barramundi juvenilesIn the present study,CPT1 and SCD expression in fish fed high lipid diet were significantly upregulated and liver lipid metabolism was more active than C.Serum biochemical parameters including serum glucose,serum triacylglycerol and serum cholesterol were significantly increased（P<0.05）.When barramundi were fed with the carbohydrate diet,GK expression was significantly downregulated and c PEPCK expression was significantly upregulated（P<0.05）,while glycolysis was weakened and gluconeogenesis was enhanced to promote liver glycogen accumulation.The fluctuation of serum biochemical parameters and the growth performance were not significantly different compared to C（P>0.05）.Although the intake of lipid and carbohydrate diets caused significant changes in metabolism,there was no obvious negative impact on the endocrine and metabolism,indicating that the barramundi juveniles are capable of adapting to non-protein diets.3.Influence of dietary non-protein energy sources on antioxidant status and immune response in barramundi juvenilesThe activity of most antioxidant enzymes in HL significantly increased（P<0.05）,and the antioxidant capacity in the HC group was significantly higher than that in other treatments（P<0.05）.In the TOR pathway,LST8 homolog（m LST8）expression in the lipid group was significantly downregulated（P<0.05）,and the mechanistic target of rapamycin（m TOR）expression in the HC was significantly downregulated and e IF4E expression was significantly upregulated（P<0.05）.The C-reactive protein（CRP）expression in the HL and HC was significantly upregulated（P<0.05）.The expression levels of heat shock protein genes in HL and HC were significantly downregulated（P<0.05）.In terms of antioxidant capacity,higher intake of lipids in juvenile sea bass in the HL induced increased antioxidant enzyme activity in the liver,while the antioxidant capacity of several antioxidant defense mechanisms in juvenile barramundi in the HC was more balanced.In the expression of immune-related genes,neither lipids nor carbohydrate caused a serious immune response in barramundi juveniles.