| The present study evaluated the effects of fish meal and soybean meal replacement by dietary different levels of Daphnia magna powder,in terms of growth performance,digestive enzyme activities in digestive tract,body color,antioxidant capacity of Pelteobagrus fulvidraco and water quality indexex,as well as intestinal microflora composition.The objective of the present study was to determine the optimal level of substitution,thus providing reference for the development of feed for Pelteobagrus fulvidraco.1.The levels of fishmeal and soybean replacement by Daphnia magna powder in feed were 0%(Y1),20%(Y2),40%(Y3),60%(Y4),80%(Y5),and 100%(Y6)respectively.Seven hundred twenty healthy male Pelteobagrus fulvidraco with an average initial body weight of 55.87±8.32 g and length of 16.76±1.21 cm were randomly divided into 6 groups,fed for 60 days with Diet Y1-Y6 respectively.Growth performance,digestive enzyme activities,muscle composition,skin carotenoid content,and water quality indexes were analyzed.The results showed that replacement of fishmeal and soybean meal by a certain level of Daphnia magna powder in the feed effectively improved the growth performance,and digestive enzyme activities of the Pelteobagrus fulvidraco,reduced the feed coefficient,at the same time increased the protein content but reduced fat content in muscle.After 30 days of feeding,the weight gain rate of Pelteobagrus fulvidraco in Y3 and Y4 group were significantly increased,especially in Y3 group,which was 47.41% higher than that in Y1 group(P<0.05),and the feed coefficient was decreased by 39.26% compared with Y1(P<0.05).The protease activities in the foregut,hindgut and stomach of the Y3 group was significantly increased(P<0.05),which was 11.14%,78.87% and 16.85% higher than that of Y1 group.Compared with the control group,the carotenoid content of Pelteobagrus fulvidraco skin significantly increased in Y4 had the maximum level(P<0.05).After 60 days of feeding,the weight gain rate of Pelteobagrus fulvidraco significantly increased and the feed coefficient decreased in the Y3 and Y4 group(P<0.05).Compared with Y1,the weight gain rate of Y4 group was increased by 2.22%,and the feed coefficient was decreased by 35.37%.The optimal substitution level of Daphnia magna powder was 58.35%,according to the regression equation developed by the fold line model using weight gain as an indicator.In Y4 samples,the intestinal protease activity and lipase activities in the foregut and hindgut remarkably increased(P<0.05)compared with the control group.In addition,the crude protein content increased by 4.3% while the fat content in muscle decreased by 6.9%(P<0.05).At the end of the culture,90% reduction of nitrite content in aquaculture water was detected in Y4 compared with the Y1 group(P<0.05).Considering the growth and feed efficiency,60% replacement of fishmeal by Daphnia magna powder was recommended.2.Effects of fishmeal substitution by Daphnia magna powder on biochemical indicators of Pelteobagrus fulvidraco.The results showed that regardless of a long-term or short-term evaluation,40%-60% replacement fishmeal and soybean meal by Daphnia magna powder significantly improved the antioxidant capacity of Pelteobagrus fulvidraco(P<0.05),revealing by stronger activities of SOD,CAT,GSH-PX,and higher levels of NO and GSH,and reduction of MDA contents in the tissue.After one month of feeding,the GOT activity decreased significantly in the hepatopancreas and serum of Y3 group(P<0.05),by 72.3% and 51.25% respectively,compared against the control.And the GPT activity greatly decreased in the Y4-Y6 group(P<0.05).After 2 months of culture,the GOT and GPT activity decreased significantly(P<0.05)against the control group,indicating that replacing 40%-60% of fishmeal and soybean meal by Daphnia magna powder could effectively improve the antioxidant capacity of and prevent liver damage of Pelteobagrus fulvidraco.3.Effects of Daphnia magna powder on serum lipid levels of Pelteobagrus fulvidraco.The results showed that the content of serum triglyceride,total cholesterol and low-density lipoprotein of Pelteobagrus fulvidraco decreased(P<0.05)gradually with the increasing level of Daphnia magna powder both in long-term or short-term.On the contrary,the high-density lipoprotein content increased continuously and reached the maximum value in the Y6 group(P<0.05),indicating that substitution of fishmeal by Daphnia magna powder could effectively lower the serum lipid level.4.Characterizing the composition of intestinal microflora in Y1 and Y4.The results showed that the dominant bacteria in the control group were of the Cetobacterium?Aeromonas?Rheinheimera?Buchnera?Plesiomonas and Flavobacterium.Compared with group Y1,Y4 group was dominated by Aeromonas,Rheinheimera,Buchnera,which had higher intestinal microbial diversity and increased proportion of beneficial intestinal bacteria such as Cetobacterium.Furthermore,the number of pathogenic bacteria such as Plesiomonas and Flavobacterium in Y4 decreased significantly.This indicated that the replacement of 60% fishmeal and soybean meal by Daphnia magna powder could improve gut health,through increasing the proportion of beneficial bacteria in the intestine and reduce the number of pathogenic bacteria. |
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