| In recent years,due to the fast increase in aquaculture and the conservation of marine fishery resources,the supply of fish meal could not meet the requirement of aquafeed.Soybean in China is highly dependent on imports,so the development of novel protein source is very urgent.This study is designed to investigate the optimal level of the replacement of dietary fish meal by the blend of cottonseed protein concentrate,Clostridium autoethanogenum protein,Chlorella meal and yellow mealworm meal(8:5:4:3)and the optimal feeding frequency,so as to provide helpful information for the application of the novel protein source in largemouth bass feed.The main research results are as follows:1.Effects of replacing dietary fish meal by novel protein sources blend on growth performance,antioxidant status,intestinal morphology and gut microflora of largemouth bassJuvenile largemouth bass(initial body weight 14.23 ± 0.09g)was fed with the diets in which 0%(Con),15%(D1),30%(D2),45%(D3),60%(D4),75%(D5)and 90%(D6)fish meal were replaced by novel protein sources blend for 8 weeks.The results showed that,with the increase of substitution levels,the moisture content of whole fish significantly increased,while the ash content significantly decreased(P < 0.05).When fed with the diets of high fishmeal replacement,protein retention efficiency,weight gain rate,specific growth rate and condition factor significantly decreased,feed conversion ratio increased.And intestinal malondialdehyde content and total antioxidant capacity significantly increased(P< 0.05).The intestinal villus height,villus surface area,goblet cell number and muscle thickness of largemouth bass in D6 group significantly decreased,while the crypt depth significantly increased(P < 0.05).With the increase of replacement levels,the plasma contents of D-lactic acid and diamine oxidase increased(P < 0.05).The expression levels of claudin-1,il-10 and keap1 genes in the gut of largemouth bass in D5 and D6 groups significantly decreased,while the expression level of nrf2 significantly increased(P < 0.05).The abundance of Firmicutes decreased and Fusobacteria abundance increased in the group of high dietary replacement of fish meal(P < 0.05).The species diversity of intestinal microflora in group D1 was the highest(P > 0.05).These results indicate that the appropriate substitution level would not obtain significant negative effects on growth and feed utilization of largemouth bass,and could improve the intestinal health of largemouth bass,and enhance the level of intestinal microflora diversity.Overall,high substitution level could have significant negative effects on growth and feed utilization o,damage the intestinal tract of largemouth bass,increase oxidative stress,intestinal permeability and the abundance of harmful bacteria,and thus adversely affect growth performance of largemouth bass.2.Effects of replacing dietary fish meal with novel protein sources blend at different feeding frequencies on growth performance and health of largemouth bassIn order to investigate the effects of novel protein diets at different feeding frequencies on growth performance,plasma biochemistry,intestinal morphology and antioxidant state of largemouth bass(initial body weight: 14.23 ± 0.09 g).Four novel protein sources:cottonseed protein concentrate,yellow mealworm meal,Clostridium autoethanogenum protein and Chlorella meal,were blended to replace 0%,30%,60% and 90% fish meal,respectively,to prepare isonitrogenous and isolipidic diets(D1,D2,D3,D4),and fed 1meal/day,2 meals/day and 4 meals/day,respectively.The fish were fed for 8 weeks.The results showed that,at different feeding frequencies,specific growth rate(SGR)and protein retention efficiency(PRE)of largemouth bass in D4 group were significantly lower than those in D1,D2 and D3 groups,and feed conversion ratio(FCR)significantly increased(P< 0.05).Between the fish fed with different diets,SGR in the group fed 1meal/d was significantly lower than that in the groups fed at 2 meals/d and 4 meals/d(P < 0.05).At different feeding frequencies,plasma total cholesterol(TC)and high density lipoprotein cholesterol(HDL-C)contents of largemouth bass fed D4 diet were significantly lower than those in D1 group,and alanine aminotransferase(ALT)content was significantly higher than that in D1 group(P < 0.05).The contents of aspartate aminotransferase(AST),glucose(GLU)and alkaline phosphatase(AKP)in plasma of largemouth bass fed 1 meal/d were significantly higher than those of 2 meals/d and 4 meals/d(P < 0.05),while the contents of total protein(TP)and albumin(ALB)were significantly lower than those of the other two groups(P < 0.05).The contents of crude lipid and plasma low density lipoprotein cholesterol(LDL-C)in whole body of D4 diet group were significantly decreased,and the moisture content of D4 group was significantly higher(P < 0.05).The content of malondialdehyde(MDA)in intestinal tissue of largemouth bass in D4 group was significantly higher than that in D1 group,while the activity of superoxide dismutase(SOD)was significantly lower than that in D2 group(P < 0.05).The total antioxidant capacity(TAOC)and catalase(CAT)activities in the group fed 2 meals/d were significantly higher than those in the group fed 1 meal/d(P < 0.05).Histological results showed that the intestinal villus height,goblet cell number,muscle thickness and villus surface area of largemouth bass fed 2 meals/d were significantly higher than those fed 1 meal/d,while the intestinal crypt depth of largemouth bass fed 4 meals/d was significantly lower than that fed 1meal/d(P < 0.05).Overall,the 60% replacement of dietary fishmeal by the blend of four novel proteins fed at 2 meals/d could obtain better growth and feed utilization in largemouth bass.In conclusion,replacing 60% fish meal in the diet with novel protein blend can improve the intestinal health and intestinal flora structure of largemouth bass,thus improving the growth of largemouth bass,and 2 meals/d is beneficial to the growth and health of largemouth bass. |
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