| Fish meal is one of the main protein sources in fish feeds.In recent years,the increasing demand for fish meal,high prices and the shortage of fish meal resources worldwide have highlighted the requirement of finding alternative protein sources to fish meal in aquafeeds.Black soldier fly(Hermetia illucens)is rich in protein and fat and other nutrients.It is considered as an ideal fish meal protein replacement source.In the present study,six types of pelleted diets with equal nitrogen and fat were prepared for largemouth bass(Micropterus salmoides).The effects of defatted black soldier fly larvae meal(BSFL)on the growth performance,tissue morphology,muscle quality and mechanism of action were evaluated by replacing fish meal in the diet of the whole fish meal group(BSFL0)with 15%(BSFL15),30%(BSFL30),45%(BSFL45),75%(BSFL75)and 100%(BSFL100)and feeding them to largemouth bass(average body weight 135.6 g)for 90 days.The main findings of the study were followed.1.Effect of fish meal replacement by BSFL on growth performance and tissue structure of largemouth bass.The results showed that:(1)In terms of growth performance and biological traits,the final body weight and specific growth rate were significantly lower in the BSFL75 group compared with the BSFL0 group(P < 0.05),but there were no significant differences in weight gain rate,survival rate,protein efficiency ratio,feed conversion ratio,condition factor,average daily feed intake,viscera somatic index,hepato-somatic index,intraperitoneal fat body index and flesh content among the largemouth bass groups(P > 0.05);(2)In terms of intraperitoneal fat,liver and villus tissue structure,the area of the intraperitoneal fat cells tended to increase with the increase of BSFL replacement;liver cells of largemouth bass were neatly arranged,with obvious cell membrane boundaries and uniform size,and no histological lesions in all groups;villus tissues were intact,and the villus length of largemouth bass in the BSFL45 ～ BSFL100 group increased significantly compared with the BSFL0 group(P < 0.05).The villus width in each group decreased significantly compared with the BSFL0 group(P < 0.05),and the villus thickness in BSFL15,BSFL45,BSFL75 and BSFL100 groups increased significantly compared with the BSFL0 group(P < 0.05).2.Effect of fish meal replacement by BSFL on muscle quality of largemouth bass.The results showed that:(1)In terms of muscle nutritional value,there were no significant differences in crude protein,crude fat,crude ash and water content in muscle among these dietary groups(P > 0.05).There were no significant differences in total essential amino acids,total non-essential amino acids,total delicious amino acids and total amino acid in muscle among these dietary groups(P > 0.05).Compared with the BSFL0 group,the saturated fatty acids were significantly lower and linoleic acid were significantly higher in the BSFL30 ～ BSFL100 groups(P < 0.05),the eicosapentaenoic acid(EPA)were significantly lower in the BSFL75 and BSFL100 groups(P < 0.05).The atherosclerotic index were significantly lower in the BSFL45 ～ BSFL100 groups compared with the BSFL0 group(P < 0.05).There were no significant differences in thrombosis index among six groups(P > 0.05).(2)In terms of muscle sensory characteristics,there were no significant differences in the brightness,redness and yellowness values of the dorsal,lateral and ventral skin of largemouth bass among these dietary groups(P > 0.05),and no significant differences in muscle brightness and redness values among these dietary groups(P > 0.05).The muscle yellowness values in the BSFL45 ～ BSFL100 group were significantly lower than those in the BSFL0 group(P < 0.05).There were no significant differences among these groups in the texture parameters of muscle hardness,chewiness and shear force(P > 0.05),and no significant differences among the groups in muscle water holding capacity,p H,hydroxyproline content,collagen content and muscle tissue morphology(P > 0.05);(3)In terms of muscle safety properties,copper content was 0.18 ～0.29 mg/kg,which were not significantly different among these groups(P > 0.05),and cadmium and arsenic of the muscles were not detected in largemouth bass.3.Effect of fish meal replacement by BSFL on liver transcriptome of largemouth bass.A total of 408907940 raw reads were obtained by sequencing the liver tissue transcriptome of BSFL0 and BSFL75 groups,and a total of 328181878 high-quality sequences were obtained after quality control,accounting for 80.3% of the total raw reads data.A total of1229 differential expression genes were screened in BSFL75 group compared with BSFL0 group,including 970 were up-regulated and 259 were down-regulated.The differential expression genes between the two groups were mainly enriched in signaling pathways such as complement and coagulation cascades,PPAR signaling pathway,tight junction,cholesterol metabolism,leukocyte transendothelial migration,cell adhesion molecules,arachidonic acid metabolism and fat digestion and absorption.It was found that the BSFL75 group induced up-regulation of Grb14,down-regulation of mLST8 in the mTOR signaling pathway and down-regulation of Pi3 kcb in the growth hormone synthesis,secretion and action pathway compared to the BSFL0 group,resulting in reduced growth performance of largemouth bass.In summary,the results of this study showed that the use of BSFL completely replacing fish meal did not affect the biological traits,liver tissue morphology and muscle quality of largemouth bass,while it would improve the villus tissue morphology;however,excessive BSFL(≥ 75%)replacing fish meal would reduce the growth performance of largemouth bass,lower the muscle EPA content and muscle yellowness value,and increase the lipid accumulation at the same time.On balance,the proportion of fish meal substituted by BSFL in largemouth bass feed should be less than 75%(accounting for 52.5% of dry matter base of feed). |
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