High inclusion of soybean meal (SBM) exhibit soybean induced enteritis in carnivore fish including rainbow trout. Our goal was to evaluate the effects of whole/defatted black soldier fly larvae (WB/DB) as complementary feed ingredient in soybean meal based diets on growth performance, disease resistance, gut histology and gut microbiome in rainbow trout.
Experiment was conducted in two phases, Phase I: feeding trial for 10 weeks. Phase II: bacterial (Flavobacterium psychrophilum) challenge study for 4 weeks. Six experimental diets (42% crude protein and 20% lipid): fish meal based diet (FM), SBM based diet (SBM), SBM+2.5% and 5% WB (WB2.5 and WB5) and SBM+2.5 and 5% DB (DB2.5 and DB5) were fed twice at satiation level for 10 weeks. Each diet was fed with four replicate 30 fish (~5 g) per tank (60-L).
Supplementation of WB and DB improved (p<0.05) the performance of rainbow trout. Feed efficiency was negatively corelated with growth performance. Fish fed diet WB5 had the lowest cumulative percent mortality (CPM) at 32.17%, while fish fed diet SBM had the highest mortality (64.10%). Mortality of fish in group WB5 and DB5 were significantly lower than group SBM (p<0.05). Soy based diets caused enteritis whereas supplementation of insect meal mitigates the enteritis. Gut microbiota at phylum, class, genera and species level were significantly affected by dietary treatments in pre and post challenge study.
The firmicutes was the predominant microbiota in the pre-challenge study however, after post-challenge study the ratio of firmicutes decreased concurrent with the increase in proteobacteria and penericutes in most of the groups. The variation in microbial composition could lead to change in microbial metabolism including carbohydrate, lipid, protein and energy, and glycan biosynthesis. Summary of variation in microbial composition and nutrient and energy metabolism is depicted in figure 1. Conclusively, insect meal could improve the utilization of plant protein in trout via improving the growth and health of fish.