Insect meal and poultry by-products as innovative ingredients for rainbow trout feed: impact on intestinal microbiota and gut health

Fish consumption and fishing levels rose sharply in the last decades leading to dramatic consequences on marine habitats. Aquaculture represents the fastest growing food production sector although its sustainability is strictly linked to provenance of feed ingredients. In the search for fish meal (FM) and fish oil (FO)-free formulations, insect meal (IM) and poultry by-product meal (PBMs) ingredients have been proposed as candidate protein sources. The aim of this study was to evaluate the effect of partial replacement of vegetable meal (VM) with IM and PBM in rainbow trout (Oncorhynchus mykiss) on gut health, by focusing on microbiota composition, markers of inflammation and gut barrier integrity. Animals were fed eight dietary treatments with different percentages of Hermetia illucens-IM or PBM to replace FM and VM. Test diets showed comparable growth performances to FM and VM. Microbial 16S rRNA analysis revealed that IM increased microbiota alpha diversity and led to a significant increase of chitin degraders Actinomyces and Bacillus genera. Analysis of feed microbiota revealed shared OTUs between IM feed and intestinal microbiota of IM fed fish supporting connectivity between food-chain microbiomes. IL-1ß, IL-10, TGF-ß, COX-2 and TCR-ß gene expression in midgut and head kidney, together with plasma LPS as biomarker of gut permeability, revealed that the experimental diets were well tolerated and did not compromise gut barrier function or induce inflammation. Both IM and PBM singly or in combination could be considered as valid alternatives to VM and FM in the aquaculture practice.