Background: Black soldier fy (Hermetia illucens) is a promising insect species to use as a novel ingredient in fsh feeds. Black soldier fy larvae consists of three major fractions, namely protein, lipid, and exoskeleton. These fractions contain bioactive compounds that can modulate the gut microbiota in fsh such as antimicrobial peptides, lauric acid, and chitin. However, it is not certain how, or which fractions of black solider fy would afect gut microbiota in fsh. In the present study, black soldier fy larvae were processed into three diferent meals (full-fat, defatted and de-chitinized) and two fractions (oil and exoskeleton), and included in diets for Atlantic salmon (Salmo salar). Atlantic salmon pre-smolts were fed with these diets in comparison with a commercial-like control diet for eight weeks to investigate the efects of insect meals and fractions on the composition and predicted metabolic capacity of gut microbiota. The gut microbiota was profled by 16S rRNA gene sequencing, and the predicted metabolic capacities of gut microbiota were determined using genome-scale metabolic models. Results: The inclusion of insect meals and fractions decreased abundance of Proteobacteria and increased abundance of Firmicutes in salmon gut. The diets that contained insect chitin, i.e., insect meals or exoskeleton diets, increased abundance of chitinolytic bacteria including lactic acid bacteria and Actinomyces in salmon gut, with fsh fed full-fat meal diet showing the highest abundances. The diets that contained insect lipids, i.e., insect meals and oil diets enriched Bacillaceae in fsh gut. The fsh fed diets containing full-fat insect meal had a unique gut microbiota composition dominated by benefcial lactic acid bacteria and Actinomyces, and showed a predicted increase in mucin degradation compared to the other diets. Conclusions: The present results showed that the dietary inclusion of insect meals and fractions can diferently modulate the composition and predicted metabolic capacity of gut microbiota in Atlantic salmon pre-smolts. The use of full-fat black soldier fy larvae meal in diets for salmon is more favorable for benefcial modulation of gut microbiota than larvae processed by separation of lipid or exoskeleton fractions.
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