Biofortified black soldier fly larvae meal as an alternative to crystalline amino acid supplementation in diets for rainbow trout (Oncorhynchus mykiss)

Abstract

Black soldier fly larvae (BSFL, Hermetia illucens) are promising alternatives to fishmeal in aquafeeds, but their use in salmonid diets may be limited by deficiencies in essential amino acids. This study evaluated whether substrate-biofortified BSFL could serve as a nutritional alternative to crystalline amino acid supplementation in diets for rainbow trout (Oncorhynchus mykiss). Four isonitrogenous and isolipidic diets were tested: a fishmeal control (D1), a diet with complete fishmeal replacement by defatted BSFL (D2), a BSFL diet supplemented with crystalline amino acids (D3), and a diet containing biofortified BSFL produced on spirulina- and flaxseed-enriched substrates (D4). Juvenile trout (initial weight 28.6 ± 3.9 g) were reared for 8 weeks in a recirculating aquaculture system. Overall biomass gain was not significantly affected by dietary treatment. In contrast, feed intake, feed conversion ratio (FCR), and specific growth rate (SGR) differed significantly among diets (p < 0.001). Fish fed the BSFL diet without amino acid correction (D2) showed higher feed intake and a higher feed conversion ratio (FCR = 1.26) than the other treatments. The hepatosomatic index was also higher in D2 (p = 0.009), whereas fish fed the biofortified BSFL diet (D4) exhibited the lowest values. Whole-body composition was largely unaffected by dietary treatment, although fish fed D4 showed slightly higher whole-body lipid content than fish in the other treatment groups (p = 0.041). Plasma AST and ALT activities were elevated in D2 (p < 0.05), while other biochemical parameters remained similar across treatments. Histological observations indicated reduced mucosal fold width and mucosal thickness in fish fed D2, whereas fish fed D3 and D4 showed intestinal morphology comparable to the control. Taken together, these results suggest that amino acid limitations in BSFL-based diets can affect feed efficiency and intestinal structure in rainbow trout. However, these effects were mitigated by either crystalline amino acid supplementation or substrate biofortification of the insect biomass. Biofortified BSFL therefore appears to be a promising approach to improve the nutritional value of insect-based ingredients for salmonid aquafeeds.