Dietary inclusion of full-fat Hermetia illucens prepupae meal in practical diets for rainbow trout (Oncorhynchus mykiss): Lipid metabolism and fillet quality investigations

Insects are able to bio-convert organic by-products into a sustainable biomass for aquafeed formulation. Specifically, among several insect species, Hermetia illucens (H) is particularly interesting for its nutritious traits but, unfortunately, the lipidic fraction is poorly represented by polyunsaturated fatty acids n-3 and poses some limits in its application in aquafeed formulation. The present study undertook an interdisciplinary approach to explore the effects of three experimental diets containing increasing levels of full-fat H meal (H0 diet based on fishmeal and purified protein-rich vegetable ingredients; H25 and H50 diets containing 25% or 50% of full-fat H meal replacing fishmeal, respectively), on rainbow trout (Oncorhynchus mykiss) fed over a 98 days experimental period. The expression of genes related to lipid metabolism by RT-qPCR, liver histology, as well as the qualitative traits of fillets and fatty acid (FA) composition were investigated. Interestingly, fads2 gene expression in pyloric caeca increased in fish fed diets containing the highest full-fat H meal inclusion (H50 > H0; p < .05). Liver histological examinations showed normal morphological aspect even though hepatic FA profiles seemed to resemble those of the diets. However, liver docosahexaenoic acid did not significantly differ between the dietary groups and showed a mean value of 11.07 g FA methyl esters/100 g total FA methyl esters. Despite the FA profile of the three diets differed depending on the H meal inclusion level, biometrics, fillet physical traits, total lipids and the overall FA profile were not jeopardised, not even eicosapentaenoic and docosahexaenoic acids. The overall results showed that the dietary full-fat H meal inclusion under study did not impair fish fillet quality, guaranteeing its nutritional value. Some effects on lipid metabolism were observed, as suggested by liver, pyloric caeca and mid intestine gene expression and liver FA profile. Future studies on the biological mechanisms behind the macroscopic traits of fish fed unprocessed insects are warmly encouraged.