Innovative protein sources in feed for salmonids: Effects on lipid metabolism, gut microbiota and fillet quality

The purpose of aquaculture is to contribute to feed an increasing world population and to become more sustainable. Consequently, the aquafeed sector must adapt to supply more feed with less expensive and more eco-friendly ingredients. Insects have been studied as one of the promising and innovative protein sources. Evidence showed that their production leads to a circular bioeconomy, and generates positive societal externalities. Moreover, insects are considered fairly nutritious as aquafeed ingredients. Administering a new diet to fish rest on the assumption that animal welfare and final eating quality are preserved. The present thesis analyses the effects of feeding the Salmonidae Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) with diets containing the larvae of Hermetia illucens, belonging to the Diptera order. The ultimate goal of the present research is to provide a meaningful insight into the decision making process for feeding and farming in order to optimise the overall production process. To fulfil this ambition, three topics were taken into consideration: - gut microbiota, for its countless functions and widespread impacts that it can have on the hosts; - lipid metabolism, for better understanding the laws behind the lipid constitution in the final product; - final product quality, specifically addressing its fatty acid composition, for the importance of fish fatty acids in human nutrition. The results on gut microbiota of either A. salmon or rainbow trout indicated that microbiota composition is modulated by dietary insect and that it differs depending on sample origin (mucosa or digesta). In addition to changes in composition, microbiota of fish fed diets containing insects was also marked by higher alpha- and beta-diversity. Additionally, mucosa samples tended to show lower alpha-biodiversity in comparison to digesta samples, fact that could be explained by positing that mucosa exercises an active selection. The examinations on lipid metabolism were performed on rainbow trout by calculating the indices of fatty acid metabolism and the ratio of products:precursors in liver and fillet, as well as by qPCR gene expression of pyloric caeca, mid intestine and liver. The three approaches agreed on the fact that lipid metabolism was affected by the diets containing the insect. Specifically, pyloric caeca elovl2 relative expression seemed to show an increased trend when rainbow trout were fed diets containing the insect. Also pyloric caeca and mid intestine fads2 expression seemed to increase following the increasing dietary insect. Minor effects on protein, fatty acid composition and fillet quality were noticed. Fillet fatty acid profile of A. salmon and rainbow trout fed diets containing the insect partly resembled the dietary fatty acid profile, commonly by showing an increase in saturated fatty acids. The cardioprotective fatty acids, namely polyunsaturated of the n-3 series, were successfully retrieved in fish fillets in satisfying amounts, not reflecting the dietary content. In addition, consumers appreciated steamed A. salmon fillets irrespective of the dietary treatment, though small weaknesses related to color intensity and textural attributes were raised in the fillets of the insect group. All things considered, diets containing insect guaranteed a nutritious and appreciated food. To conclude, as some bacteria may protect fish from pathogens as well as enhance physiological functions, it is highly desirable to delve into the effects that bacterial communities have on fish biology. Likewise, an interaction of microbiota with lipid metabolism cannot be excluded. H. illucens larvae used as dietary source for A. salmon and rainbow trout modulated lipid metabolism, but the final eating quality of fillets was highly preserved. It seems probable that the modulation of other oily ingredients may play a role in generating such an outcome, and further studies would be necessary to unravel the underlying dynamics. Additionally, tailoring H. illucens larvae by rearing them on a substrate from marine origin could help improve the final result.