Abstract :
[en] Worldwide fish demand for food market continuously increases while fishery resources reached a ceiling in the 1990s and wild fish stocks are subject to increasing pressure which endangers them. Therefore, aquaculture continues to develop as an alternative to contribute to the global fish production. Nevertheless, this production sector requires large quantities of feed ingredients for fish species that are mostly carnivorous and require large amounts of proteins and lipids. Feed sources for the fish farm largely rely on industrial fisheries whose stocks are limited and already overexploited. In this context, a diversification of nutritional resources must be considered in order to perpetuate the sector development and ensure its sustainability. Vegetable meals inclusion in fish feed has already made it possible to substitute a significant part of the fishing resources. Nevertheless, their nutritional profiles do not meet all the requirements of most fish species, which limits their inclusion rate in fish feed. Insects present a new source of proteins and lipids that could be produced locally to feed farmed fish. Compared to conventional farms, they require less space, consume less water and have a high conversion rate, especially for organic waste. Among potential species, the black soldier fly (Hermetia illucens L. 1758- Black Soldier Fly - BSF) is a prime candidate because of its rapid production cycle, its voracity for a wide range of organic matter and its amino acid profile, making it an excellent substitute for fish meal. Its fatty acid profile characterized by a high level of saturated fatty acid (SFA) will be the limiting factor for its inclusion in fish feed. The production of this new fish feed ingredient is still in its infancy and the BSF zootechnical data, as well as the impacts of its nutritional composition on fish feed, are still limited.
In this context, this thesis research focused on the prepupae production of BSF through the valorization of agricultural by-products and their inclusion in rainbow trout (Oncorhynchus mykiss Walbaum 1792) feed.
The first experimental axis of this work focuses on the development of the BSF rearing in a controlled environment. Based on the natural behaviors of the species, specific rearing methods and infrastructures for each stage of its life cycle (reproduction, egg incubation, larval pre-fattening, larval fattening and prepupae self-collection) have been developed. This research has led to the setting up of a mass production unit of BSF larvae and prepupae in a converted transport container. A particular emphasis was put on the BSF artificial reproduction, the most sensitive rearing phase. Breeding experiments were conducted to evaluate the impact of sex-ratio and density of broodstock as well as variation in the nycthemeral cycle on breeding effort. In order to carry out these experiments, a broodstock selection model and an oviposition system were first developed. Sexing of imago populations resulting from prepupae mass production revealed sexual growth dimorphism, wherein males exhibit faster growth. These observations allowed us to establish a selection of broodstock populations according to their sex-ratio based on their progressive self-collection at the prepupal stage. The oviposition device consisting of artificial supports placed on an attraction system has allowed to centralize the ovipositions within the breeding cages in order to collect and quantify egg production. This research has shown that a population with a dominant female sex-ratio should be favored from a density of 6500 individuals/m³ and that a daily light duration of minimum 6h should be applied to maintain the reproduction. The compilation of the results obtained allowed the development of an efficient method of BSF reproduction in a controlled environment.
The second experimental axis of this work focuses on the modulation of the BSF prepupae fatty acid profile through larval feeding. The objective of this research was to evaluate the potential of modulating the prepupae fatty acid profile by enriching diets with available agricultural by-products and to determine the limits of these modulations through the lipid metabolism that conditions them. Two larval nutrition experiments consisting of the enrichment of diets by the progressive incorporation of oilseed cakes and the substitution of water brought to diets by heavy water (D2O) were successively carried out. The diets enrichment has partially balanced the prepupae fatty acid profiles by decreasing their SFA contents and increasing their unsaturated fatty acid (USFA) contents. The D2O diet inclusion led to a mass labelling of the biosynthesized fatty acids by the deuterium atoms incorporation highlighting the limited modulation of fatty acid profiles due to high levels of neosynthesized SFAs, the inability of larvae to synthesize α-linolenic acid (ALA: C18:3n3) targeted by diet enrichments, and the underestimation of the involvement of carbohydrates in larvae fatty acid synthesis. A scheme of the fatty acid synthesis pathways has been established and will require further research to confirm the identification of the key enzymes involved.
The third experimental axis of this work focuses on the inclusion of BSF prepupae meals in the diets of rainbow trout. The objective of this research was to evaluate the impact of this inclusion on the trout growth performance and on the proximal compositions and fatty acid profiles of their fillets. Through the experiments conducted, the entire production cycle of BSF-based trout feed was studied (insect production and processing, feed formulation and extrusion, and fish nutrition trial). These experiments validated the applicability of the BSF rearing methods developed which led to the prepupae mass production with differentiated fatty acid profiles from plant by-products. The inclusion of prepupae meals in trout diets did not modify the proximal composition of trout fillets but modified their fatty acid profile and decreased their growth. These results show the limitations of the prepupae fatty acid profiles obtained by the valorization of plant by-products to maintain the levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the trout fillets. Regarding the decrease in trout growth, the amino acid profile, chitin content and SFA content of BSF-based trout feed have been incriminated and it is reasonable to suppose that these three factors could have influenced trout growth concomitantly. Different ways to improve the prepupae meal nutritional quality through methods of fatty acid profile enrichment and processing of prepupae are suggested.
The research carried out during this study has allowed the exploration of different areas of this new model of feed ingredients production. The results obtained highlight several potentialities and limits of application of this new resource. They constitute a basis of reflection to allow the development of this new sector of activity.
