CHITIN IN BLACK SOLDIER FLY MEAL-BASED AQUAFEEDS: NUTRITIONAL OPPORTUNITIES AND PHYSIOLOGICAL CHALLENGES

Background and knowledge gap: Global capture fisheries have declined since the late 1980s due to overfishing, whereas aquaculture has become the primary source of fish for human consumption. Production efficiency in aquaculture remains critically dependent on feeds containing fishmeal (FM) as the principal protein source. However, rising fishmeal prices and demand have intensified the need to reduce the use of fishmeal in aquafeed formulation and identify more sustainable and cost-effective alternatives. Insect-derived ingredients, particularly black soldier fly (Hermetia illucens) larvae (BSFL) meal, have emerged as promising fishmeal substitutes due to their high protein content, favourable amino acid profile, and low environmental footprint; however, high dietary inclusion of BSFL is constrained by chitin content, a structurally resistant polysaccharide that limits nutrient digestibility and may hinder fish growth (see Chapter 1). The specific mechanisms to mitigate chitin’s anti-nutritional effects in insect-based aquafeeds remain underexplored. Objective: This thesis aimed to develop and evaluate innovative strategies to mitigate the chitin-induced anti-nutritional effects in insect-based aquafeeds, with emphasis on improving nutrient utilization and physiological responses in rainbow trout (Oncorhynchus mykiss). Methods: A multidisciplinary approach integrating in vitro and in vivo methodologies was employed. In Chapter 2 (in vitro study), the chitinolytic potential of selected Bacillus species was assessed using BSFL-derived colloidal chitin substrates, along with probiotic characterization (gastrointestinal tolerance and antimicrobial activity). The results demonstrated that Bacillus species, particularly B. licheniformis (BL) and B. megaterium, exhibited significant chitinolytic activity. Based on these findings, BL, exhibiting both chitinolytic and probiotic properties, was selected for subsequent feeding trials (in vivo study). In Chapters 3 and 4 (in vivo study), juvenile rainbow trout were fed experimental diets replacing 0%, 15%, or 30% fishmeal with BSFL meal, with or without BL supplementation, over 60 days. Growth performance, feed efficiency, immune responses, oxidative status, liver histology, intestinal morphology, and inflammatory markers were comprehensively evaluated. Key findings: Fishmeal replacement with 15% BSFL combined with BL supplementation significantly enhanced growth performance, nutrient utilization, gut integrity, immune responses, and antioxidant capacity while modulating inflammatory pathways. On the contrary, high BSFL inclusion (30%) without probiotic supplementation negatively affected growth, induced intestinal inflammation, and impaired tissue integrity, confirming chitin content as the limiting factor. Conclusion and significance: This thesis demonstrated that the strategic dietary administration of chitinolytic probiotics can effectively enhance chitin degradation and the nutritional value of BSFL-based aquafeeds, enabling partial fishmeal replacement without compromising fish health or performance (see Chapter 5). These findings provide a novel, sustainable framework for aquafeed formulation, establishing chitinolytic probiotics as a key intervention to overcome physiological bottlenecks associated with insect-based aquafeeds and to advance sustainable aquaculture production.