Insect Larvae Meal as a Complementary Functional Ingredient in High Soybean Meal-Based Diets Improve the Health of Rainbow Trout (Oncorhynchus mykiss).

This study explores the integration of black soldier fly larvae (BSFL) as a complementary functional ingredient to increase the soybean meal (SBM) inclusion in rainbow trout (Oncorhynchus mykiss) diets. Six experimental diets were formulated including a control diet as fishmeal (FM), an SBM-based diet, and 2.5% and 5% whole-body (WB) or defatted (DB) BSFL-supplemented SBM-based diets. Results revealed that BSFL inclusion positively influences gut health, immune response and survival rates following challenge with Flavobacterium psychrophilum. Dietary lauric acid content significantly impacted whole-body lauric acid levels in a dose-dependent manner, with BSFL diets showing higher levels than FM and SBM diets. Fish fed diets with BSFL exhibited enhanced survivability against F. psychrophilum infection compared to SBM-fed fish, with the highest survival rates observed in the WB5 (5% whole-body BSFL) group. Histological analysis demonstrated improved intestinal morphology in BSFL-fed fish, particularly evident in the absence of pathogenic enteritis. Gene expression analysis revealed upregulated proinflammatory markers (IL-8, TNF-α, C5) in BSFL-fed fish post-challenge, indicating an enhanced immune response. These findings indicate the potential of BSFL as a functional feed ingredient to complement high SBM inclusion in rainbow trout with improved health and performance. By strategically reducing inclusion levels and repositioning BSFL as a functional feed ingredient instead of fishmeal replacer in aquafeed formulations, one can mitigate economic concerns while maximising the benefits of this sustainable alternative.