Abstract
This study explores the use of black soldier fly (Hermetia illucens) larvae meal (BSFL) as a ‘functional feed ingredient’ in aquafeed formulations, aiming to mitigate the adverse effects of high soybean meal (SBM) inclusion in the diet of rainbow trout. Two forms of BSFL, whole (WB) and defatted (DB), were supplemented at different levels (2.5 % and 5 %) in a high SBM diet. Six isonitrogenous (crude protein 50 %) and isolipidic (20 %) diets include fishmeal-based diet (FM), high SBM (SBM), WB at 2.5 and 5 %, and DB at 2.5 and 5 %. Fish (5.2 g) were randomly distributed in twenty-four tanks (30 fish/tank), maintaining four replications for each diet, and fed six experimental diets at apparent satiation for 10 weeks. The research investigated the impact of BSFL supplementation on growth, feed utilization, proximate and nutrient (amino acids, fatty acids, and minerals) compositions, immune-related gene expression, and intestinal histopathology. The results indicated that a 5 % inclusion of whole BSFL in the diet outperforms all other dietary treatments regarding growth. Fatty acid composition analysis shows higher levels of lauric acid in BSFL-supplemented groups, notably in WB at 5 %. Mineral composition analysis suggests that BSFL supplementation, especially WB5, can restore levels of sulfur and zinc affected by high SBM inclusion. Histopathological analysis of the intestine demonstrates that whole BSFL supplementation, particularly at the 5 % level, reduces inflammation and mucous cell populations, enhancing intestinal health. Gene expression analysis indicates a higher level of the complement factor 5 (C5) gene in the highest whole BSFL-supplemented group, suggesting an increase in the activity of the complement system, while the expression of IL8 and TNFα did not show any difference. Overall, this study demonstrates the potential of BSFL as a functional feed ingredient to enhance rainbow trout performance and gut health in high SBM diets.