Comparative effect of dietary full-fat or defatted Hermetia illucens larvae on growth and fatty acid profiles of juvenile Oncorhynchus mykiss

Abstract Although black soldier fly (BSF) larvae meal (BSFL) has been a promising alternative ingredient, limited information is available on full-fat BSF larvae meal (FBSF), which may offer cost-savings in aquafeed. This study evaluated the effects of FBSF compared with defatted BSF larvae meal (DBSF) as fishmeal substitutes on the growth performance and biochemical responses of juvenile rainbow trout. A control diet contained fishmeal at 360 g/kg diet, and four experimental diets replaced fishmeal with FBSF or DBSF at 36 or 108 g/kg diet (designated as FBSF10, FBSF30, DBSF10, and DBSF30, respectively). Following an 8-week feeding trial, only FBSF30 resulted in reduced growth and an increased feed conversion ratio compared to the control. Proximate compositions of carcass and viscera were unaffected. Tissue fatty acid profiles mirrored dietary composition; polyunsaturated fatty acids (including C20:5n-3 and C22:6n-3) decreased with higher BSFL inclusion, whereas saturated fatty acids increased, with C12:0 levels significantly elevated in the FBSF30 group. Principal component analysis associated FBSF30 strongly with saturated fatty acid vectors (C12:0 and C14:0), demonstrating that high FBSF inclusion markedly shifts tissue lipid composition toward saturation. Serum cortisol, glutathione peroxidase, superoxide dismutase, lysozyme, and immunoglobulin M levels were unaffected across treatments. Overall, up to 30% DBSF and 10% FBSF can replace fishmeal in juvenile rainbow trout diets without compromising growth or physiological health. While FBSF offers economic advantages, inclusion levels must be optimized to prevent growth impairment and lipid alterations.