Abstract
The sustainable expansion of salmonid aquaculture necessitates the identification of environmentally viable protein alternatives to traditional fishmeal. This study evaluated the effects of dietary inclusion of defatted black soldier fly (Hermetia illucens) larvae meal (BSFLM) on the growth performance, systemic and mucosal innate immune responses, and distal intestinal microbiota in juvenile rainbow trout (Oncorhynchus mykiss). Four experimental isoproteic (46% crude protein) and isolipidic (18% crude lipid) diets were formulated to replace fishmeal at 0% (BSFL0, control), 10% (BSFL10), 20% (BSFL20), and 30% (BSFL30) on a dry matter basis. A total of 360 juvenile trout (initial body weight: 15.42 ± 0.35 g) were distributed into 12 flow-through tanks across four triplicated treatments and fed to apparent satiation for 8 weeks. Growth parameters, including specific growth rate and feed conversion ratio, remained statistically comparable up to 20% inclusion, while fish fed BSFL20 exhibited significantly enhanced serum lysozyme activity, alternative complement activity (ACH50), and respiratory burst activity compared to controls (P < 0.05). High-throughput 16S rRNA gene sequencing revealed that BSFLM dietary inclusion promoted microbial richness and modulated taxonomic composition by enriching beneficial Firmicutes, notably lactic acid bacteria (Lactobacillus and Enterococcus spp.), while reducing the relative abundance of opportunistic Vibrio and Aeromonas taxa. These findings demonstrate that dietary BSFLM at up to 20% replacement provides a biologically functional alternative protein source that sustains growth performance while bolstering non-specific immunity and gut microbial homeostasis in juvenile rainbow trout.
