Dietary inclusion of frass from black soldier fly (Hermetia illucens) larvae modulates gut microbiota without compromising the growth performance and health status of catfish (Ictalurus punctatus) juveniles.

The present study evaluated the potential of BSFL frass (BF), a by-product composed of larval excretions, undigested substrate, and exoskeletons, as a dietary ingredient for juvenile channel catfish (Ictalurus punctatus). The experiment included a control diet and three test diets with BSFL frass 1.25% (BF1.25%), 2.5% (BF2.5%), and 5.0% (BF5.0%). All diets were isonitrogenous (37% crude protein), isolipidic (6% crude lipid), and isoenergetic (15 MJ gross energy/kg). Six hundred channel catfish juveniles (~ 1.85 g) were equally distributed to 20 aquaria (30 fish/tank) in a recirculating aquaculture system and fed the experimental diets for 60 days. Dietary inclusion of BSFL frass did not significantly affect growth performance, viscerosomatic index, intraperitoneal fat, and hematological parameters. However, fish-fed BF5.0%diets showed a higher hepatosomatic index, suggesting effects on liver metabolism. No significant differences were observed in intestinal histomorphology. Fish-fed BF1.25% exhibited a higher relative abundance of Lactococcus spp. (~ 47%), along with increases in Streptococcaceae, Lactobacillales, and Weissella, accompanied by reduced Cetobacterium spp. and Plesiomonas spp. BSFL frass did not improve performance but significantly altered the catfish gut microbiota. This finding warrants further research to better understand the components of the BSFL frass that can serve as a substrate for commensal bacteria and which bacterial species are benefiting from this supplementation. Additionally, evaluating the feasibility of BSFL frass as an unconventional ingredient in aquafeeds could provide insights into its potential functional benefits and practical application in sustainable fish nutrition.