Black soldier fly larvae (Hermetia illucens, BSFL) can reduce waste by 50–60% and generate raw feedstuffs such as BSFL meal, oil, or frass. BSFL frass contains relatively high protein and lipid concentrations (21.6% and 6%, respectively), and is typically considered a by-product by the BSFL industry. Frass contains chitin, a polymer present in the exoskeleton of insects that presents antimicrobial properties and can improve intestinal health. Thus, the present study evaluated the potential of BSFL frass as an ingredient in the diet of channel catfish (Ictalurus punctatus) juveniles.
Four experimental diets with 0, 1.5%, 2.5%, and 5.0% inclusion levels of BSFL frass were manufactured to meet the nutrient requirements of channel catfish. Diets were formulated to be isonitrogenous, isolipidic, and isoenergetic, containing 37% crude protein, 6% crude lipid, and 18.2 MJ crude energy /kg. Six hundred channel catfish juveniles (average initial weight of ~1.8 g) were equally distributed into twenty experimental aquaria (110-L, 30 fish/tank, n=5), operating as a recirculating system. Fish received a ration according to the tank biomass, and the amount of feed was adjusted biweekly. After feeding for 63 days, fish were weighed and three fish per tank were anesthetized for blood collection, then euthanized to collect digesta samples for intestinal microbiota. Dietary BSFL frass increased fish liver weight and fish-fed diets containing 5% BSFL frass had a higher hepatosomatic index. No differences were observed for weight gain, feed efficiency, survival, feed intake, viscerosomatic index, intraperitoneal fat, hematocrit, hemoglobin, or erythrocyte count. The posterior intestine of channel catfish fed the control diet was mainly composed of the genera Cetobacterium (67%), Lactococcus (16%), and Plesiomonas (15%) (Figure 1A). Higher relative abundances of some gram-positive bacteria were observed in channel catfish fed the containing frass at 1.25% of diet (Figure 1B). Gut bacterial community structure and diversity were different among dietary treatments. The findings from this study suggest that BSFL frass can modulate the intestinal microbiota, but still be included up to 5% in catfish diets without compromising growth performance or overall health.