Effects of black soldier fly (Hermetia illucens) pre-pupae meal on the growth, stress, and immune responses of juvenile rainbow trout (Oncorhynchus mykiss) reared at different stocking densities

Abstract

The present study investigated the effects of black soldier fly (Hermetia illucens) pre-pupae meal (BM) as a feed ingredient on the growth performance and health status (stress and immune response) of rainbow trout (Oncorhynchus mykiss) reared at different stocking densities. BM was used to replace 5 % of soybean meal in the control diet (CON). Juvenile rainbow trout (4.9 ± 0.2 g) were acclimated and placed in experimental tanks, at stocking densities of 4.9 (low density, LD) and 9.8 (high density, HD) kg/m3. Trout were assigned to four treatments (CON/LD, CON/HD, BM/LD, and BM/HD) in triplicates and reared for 6 weeks. After the feeding trial, fish reared in low-density tanks showed significantly higher weight gain regardless of diet (P < 0.05). In HD groups (CON/HD and BM/HD), however, fish fed the BM diet showed a significantly higher weight gain than those fed the CON diet. The lauric acid (12:0) content of BM groups (BM/LD and BM/HD), in the whole-body, was closely correlated with diets and significantly higher than that of the CON groups (P < 0.05). Fish from the BM/LD group showed significantly higher expression of interleukin 1β (IL-1β), interleukin 8 (IL-8), and immunoglobulin M (IgM) genes compared to the CON groups (P < 0.05). Although no statistical difference was detected, the overall response patterns of liver tissue in antioxidant and immune parameters, including superoxide dismutase (SOD), lysozyme activity (Lyz), IL-1β, and IgM, followed the expression of immune-related genes in response to the different diets and stocking density. In a challenge test with Aeromonas salmonicida for 10 days, fish fed the BM diet showed a significantly higher cumulative survival rate than those fed the CON diet (P < 0.05). These results suggest that BM could be a beneficial feed ingredient for juvenile rainbow trout, but that high stocking densities can negatively influence the growth and immune responses.