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.
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