Evaluation of black soldier fly (Hermetia illucens) larvae oil as a potential lipid source for largemouth bass (Micropterus salmoides): Growth performance, myofiber development and glucolipid metabolism

Black soldier fly larvae oil (BSFLO) is considered as a by-product of the production process of defatted black soldier fly larvae meal at present, and may show the potential to be used as a lipid source for aquafeeds. In this study, a total of 300 largemouth bass (initial body weight 7.95 g) were fed diets containing different levels of BSFLO for 56 d to evaluate its effects on the growth performance, fatty acid composition, antioxidant capacity and glucolipid metabolism. Five isonitrogenous and isolipidic diets were designed with fish oil supplemented feed (FO) as the control diet and then feeds with BSFLO replacing 12.5 %, 25 %, 50 % and 100 % of the fish oil (BSFLO12.5, BSFLO25, BSFLO50, BSFLO100) as the treatment diets. No significant changes in weight gain and specific growth rate were observed between groups, but BSFLO100 showed a significant change in feed conversion ratio, higher than the control group (P < 0.05). The muscle and liver C12:0 and ∑n-6 PUFA content increased with the addition of BSFLO, while C20:5n-3, C22:6n-3 and ∑n-3 PUFA content gradually decreased, and significant differences were observed between FO and BSFLO100 (P < 0.05). Expression of hepatic Δ6fad and FABP genes was lower in the FO group and the highest in BSFLO50 and BSFLO100 (P < 0.05). In terms of myofiber development, Dietary BSFLO led to e a progressive increase in myofiber diameter and area, while the expression of smad-2 gene in muscle was significantly reduced in BSFLO25 and BSFLO50 (P < 0.05). Besides, the serum glucose levels were significantly lower in BSFO12.5, BSFLO50 and BSFLO100, and dietary 8 % BSFLO (BSFLO100) resulted in a decrease in hepatic glycogen content from 94 mg g⁻¹ to 71.78 mg g⁻¹, along with the expression of glycolysis-related genes (GK, PK and PFK) and enzyme (PK and PFK) activities (P < 0.05), while the expression of gluconeogenesis-related genes (G6Pase) was significantly increased (P < 0.05). Dietary 4 % BSFLO (BSFLO50) increased muscle and liver T-SOD activity and simultaneously reduced muscle MDA content (P < 0.05), which has the best antioxidant properties. One-variable linear regression analysis revealed that various parameters of feed utilization, glucose metabolism, and muscle development in fish were significantly and linearly affected by dietary BSFLO levels (P < 0.05). In summary, if only optimal glucose lowering is considered, 8 % BSFLO is recommended. However, if feed utilization efficiency is taken into account, it is recommended that dietary BSFLO be added at no more than 4 %.