Effects of full-fat black soldier fly larvae and yellow mealworm meals on blood composition and liver health in Atlantic salmon (Salmo salar)

Abstract

Insects, such as black soldier fly larvae (BSFL) and yellow mealworms (MW), are rich in fats and can potentially serve as sustainable sources of lipids in aquafeeds. This study aimed to investigate the effects of full-fat insect meal (IM) on blood serum composition, erythrocyte structure, liver histology, liver immunohistochemistry, and gene expression in post-smolt Atlantic salmon (Salmo salar).

Five experimental diets were prepared based on the nutritional requirements of the species. The control diet (CO) contained 20 % fish meal (FM), 13 % fish oil (FO), 6 % rapeseed oil (RO), 20 % soy protein concentrate (SPC), and 14 % wheat gluten (WG). The remaining four diets were formulated using full-fat BSFL and MW with varying inclusion rates: LB (5 % low BSFL), HB (10 % high BSFL), LM (15 % low MW), and HM (30 % high MW), substituting RO, SPC and WG in the control diet. Quadruplicated groups of Atlantic salmon (143 ± 12.8 g) were fed with the diets for 74 days. After the feeding trial, blood samples were collected to evaluate enzymatic activity, erythrocyte structure, and differential leucocyte counts, whilst adipose tissue and liver samples were collected to assess lipid metabolism and gene expression.

Higher frequencies of erythrocyte nuclear and cellular abnormalities were noted in the HM dietary group. A significant increase in cholesterol, triglyceride, and LDL levels was found in the same group, while HDL and total protein levels were significantly elevated in the HB dietary group. Total glucose levels did not differ among the dietary groups. Fish fed full-fat MW diets had a notable increase in vacuolation and neutral lipid droplets in the liver. Major histocompatibility complex II (MHC II) positive cells in the liver, such as erythrocytes, monocytes, dendritic cells, and macrophages, increased in the HB diet. In contrast, only erythrocytes increased significantly when compared to the CO diet. The adipose cell size index significantly increased in the HM diet. The study also revealed that immune-related genes such as tlr22, tnfα, and nrf2 were significantly upregulated in the liver of fish fed the HB diet. In contrast, genes associated with lipid metabolism, including pparα, cpt1α, and fas, showed an upward trend in the HM diets, although only fas was significantly upregulated. A significant correlation was established between neutral lipid droplets and liver vacuolation. Overall, the results suggest that full-fat BSFL diets demonstrated superior lipid utilization compared to the full-fat MW diet, which appeared to link lipid droplet accumulation to fatty liver development. This excessive lipid accumulation in the liver could potentially disrupt lipid metabolism and compromise the overall health of Atlantic salmon.