Assessing the impact of full-fat black soldier fly and yellow mealworm meals on the mucosal health and immune responses in Atlantic salmon (Salmo salar).

The aquaculture industry is exploring sustainable alternatives to fishmeal, with insect meals (IMs) emerging as promising options due to their high-quality proteins, lipids, and immunostimulatory compounds, such as chitin. This study evaluated how full-fat IMs affect the histomorphometry of Atlantic salmon (Salmo salar) mucosal tissues (intestine, skin, and gills), their barrier function, and immune gene expression to explore their immunomodulatory potential. Two insect species were tested: black soldier fly, Hermetia illucens (HI), and yellow mealworm, Tenebrio molitor (TM), formulating into five diets: a control diet (CTL) with fish meal 20%, fish oil 13%, rapeseed oil 6%, soy protein 20%, and wheat gluten 14%; and four insect-based diets- LHI (5% low HI), HHI (10% high HI), LTM (15% low TM), and HTM (30% high TM)- where the mentioned other ingredients were replaced. The quadruplication of each dietary group continued for 74 days. Post-trial, samples from the distal mid-intestine (DI), skin, and gills were analysed for structure, barrier function, and immune gene expression related to mucin and antimicrobial peptides (AMPs). Histomorphometric analysis of the intestine, skin, and gills revealed notable changes in tissue structure in the HHI group, as indicated by principal component analysis, suggesting a greater beneficial effect in this group compared with the other groups. In addition, increased mucous cell area, volumetric density, and barrier integrity, were observed in the same group compared with the other groups. Immune cell counts (rodlet-like cells, intraepithelial lymphocyte cells) in the intestine and skin (keratinocyte cells) were higher in the HHI group, along with increased cell proliferation in the DI. While no significant differences were observed between the CTL and TM (both LTM & HTM) dietary groups. PCNA-positive cells appeared higher in the HHI than in the other groups. Lymphocyte and monocyte/macrophage cell size and their percentage were higher in the same dietary group. Mucin, AMP, and other key immune-associated gene expression were elevated across all dietary groups compared to CTL, particularly in the HHI group, which also showed significant upregulation of muc2 and cath1 in the distal intestine; muc5b in the skin; and muc2 in the gills. Pearson's correlation between the condition factor and 'PC1 intestinal health', 'PC1 skin health', and 'PC1 gill health' was positive, whilst 'PC1 skin health' demonstrated a significant (p = 0.034) correlation. Overall, the results indicate that the HHI diet significantly modulated structural health, mucosal barrier function, and immune responses in Atlantic salmon compared with the other diets.