Rearing substrate modulates mealworm (Tenebrio molitor) bioactivity: Implications for rainbow trout (Oncorhynchus mykiss) innate immune and oxidative status.

The yellow mealworm (Tenebrio molitor) is a promising alternative ingredient for aquafeeds. The rearing substrate can influence both the nutritional composition and functional properties of mealworm larvae, potentially altering their effects on fish health. Spent mushroom substrate (SMS), the main waste generated by mushroom agriculture, represents a sustainable alternative substrate for mealworm rearing. This study aimed to evaluate the impact of rearing substrate on the bioactive properties of mealworm meal by comparing the in vitro bioactivity of commercial mealworm (CC-MM) with mealworm partially reared on SMS (SMS-MM), and their functional potential for rainbow trout (Oncorhynchus mykiss) aquafeeds. Fish were fed a control diet (CTRL) and four experimental diets containing 12.5% or 25% of CC-MM or SMS-MM, replacing 50% or 100% of fishmeal (CC12.5, CC25, SMS12.5, and SMS25). In vitro assays showed that extracts from both mealworm meals induced a similar anti-inflammatory potential. However, differed in antioxidant capacities and distinctively modulated the growth of both probiotic and pathogenic bacterial strains. Rainbow trout juveniles fed CC25 showed higher haemoglobin concentrations than the CTRL group. The 25% inclusion of both mealworm meals reduced plasma lactate levels and alternative complement pathway activity, while increasing cholesterol levels. The 12.5% mealworm meal diets, especially the CC-MM, increased hepatic catalase activity. SMS-MM based diets increased intestinal glutathione S-transferase activity, while the CTRL diet led to the highest intestinal glutathione reductase activity. Overall, despite differences in in vitro bioactivity, both CC-MM and SMS-MM induced similar baseline innate immune and oxidative responses in rainbow trout juveniles, with no indication of adverse physiological effects.