Black soldier fly (BSF, Hermetia illucens) and yellow mealworm (YM, Tenebrio molitor) larvae meals have been tested individually as fishmeal (FM) substitutes in fish diets, due to their potential for sustainable production, high nutritional value, and composition in bioactive compounds with immunomodulatory or antimicrobial properties. However, combining both insect meals in a mixture (IM) could provide feed companies with increased flexibility for formulation, ensuring a well-balanced dietary amino acid profile while allowing for a strategic use of FM. This study assessed the potential of combining BSF and YM larvae meals as a protein source and functional ingredient in practical diets for European seabass (Dicentrarchus labrax) juveniles.
Four isoproteic and isolipidic diets were formulated. The control (CTRL) followed commercial trends for seabass diets, incorporating plant proteins (55.3%), processed animal proteins (9%), and moderate levels of FM (15%). The remaining diets included a 50:50 mixture of defatted BSF and YM larvae meals, replacing 3% (IM3), 25% (IM25), or 50% (IM50) of FM in CTRL. Amino acid supplementation was not necessary, and equivalent levels of EPA and DHA were maintained across diets by adding extra 0.4 and 1.1% salmon oil to IM25 and IM50, respectively, at the expense of rapeseed oil. Diets were tested in triplicate in a 75-day feeding trial conducted in a RAS with water at 22 ºC and 35 ppt salinity, under a 12 h light/12 h dark photoperiod.
The diets containing IM promoted similar growth, feed efficiency, and nutrient digestibility to the CTRL diet, while also reducing phosphorus fecal losses. Moreover, intestinal integrity was maintained in all fish, but those fed IM50 displayed the longest villi. All diets ensured a high muscle nutritional value, with 360-400 mg of EPA+DHA per 100 g of fillet. The muscle fatty acid profile of fish fed IM closely resembled that of those fed CTRL. However, the IM25 and IM50 diets promoted a higher deposition of lauric acid (2.4 and 3.5 mg per 100 g, respectively) compared to both the CTRL and IM3 diets (0.9 mg per 100 g). Given that lauric acid has been suggested to possess antioxidant properties, its potential impact on muscle lipid peroxidation and antioxidant capacity is presently being explored. IM50 also promoted a higher hepatic response of the glutathione antioxidant system after a salinity stress challenge. Overall, our findings demonstrate the potential of utilizing a combination of BSF and YM to partially substitute FM in practical diets for European seabass. Future work will evaluate fish health and resilience upon stress-inducing conditions.
Acknowledgments
This work has received funding from the InFishMix project (PT-INNOVATION-0094), funded by Iceland, Liechtenstein and Norway through the EEA and Norway grants, and Rafaela Costa is supported by Fundação para a Ciência e Tecnologia (FCT), I.P., through a Ph.D. grant (2023.01899.BD).