Fishmeal Dietary Replacement Up to 50%: A Comparative Study of Two Insect Meals for Rainbow Trout (Oncorhynchus mykiss)

Simple Summary The reduction of dependence on fishmeal as a main protein source for aquafeeds remains a big problem in reaching sustainable aquaculture. Several alternatives to this ingredient are being tested and developed, insects being one of the most promising. The present study included two different insect species (black soldier fly, Hermetia illucens, and yellow mealworm, Tenebrio molitor) in the formulation of diets for rainbow trout (Oncorhynchus mykiss) against one typical fishmeal-based diet. Different parameters related to both the efficiency of these diets and their physiological repercussions were analysed. Yellow mealworm proved to be the best alternative for the growth and nutrition of rainbow trout, possibly due to some changes described in protein utilization and intestine histology, while other parameters revealed the possible usage of insect meals as functional ingredients due to their repercussions on preventing tissue damage. The demand of optimal protein for human consumption is growing. The Food and Agriculture Organization (FAO) has highlighted aquaculture as one of the most promising alternatives for this protein supply gap due to the high efficiency of fish growth. However, aquaculture has been facing its own sustainability problem, because its high demand for protein has been traditionally satisfied with the use of fishmeal (FM) as the main source. Some of the most promising and sustainable protein substitutes for FM come from insects. The present manuscript provides insight into an experiment carried out on rainbow trout (Oncorhynchus mykiss) with a 50% replacement of FM with different larvae insect meals: Hermetia illucens (HI), and Tenebrio molitor (TM). TM showed better results for growth, protein utilization and more active digestive function, supported by intestinal histological changes. Liver histology and intermediary metabolism did not show relevant changes between insect meals, while other parameters such as antioxidant enzyme activities and tissue damage indicators showed the potential of insect meals as functional ingredients.