Digestibility of defatted insect meals in rainbow trout aquafeeds

The assessment of the apparent digestibility coefficients (ADCs) of defatted insect meals (IMs) is fundamental for the correct inclusion of these innovative protein sources in diets for carnivorous fish species. This study aimed at assessing the ADCs of main nutrients, amino acids and fatty acids (FA) of four defatted IMs (two from yellow mealworms, one from black soldier fly and one from lesser mealworm) in rainbow trout (Oncorhynchus mykiss). Five experimental diets were prepared, a control diet and four diets containing 30% of each tested IM based on the substitution method. Celite® was used as digestibility marker. The trial was performed on 80 rainbow trout (initial body weight: 140±5.6 g) kept into tanks connected to in flow-through open water system. After 10 days of diet adaptation, fish faeces were collected across four consecutive weeks using an automatic Choubert device and then analysed. Significant differences were observed among the IMs for the ADCs of dry matter, crude protein, gross energy, methionine, cysteine, glycine and tyrosine, with the lesser mealworm meal showing the overall lowest digestibility values. Such differences were imputed to both insect species and meal processing techniques. The ADCs of ether extract (in all cases higher than 96%) and main FA (namely lauric, myristic, palmitic, oleic, linoleic and ?-linolenic acids; in all cases higher than 85%) did not significantly vary among the IMs. Based on the results, the tested IMs resulted highly digestible and could be a valuable protein source for the formulation of sustainable rainbow trout feeds.