A 71-day-long experiment was conducted to evaluate the inclusion of 20% Hermetia illucens (HI) meal; Tenebrio molitor (TM) meal; Gryllodes sigillatus (GS) meal; and Blatta lateralis (BL) meal in comparison to a control diet without any insect-based materials that used fish meal as the main source of protein. A total of 1950 rainbow trout juveniles (53.39 ± 3.74 g) were used. The formulated diets were isonitrogenous (45%) and isoenergetic (10 MJ kg−1). The inclusion of a full-fat insect meal did not affect the survival rate during the experimental period. The growth performance was significantly improved in the BL and TM treatments, while in the HI treatment was not affected. However, the GS treatment had a negative effect on the growth performance. The villus height decreased in the TM and GS treatment groups and increased in the BL diet group. The total number of bacteria increased in all insect meal diet groups. The results of the experiment show that B. lateralis, T. molitor and H. illucens full-fat meals can be used as a partial fish meal replacement without negative effects on survival or growth performance parameters. Moreover, full-fat insect meals may be considered as a protein source and a functional feed component that may positively affect the histomorphological structure of the fish gastrointestinal tract and stimulate the expansion of beneficial bacterial populations in the gut.
