Digestibility of defatted insect meals for rainbow trout aquafeeds

The apparent digestibility coefficients (ADCs) of the dry matter (DM), crude protein (CP), ether extract (EE), gross energy (GE), amino acids (AA), and the main fatty acids (FA) of four defatted insect meals in rainbow trout (Oncorhynchus mykiss) have been assessed. The tested meals were obtained from two yellow mealworms (Tenebrio molitor - TM1 and TM2), one black soldier fly (Hermetia illucens - HI) and one lesser mealworm (Alphitobius diaperinus - AD). The experimental diets were prepared by means of the substitution method, with each test ingredient included in the diet at 30% on an as fed basis and using Celite® as an inert digestibility marker. Eighty rainbow trout (140±5.6 g) were stocked in tanks connected to an open water system. Faeces were collected over four consecutive weeks using an automatic collection device after feeding the fish. The ADCs of the DM, CP and GE of the insect meals differed significantly, with the AD meal displaying the lowest values. The ADC of the EE in the different meals did not vary. As far as AA digestibility is concerned, the ADC of methionine changed according to the following pattern TM2=HI>TM1>AD, whereas the ADCs of cysteine and tyrosine were significantly lower in the AD meal than in the other meals. The ADCs of the main FAs (C12:0, C14:0, C16:0, C18:1 c9, C18:2 n-6 and C18:3 n-3) were higher than 85% and did not differ significantly in the insect meals. Overall, the tested insect meals resulted to be highly digestible and the differences among them depended on both the insect species and the specific production techniques of the meals. These results provide useful data that may be considered to properly formulate compound diets for rainbow trout using innovative protein sources.