Efficacy of Utilization of All-Plant-Based and Commercial Low-Fishmeal Feeds in Two Divergently Selected Strains of Rainbow Trout (Oncorhynchus mykiss): Focus on Growth Performance, Whole-Body Proximate Composition, and Intestinal Microbiome

The present study aimed to investigate the growth performance, whole-body proximate composition, and intestinal microbiome of rainbow trout strains when selected and non-selected for weight gain on all-plant protein diets. A 2x2 factorial design was applied, where a selected (United States) and a non-selected (ITA) rainbow trout strain were fed using either an all-plant protein (PP) or a commercial low-FM diet (C). Diets were fed to five replicates of 20 (PP) or 25 (C) fish for 105 days. At the end of the trial, growth parameters were assessed, and whole fish (15 pools of three fish/diet) and gut samples (six fish/diet) were collected for whole-body proximate composition and gut microbiome analyses, respectively. Independent of the administered diet, the United States strain showed higher survival, final body weight, weight gain, and specific growth rate when compared to the ITA fish (p < 0.001). Furthermore, decreased whole-body ether extract content was identified in the PP-fed United States rainbow trout when compared to the ITA strain fed the same diet (p < 0.001). Gut microbiome analysis revealed the Cetobacterium probiotic-like genus as clearly associated with the United States rainbow trout, along with the up-regulation of the pathway involved in starch and sucrose metabolism. In summary, the overall improvement in growth performance and, to a lesser extent, whole-body proximate composition observed in the selected rainbow trout strain was accompanied by specific, positive modulation of the intestinal microbiome

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 (R) 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