Hermetia illucens meal inclusion in low-fishmeal diets for rainbow trout (Oncorhynchus mykiss): Effects on the growth performance, nutrient digestibility coefficients, selected gut health traits, and health status indices

The effects of including Hermetia illucens (HI) meal in rainbow trout diets have been widely characterized, but data related to its utilization in low-fishmeal (FM) diets are quite scarce. The current research investigated the impact of HI meal inclusion in low-FM diets for rainbow trout by assessing fish growth performance, nutrient digestibility, histomorphological traits of intestine and main organs, and gut microbiota. In the 133-days growth trial, 600 rainbow trout were randomly distributed to 4 dietary treatments (3 replicate tanks/treatment, 50 fish/tank): a low-FM diet as control (HI0), and three experimental diets including 80, 160 and 320 g/kg of HI meal as fed as replacement of 25%, 50% and 100% of FM (HI25, HI50 and HI100, respectively). At the end of the trial, growth parameters, condition factor and somatic indices were assessed, and gut, stomach, liver and spleen samples (12 fish/diet) were collected for histomorphological analyses. Feed and posterior intestine content were also sampled to characterize their microbiota. In the digestibility trial, 216 fish (3 tanks/treatment, 18 fish/tank) allowed evaluating the apparent digestibility coefficients (ADC) of the dietary nutrients. Unaffected growth performance, condition factor, somatic indices, nutrient digestibility, and histomorphological features were observed in the HI-fed rainbow trout (P > 0.05). Increasing percentages of HI meal in the feeds determined a progressive increase in the relative abundance of Firmicutes and Actinobacteria phyla, and Staphylococcus, Enterococcus, Oceanobacillus and Actinomyces genera, whereas Proteobacteria - as well as Lactobacillus and Listeria - displayed a gradual reduction. The Chao1 index of the fish gut microbiota increased when including HI meal, while the Shannon index displayed the opposite trend (P < 0.05). The HI25 and HI50 fish showed enrichment of Actinobacteria, but a reduction in Bacteroidetes (False Discovery Rate [FDR] < 0.05). Furthermore, Bacillus, Actinomyces, Staphylococcus, Enterococcus, and Oceanobacillus displayed higher relative abundance in the HI-fed fish than the other groups (FDR < 0.05). On the contrary, HI meal utilization was accompanied by a decrease in Campylobacter and Listeria, as well as Lactobacillus, Clostridium, Pediococcus, Leuconostoc, unclassified members (U.m.) of Peptostreptococceae, Vagococcus, Lactococcus, and Weissella (FDR < 0.05). In conclusion, HI meal can be used in low-FM diets for rainbow trout up to high inclusion levels (32%) without negatively affecting the fish nutrient digestibility, growth performance, somatic indices and histomorphological features. A positive shift of the gut microbiota towards the selection of short-chain fatty acids (SCFAs)-producing bacteria and reduction of foodborne disease-causing pathogens was also observed.