Characterisation of the intestinal microbial communities of rainbow trout (Oncorhynchus mykiss) fed with Hermetia illucens (black soldier fly) partially defatted larva meal as partial dietary protein source

Scientific research has examined the possibility of replacing fishmeal with alternative protein sources in feed for fish. The literature indicates that insects are an eco-friendly nutrient-rich alternative to fishmeal. The purpose of this study was to investigate the effects of including insects in a diet for rainbow trout (Oncorhynchus mykiss), by analysing organosomatic parameters, fillet yield and intestinal bacterial communities. Three experimental diets were formulated: a control diet with fishmeal as main protein source, and diets Hi25 and Hi50 where 25% and 50% of fishmeal, respectively, was replaced with partially defatted meal of larvae of Hermetia illucens (Hi; Diptera: Stratiomydae). At the end of the trial, organosomatic parameters and fillet yield were recorded. To profile the complex intestinal mucosa- and digesta-associated bacterial communities (MAB and DAB), denaturing gradient gel electrophoresis was performed on bacterial DNA extracted from intestinal mucosa and digesta, followed by sequencing of selected bands. Rainbow trout fed the alternative diets showed the same organosomatic indices and fillet yields as the control group, but increased bacterial community biodiversity, structure and composition, with ANOSIM p < .05 and p < .01 for MAB and DAB, respectively. The sequencing highlighted a clear prevalence of γ-Proteobacteria in all samples, though α- and β-Proteobacteria and Actinobacteria were also present in MAB of insect-fed fish; DAB of insect-fed fish showed a clear increase in the Firmicutes phylum compared to the control group. The results suggest that H. illucens partially defatted larva meal is a valid alternative protein source and can replace up to 50% of fishmeal in rainbow trout feed without impairing organosomatic indices nor fillet yield. The microbiological assays revealed that the intestinal bacterial communities were sensitive to dietary changes, showing modified community structure and increased biodiversity in the Hi-fed groups. We discuss the effects that modified bacterial communities could have on fish biology. There is a good possibility of further studies on the functional role of bacteria.