A community of fish, a part of the biohydrocenosis, interconnected trophically and informationally into a single whole, which functions as a relatively separate subsystem and undergoes regular changes in accordance with changes in the biohydrocenosis.

A set of fish as part of a single community of fish occupying a certain area of the reservoir and united by various (mainly food) relationships. The name of the ichthyocenosis is given for the predominant fish or fish that make up more than 50% of the total ichthyomass. In perch-roach lakes, the ichthyomass of perch is higher than the ichthyomass of roach, and in roach-perch lakes — on the contrary. In all types of ichthyocenoses with a double name, the name of the fish species whose ichthyomass occupies the first place is in the first place.

An analysis of the ichthyocenosis of any reservoir where more than three species of fish occur reveals two structural parts. One of them is represented by species that are invariably and necessarily present in all ichthyocenoses of this territory. The other part does not have a definite composition and creates differences in communities, their diversity. Thus, the ichthyocenosis can be divided into a core consisting of species that are necessarily present in a given region, and a marking part, which includes species that create a community identity.