From the Latin populatio - population.
A population is a group of individuals of the same species occupying a certain territory for a long time, freely interbreeding with each other and forming their own ecological niche. A population is understood as an aggregate of fish of the same species, but of different age, weight, sex or condition, united by some trait or set of traits.
A population can be considered in isolation or as part of a larger system of the ecological system of a water body. Such a system as elements includes individual populations of fish, aquatic animals, plants, etc. Various relationships may exist between the elements of a biocenosis as a system, such as competition, predator and prey.