Plant or animal species that quantitatively dominate an aquatic ecosystem and play a critical role in its functioning.

A species whose individuals are at least 50% of those of all other species inhabiting a given area, so that it influences neighboring species and heads the food chain.

Species that dominate in numbers are dominant. The dominant species dominate the community and constitute its species nucleus. However, not all dominant species have the same effect on the biocenosis. Among them, there are some that create the environment for the whole community and without which the existence of most other species is impossible. Removal of such a species from the biocoenosis usually causes changes in the environment, in particular in the microclimate of the biotope. Rare and small species are also important for the life of a biocenosis: they create its species diversity, increase the number of biocenotic connections, serve as a reserve for filling and replacing the dominants, i.e. they give the biocenosis stability in different conditions.

Examples of dominant fish are: Cichlids, Moon Goop, Mozambique Tilapia.