Species
A group of living things that can breed with each other and are distinct from other such groups; about 36,000 fish species are known.
Also: subspecies, population
A species is the basic unit of naming in biology, usually a group of populations that interbreed and are kept apart from others by geography, behaviour or biology. Fish make the idea hard at times: some species hybridise freely, and the cichlids of the African lakes have split into hundreds of species very recently. More than 36,000 valid fish species are recognised, and several hundred new ones are described each year, many from tropical rivers and the deep sea. Some species are split into subspecies or distinct populations, such as separate salmon runs.