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Salmonidae (salmon, trout; superorder Protacanthopterygii, order Salmoniformes) A fairly large family of freshwater fish (although a number of species spend several years in the sea before returning to the river to spawn) that have an elongate body, with the soft-rayed, single, dorsal and pelvic fins placed about midway along the trunk, a small adipose fin, and a slightly forked tail fin. Most are medium to large fish, inhabiting the cold to temperate freshwater systems of the northern hemisphere. Many species are of considerable commercial importance and a number have been introduced into other countries in order to improve the local fisheries. There are about 70 species.