Trout

trout a group of fish in the genus Salmo, which is where salmon also belong. Because trout are a favourite of anglers and are food fishes of high value, they have been introduced from one continent to another on a large scale, and have also become a major subject of fish farming. Trout of one kind or another are now found in every continent except Antarctica, and some confusion over nomenclature has resulted. The three species of true trout, with their common names in English, regions of origin, present distribution, and maximum lengths, are listed below. So are two related fish to which the name trout applies, albeit less correctly.

• Salmo trutta, the brown trout in its freshwater version, sea trout or salmon trout in the migratory, sea-run version (see sea trout). Originally a European species, found at sea in the NE Atlantic, and in fresh waters as far east as the Caspian Sea. Now introduced to N. and S. America, Australasia, S. Africa, E. Africa, India, etc. Up to 140 cm (55"). The sea-run fish, which migrate to sea at any age from 1 to 5 years and spend anything from six months to five years at sea before returning, grow larger than the freshwater fish.

• Oncorhynchus mykiss (until recently known as Salmo gairdneri), rainbow trout, also known in its sea-run version as steelhead. Native to NW America. Introduced to Europe, where it is farmed extensively, now known to be identical with what was formerly called Salmo mykiss in Asia. As with the preceding species, it is the sea-run specimens which are the largest.

• O. clarki clarki, the cutthroat trout. Native to NW America. In the headwaters of some American and Canadian rivers it is represented by a subspecies, O. c. lewisi, the yellowhead cutthroat trout. Up to 1 m (39").