Salmo trutta morpha trutta
Salmo trutta morpha fario
Salmo trutta morpha lacustris
The brown trout (Salmo trutta) is an originally European species of salmonid fish. It includes both purely freshwater populations, referred to Salmo trutta morpha fario and S. trutta morpha lacustris, and anadromous forms known as the sea trout, S. trutta morpha trutta. The latter migrates to the oceans for much of its life and returns to freshwater only to spawn. Sea trout in the UK and Ireland have many regional names, including sewin (Wales), finnock (Scotland), peal (West Country), mort (North West England) and white trout (Ireland).
The lacustrine morph of brown trout is most usually potamodromous, migrating from lakes into rivers or streams to spawn, although evidence indicates stocks spawn on wind-swept shorelines of lakes. S. trutta morpha fario forms stream-resident populations, typically in alpine streams, but sometimes in larger rivers. Anadromous and nonanadromous morphs coexisting in the same river appear not to be genetically distinct. What determines whether or not they migrate remains unknown.
The brown trout is normally considered to be native to Europe, but the natural distribution of the migratory forms may be, in fact, circumpolar. Landlocked populations also occur far from the oceans, for example in Greece and Estonia.