The brown trout (Salmo trutta morpha fario and S. trutta morpha lacustris) and the sea trout (S. trutta morpha trutta) are fish of the same species. They are distinguished chiefly by the fact that the brown trout is largely a freshwater fish, while the sea trout shows anadromous reproduction, migrating to the oceans for much of its life and returning to freshwater only to spawn.

The lacustrine morph of brown trout is most usually potamodromous, migrating from lakes into rivers or streams to spawn, although there is some evidence of stocks that spawn on wind-swept shorelines of lakes. S. trutta morpha fario form stream-resident populations, typically in alpine streams but sometimes in larger rivers. There is evidence that anadromous and non-anadromous morphs coexisting in the same river can be genetically identical.[2] In common usage, the name "brown trout" is often applied indiscriminately to the various morphs.