Trout is the name for a number of species of freshwater fish belonging to the genera Oncorhynchus, Salmo and Salvelinus, all of the subfamily Salmoninae of the family Salmonidae. The word trout is also used as part of the name of some non-salmonid fish such as Cynoscion nebulosus, the spotted seatrout or speckled trout.
Trout are closely related to salmon and char (or charr): species termed salmon and char occur in the same genera as do trout (Oncorhynchus - Pacific salmon and trout, Salmo - Atlantic salmon and various trout, Salvelinus - char and trout).
Most trout such as Lake trout live in freshwater lakes and/or rivers exclusively, while there are others such as the Rainbow trout which may either live out their lives in fresh water, or spend two or three years at sea before returning to fresh water to spawn, a habit more typical of salmon. A rainbow trout that spends time in the ocean is called a steelhead.
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.