The Brown Trout is one of the key game fish, along with Salmon, in Irish waters, and adds a huge value to the economy through tourism (e.g. accommodation, transport, equipment etc. for anglers who come to Ireland from across the world to fish.) It is a close relative of the Salmon, but unlike the Salmon, most Brown Trout spend their entire lives in the fresh water of lakes and rivers. However in some places, Brown Trout become migratory, and will follow the same breeding cycle as Salmon: young fish heading out to sea as smolts to feed and grow, before returning to the river, usually the following autumn, to breed.

Brown Trout which migrate to sea are known as Sea Trout, but they are biologically the same fish. Brown Trout are unusual in that they have been found to occur in reproductively isolated populations within the same water body. This basically means that there can be two or more populations of Brown Trout in the same lake, with each population having developed individual characteristics. One of the most famous examples of this is in Lough Melvin in County Sligo, where there are known to be at least three individual populations of Trout: Ferox (large size), Gillaroo (red spots) and Sonaghen (black spots).