Sea trout are native to Ireland and are usually found in low-productivity rivers, particularly along the west coast. Sea trout are part of the same species as brown trout: brown trout spend their entire lives in freshwater, whereas sea trout migrate from the streams in which they are born to the sea to grow and mature before returning to their natal river to spawn. This type of migratory behaviour from river to sea and back again is called anadromous migration.
As juvenile trout in rivers where they grow as parr, sea trout are indistinguishable from brown trout, and these two ecotypes of trout interbreed in rivers where both are present. After a couple of years, however, sea trout begin their migration downstream to the sea, usually from March to June. They become smolts, changing to a silvery colour and undergoing changes in physiology to adapt to life in the sea, where they take advantage of rich marine feeding in coastal waters to grow quickly.
Sea trout have a lot of variation between individuals in their movements at sea and spawning migrations. Some sea trout known as finnock return to freshwater in the summer following migration, whereas others known as maidens may not return to spawn for at least one year. Sea trout can range widely along the coast, moving in and out with the tides and visiting other estuaries and rivers. After returning to rivers, sea trout may either spawn in the late autumn or winter and migrate back to sea as kelts or overwinter in the river without spawning.
Sea trout are highly valued for anglers as a hard-fighting, sporting fish. Stocks of sea trout have collapsed in recent years, however, and there are concerns about the impacts of sea-lice infestations. Anglers are encouraged to practice catch-and-release to help conserve sea trout.