The brown trout is native to Ireland, and its native geographic range includes Europe and parts of northern Africa. As a favourite angling species, brown trout have been widely introduced to other temperate parts of the world. They are the most widespread fish in Ireland and are found in practically every stream, river and lake in the country.
Typically, brown trout have a brown back, lighter coloured sides with black or reddish spots, pale belly and reddish-brown fins; however, they are very variable in appearance depending on habitat and can camouflage themselves by making their skin lighter or darker in colour to match their surroundings.
Brown trout usually spawn between October and December in well-oxygenated gravel beds of rivers. They thrive in lakes and rivers with cool temperatures and high oxygen levels, and they can cope with moderate levels of pollution. Small trout feed primarily on freshwater invertebrates and will also eat fish as they grow larger. Brown trout are territorial and compete for the best feeding location in a river. Trout are noted for being opportunistic feeders that exploit seasonally abundant prey, such as mayfly hatches.
Within the brown trout (Salmo trutta) species, there are two main ecotypes of trout: brown trout (Salmo trutta fario) remain resident in freshwater all their life, whereas sea trout (Salmo trutta trutta) start life in freshwater but migrate to sea to feed before returning to rivers to spawn. Despite their very different life histories, brown trout and sea trout breed together and coexist as juveniles in the same rivers.
Some varieties of brown trout that live in Ireland's lakes have distinctive life histories. Some biologists classify these trout as separate species, and there is evidence of genetic divergence between them, whereas other biologists treat them as separate subspecies, races or varieties of brown trout within the single species Salmo trutta:
- Ferox: a large, piscivorous (fish-eating) trout found in several big, deep lakes.
- Gillaroo: found only in Lough Melvin, a bottom-dwelling trout that feeds primarily on snails.
- Sonaghan: found only in Lough Melvin, a trout usually found in open, deep water that feeds primarily on plankton.
Some brown trout spend their entire life close to the stretch of river where they were born, whereas others migrate from rivers to lakes or estuaries and back again to spawn. Some of these migratory trout are also recognised as distinct varieties, such as the croneen in the River Shannon catchment, the dollaghan in Lough Neagh and its tributaries and slob trout, which are brown trout that spend part of their lives feeding in estuaries but which do not migrate to sea.
This rich variation in the biology of brown trout, its adaptability to different niches in its environment and its ability to pursue alternative life-history strategies make brown trout a fascinating species within Ireland's fish fauna.