The brown trout has a wide natural distribution from Iceland and the northern coasts of Europe in the north, southwards to countries fronting the Mediterranean Sea, the islands of Corsica and Sardinia and Algeria in North Africa. The range extends from the Atlantic drainage east to the northern slopes of the Himalayas.
This predatory fish varies much in size, colouration and habitat use. It includes both purely freshwater populations and anadromous forms. Its colonization ability and highly fragmented distribution pattern with populations living in physically isolated habitats, have contributed to a very high level of genetic variability in this species. The fine-spotted brown trout in some lakes on the Hardangervidda, is a morph with a distinct pigmentation pattern inherited in a simple mendelian pattern. Another distinct morph termed marmorated brown trout is found in River Otra.
Brown trout is the only trout species in Norway, and it is widely distributed in rivers and lakes along the coast. The species also lives in high mountain areas such as the Hardangervidda plateau at 1300 metres above sea level, where archaeological excavations have shown that the brown trout was present in the large lakes already about 5000 years ago.
Different populations of brown trout may occur in fresh water and salt water. Freshwater populations spend their entire lives in fresh water, while sea trout migrate to sea as smolts to salt-water feeding grounds. Often sea trout return to freshwater to overwinter either as immature fish or to spawn.
Individuals in some populations of sea trout may make long migrations, around the North Sea for example, but most often migrations are short and limited to 20 km seaward from the river mouth.
In many areas, populations of sea trout are severely affected by sea lice and are therefore reduced in abundance and biomass. In the Hardangerfjord, an area with abundant fish-farming operations, years with intense rates of infection by sea lice have been documented. High rates of lice infection, far above what is needed to kill the host, have also been observed at other Norwegian waters.
Due to the fragmented distribution with populations inhabiting different enviroments, trout populations tend to develop different genetic characteristics such as growth patterns, colouration and susceptibility to parasites. For example, sea trout from Simavassdraget in the inner parts of the Hardangerfjord tend to grow faster and to be more heavily infected with lice than trout from Guddalselva in the central part of the fjord.