Trout usually occur in cold, clear streams and lakes. Natural area of occurrence includes the northern hemisphere: Europe, North Asia and North America. Several species have been artificially introduced to many countries, including Australia and New Zealand, where they often displace native species of fish. Rainbow trout is the species most domesticated by man and mostly grown for human consumption. In Poland, the largest breeding rainbow trout is in Pomerania. Brown trout are bred mainly for restocking rivers of a mountain. Isolated populations are often so different in shape and coloration that are described as separate species, but genetic studies indicate that the number of species is much smaller than it would with morphological differences.