Atlantic, North, White and Baltic Sea basins, from Spain to Chosha Bay (Russia). Present in Iceland and in northernmost rivers of Great Britain and Scandinavia. In Rhne drainage, native only to Lake Geneva basin, which it entered after last glaciation. Native to upper Danube and Volga drainages. Introduced throughout Europe, North and South America, southern and montane eastern Africa, Pakistan, India, Nepal, Japan, New Zealand and Australia.
Comments: Eats aquatic and terrestrial insects and their larvae, crustaceans (especially crayfish), molluscs, fishes, and other animals. In streams, young feed mainly on aquatic and terrestrial drift invertebrates; in lakes, they feed on zooplankton and benthic invertebrates (Sublette et al. 1990). Large adults feed on fishes, crayfish, and other benthic invertebrates.
When brown trout spawn, the male and female are not monogamous. These trout mate every year, and they are not likely to have the same mate year after year. Occasionally, large males take over a bed already occupied by a smaller, less aggressive trout.
Mating System: polygynandrous (promiscuous)
Brown trout mature at about 3 or 4 years of age. They spawn in the fall from October into December. When they spawn, they head into shallow headwater brooks of the river. The female scoops out a hollow on a gravel "redd" where she can lay her eggs. As she releases the eggs on the redd, the male simultaneously releases milt to fertilize them. The pair continues this process until all of the female's eggs are spent. The female then covers the fertilized eggs with sand or gravel for protection. The eggs are then left to develop and hatch the following spring. Browns do not necessarily come back to the same gravel bed to spawn each year, but they come back to the same general area of the river.
Species Impact: Incompatible with native species such as cutthroat trout and Gila trout; protection of native species requires segregation of them from brown trout (Sublette et al. 1990).
Introduced populations in New Zealand exert strong top-down control of communty structure and ecosystem functioning via effects on individual behavior and population distribution and abundance (Townsend 2003).
Salmo trutta is a common trout known by two different common names reflecting the alternative ecological strategies and associated morphological characteristics of this species. The freshwater morphs (Salmo trutta morpha fario and S. trutta morpha lacustris) are known as brown trout. Sea trout is the anadromous morph which migrates between the ocean, where it spends most of its life, and freshwater spawning grounds. The two morphs, which often share the same breeding grounds (sympatric distribution), have in the past been classified as distinct species. The morphs do interbreed, but the extent of reproductive isolation between them varies by location and some studies have found genetic differentiation between morphs inhabiting the same territory.
Although native to Europe, northern Africa and western Asia, S. trutta has been widely introduced for aquaculture and recreational fishing purposes and is found in streams, lakes, and coastal areas throughout the world. Brown trout commonly mature at 13-16 inches long (often longer in large streams); sea-run morphs are larger and can be found up to 30 pounds and 3 feet long. An aggressive species, S. trutta has been responsible for declines in native fish populations, for example in the Great Lakes, where they displaced Arctic greyling (Thymallus arcticus) and in California, where they threaten native golden trout Oncorhynchus mykiss aguabonita and Dolly Varden (Salvelinus malma). This species was nominated as one of the world’s 100 worst invasive species by the Invasive Species Specialist Group (ISSG).