The brown trout is normally considered to be native to Europe and Asia, but the natural distribution of the migratory forms may be, in fact, circumpolar. There are also landlocked populations far from the oceans, for example in Greece and Estonia.
The fish is not considered to be endangered although, in some cases, individual stocks are under various degrees of stress mainly through habitat degradation, overharvest and artificial propagation leading to introgression. Increased frequency of excessively warm water temperatures in high summer, attributed to global warming, causes a reduction in dissolved oxygen levels which can cause 'summer kills' of local populations if temperatures remain high for sufficient duration and deeper/cooler or fast, turbulent more oxygenated water is not accessible to the fish. This phenomenon can be further exacerbated by eutrophication of rivers due to pollution - often from the use of agricultural fertilizers within the drainage basin.
Overfishing is a problem where anglers fail to identify and return mature female fish into the lake or stream. Each large female removed can result in thousands fewer eggs released back into the system when the remaining fish spawn.
CharacteristicsEditThe Brown Trout is a medium-sized fish, growing to 20 kg or more in some localities although in many smaller rivers a mature weight of 1 kg (2 lb) or less is common. The spawning behaviour of brown trout is similar to that of the closely related Atlantic salmon. A typical female produces about 2,000 eggs per kilogram (900 eggs per pound) of body weight at spawning.
Brown trout can live to ages of 20 years. But as with the Atlantic salmon, there is a high proportion of death of anadromous males after spawning and probably fewer than 20% of anadromous female kelts recover from spawning. The migratory forms grow to significantly larger sizes for their age due to abundant forage fish in the waters they spend most of their lives. Brown trout are active both by day and by night and are opportunistic feeders. While in fresh water, the diet will frequently include invertebrates from the streambed, other fish, frogs, mice, birds, and insects flying near the water's surface. The high dietary reliance upon insect larvae, pupae, nymphs and adults is what allows trout to be a favoured target for fly fishing.Freshwater brown trout range in colour from largely silver with relatively few spots and a white belly, to the more well known brassy brown cast fading to creamy white on the fish's belly, with medium-sized spots surrounded by lighter haloes. The more silver forms can be mistaken for rainbow trout. Regional variants include the so-called "Loch Leven" trout, distinguished by larger fins, a slimmer body, and heavy black spotting, but lacking red spots. The continental European strain features a lighter golden cast with some red spotting and fewer dark spots. It is important to remember that both strains can show considerable individual variation from this general description.
Brown trout rarely form hybrids, almost invariably infertile, with other species.
Young brown trout feed on insects and other invertebrates such as shrimp, flys, caddis, stonefly, mayfly, etc. Both larvae and adults are taken and the fish will eat whatever local insect life is abundant at the time. Larger fish are active predators of fish including young brown trout, suckers, sculpins, shad, whitefish and rainbow trout. Larger brown trout will also feed on small terrestrial animals that fall into the water such as baby birds falling from overhanging nests, or even swimming mice/voles. Brown trout sometimes do not actively feed until the late afternoon or early evening but when the weather is cool they will feed during the day as well. The largest browns feed under cover of darkness. Brown trout can be caught with artificial flies, jigs, plastic worm imitations, spinners and other lures.