Brown Trout

Defining characteristics of brown trout include a slender body with a long, narrow head. The mouth is large, and on its roof, vomerine teeth are developed in a zig-zag pattern. The caudal fin is deltaform without forking, unlike that of the related Atlantic salmon (Salmo salar). Dark and red spots are often present on the sides, but do not extend to the tail. Parr trout (juvenile) often have a red margin on their adipose fin, with dark blotches along their sides that also become inconspicuous with age.

The native range of brown trout extends from northern Norway and White Sea tributaries in Russia in the Arctic Ocean to the Atlas Mountains in North Africa. The western limit of their native range is Iceland in the north Atlantic, while the eastern limit is in Aral Sea tributaries in Afghanistan and Pakistan.

Field studies have demonstrated that brown trout fed on several animal prey species, aquatic invertebrates (such as crustaceans and molluscs) being the most abundant prey items. However, brown trout also feed on other taxa such as terrestrial invertebrates (e.g. Hymenoptera) or other fish (e.g. small suckers, minnows, sculpin, and darters). Moreover, in brown trout, as in many other fish species, a change in the diet composition normally occurs during the life of the fish, and piscivorous behaviour is most frequent in large brown trout. These shifts in the diet during fish lifecycle transitions may be accompanied by a marked reduction in intraspecific competition in the fish population, facilitating the partitioning of resources. Brown trout that live in lakes and reservoirs normally eat zooplankton on the lake bed, detritus, and in some cases have a diet that only consists of algae.

The fish is not considered to be endangered, although some individual stocks are under various degrees of stress mainly through habitat degradation, overfishing, and artificial propagation leading to introgression. Increased frequency of excessively warm water temperatures in high summer 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.