The cutthroat trout (Oncorhynchus clarki) is a species of freshwater fish in the salmon family of order Salmoniformes.[2] It is one of the many fish species colloquially known as trout. All subspecies of cutthroat trout are sought after gamefish, especially among anglers who enjoy fly fishing. However, several native subspecies of cutthroat are currently listed as threatened, generally due to loss of habitat and introduction of non-native species. Cutthroat trout are native to western North America. The cutthroat species has evolved through geographic isolation into many subspecies, each native to a different major drainage basin. Native cutthroat species are found along the Pacific Northwest coast, in the Cascade Range, the Great Basin, and throughout the Rocky Mountains. Some coastal populations are anadromous, living primarily in the Pacific Ocean as adults and returning to fresh water from fall through early spring to feed on insects and spawn. Most populations, however, stay in freshwater throughout their lives and are known as non-migratory, stream-resident or riverine populations. Anadromous fish may reach weights of 20 pounds (9 kg), but those fish which remain permanently in freshwater may only reach a weight of 2 pounds (1 kg). At least three subspecies are confined to isolated basins in the Great Basin and can tolerate saline or alkaline water. Cutthroat trout vary widely in size, coloration, and habitats. Though their coloration can range from golden to gray to green on the back, depending on subspecies and habitat, all populations feature distinctive red, pink, or orange marks on the underside of the lower jaw; usually the easiest diagnostic of the species for the casual observer. As adults, different populations and subspecies of cutthroat can range from 6–40 inches (15–100 cm) in length making size an ineffective indicator as to species. Cutthroat will readily interbreed with the closely related rainbow trout, producing fertile hybrids commonly called "cutbow". Cutthroat will also hybridize with the O. gilae subspecies, the Gila trout and Apache trout.

There are many subspecies of cutthroat, each native to a separate geographic area. The cutthroat trout is thought to have evolved over the past two million years from other Oncorhynchus species which migrated up the Columbia and Snake river basins. There are at least 10, and perhaps more than 14 subspecies[3][2], including: Pacific Coast Coastal cutthroat trout O. c. clarki, also known as "sea-run" cutthroat; native from northern California to Alaska.[3] Crescenti trout O. c. crescenti, no longer a recognized subspecies, but a unique population of coastal cutthroat endemic to Lake Crescent, Washington state.[4] [edit] Great Basin Likely O. c. utah, northern UtahAlvord cutthroat trout O. c. alvordensis, endemic to tributaries of Alvord Lake in southeastern Oregon; considered extinct. Named in 2002. Bonneville cutthroat trout O. c. utah, native to tributaries of the Great Salt Lake. Humboldt cutthroat trout O. c. spp., found only in the upper Humboldt River of northern Nevada.[4] Considered by some to be a population of O. c. henshawi. Lahontan cutthroat trout O. c. henshawi, western Nevada, designated as threatened.[5] Whitehorse Basin cutthroat trout, O. c. spp.[4], once considered a separate subspecies, native to southeastern Oregon.[6] Paiute cutthroat trout O. c. seleniris, endemic to eastern Sierra Nevada Mountains, designated as threatened. [edit] Northern Rockies O. c. bouvieriSnake River fine-spotted cutthroat trout, named O. c. behnkei (1995 & 2002), but some consider it a population of O. c. bouvieri. Native to the Snake River of Idaho and Wyoming. Westslope cutthroat trout O. c. lewisi, native to northern Idaho, Montana, British Columbia, and Alberta. Yellowfin cutthroat trout O. c. macdonaldi, endemic to Twin Lakes, Colorado; now extinct. Yellowstone cutthroat trout O. c. bouvieri, native to the upper Snake River, Yellowstone Lake, and Yellowstone River, Idaho and Wyoming. [edit] Southern Rockies O. c. virginalisColorado River cutthroat trout O. c. pleuriticus, native to tributaries of the Green and Colorado Rivers. Greenback cutthroat trout O. c. stomias, native to the Arkansas and South Platte Rivers in eastern Colorado; designated as threatened. Rio Grande cutthroat trout O. c. virginalis, native to New Mexico and southern Colorado.