Oncorhynchus mykiss

This highly variable species formerly was known as Salmo gairdneri, but this taxon is closely related to Pacific salmon and is conspecific with Asiatic steelhead (Oncorhynchus mykiss). For a complete taxonomic history, see Smith and Stearley (1989), Robins et al. (1991), and Behnke (1992).

"Redband" trout has been used as the name for nonanadromous populations adapted to harsh arid environments (Wishard et al. 1984); however, the term "redband" should not be used to imply a taxonomic relationship among all groups of rainbow trout in interior basins of Oregon and adjacent areas of Idaho, Nevada, and California (Wishard et al. 1984, Currens et al. 1990). Currens et al. (1990) found no evidence that all isolated groups of rainbow trout with plesiomorphic characteristics in the White River, Oregon desert basins, and northern California represent a monophyletic group.

Behnke (1992) included in O. mykiss three major groups: (1) the redband trout of the Columbia River basin east of the Cascade Mountains, and in upper Fraser River basin and the Athabasca headwaters of the Mackenzie River basin (subspecies gairdneri); (2) the redband trout of the Sacramento River basin, which he regarded as comprising two Kern River drainage subspecies (aguabonita and gilberti), plus the McCloud River subspecies (provisionally denoted as subspecies stonei); and (3) the coastal rainbow trout (nominal subspecies irideus of North America and mykiss of eastern Asia, though no known taxonomic characters separate mykiss from irideus). Behnke concluded that other forms, such as the redband trout native to Oregon desert basins, Upper Klamath Lake, the Pit River drainage, and Eagle Lake, California, cannot be consistently distinguished from the three groups listed above. He noted that their classification is a matter of personal preference and professional judgment. However, in the same publication, he stated that "the trout specialized for lacustrine conditions in Klamath Lake...is well differentiated from other groups of both redband and coastal rainbow trout and could be recognized as a subspecies, O. m. newberrii."

Hatchery rainbow trout derived mainly from coastal steelhead are widely stocked throughout the ranges of western trout (Behnke 1992). These hatchery fishes have led to hybridization with most populations of resident redband trout in the upper Sacramento River basin, the Oregon desert basins, and much of the Columbia River basin (Behnke 1992).

Oncorhynchus mykiss freely interbreeds with cutthroat trout (O. clarki) and Gila trout (O. gilae), producing fertile offspring (Sublette et al. 1990).

Very large range; abundant; secure on a range-wide scale. See also information for the many subspecies and stocks (e.g., steelhead, redband trout, etc.).

Native to streams along the Pacific coast of North America from the Kuskokwim River, Alaska, south to northern Baja California; also the upper Mackenzie River drainage (Arctic basin), Alberta and British Columbia, and endorheic (i.e. having no outflow of water) basins of southern Oregon (Page and Burr 1991). The species has been widely introduced and established in suitable habitats all over the world (Lee et al. 1980). At sea, O. mykiss occurs throughout the North Pacific above 40° N from the North American coast to the Sea of Okhotsk (Burgner et al. 1992); it is most abundant in the Gulf of Alaska and eastern part of the North Pacific, conforming to the 5°C isotherm in the north and 15°C isotherm in the south. Seasonal shifts in distribution are correlated with changes in water temperature (Sutherland 1973).

This species is represented by a very large number of occurrences (subpopulations).

In a survey of populations in the contiguous U.S., Huntington et al. (1996) identified 28 healthy native stocks of winter steelhead (20 in Washington, 7 in Oregon, 1 in California) and 6 healthy native stocks of summer steelhead, all in Oregon.

On a range-wide scale, this species is not significantly threatened. However, many subspecies and populations face serious threats (see separate accounts).

Declines in winter steelhead stocks from the Siuslaw River north to Tillamook Bay, Oregon, may have resulted from deterioration of ocean feeding conditions, widespread use of hatchery stock, predation by marine mammals, and ocean drift-net fishing (Nehlsen et al. 1991). Declining winter catches on the Illinois River (tributary to the Rogue River), Oregon, since the mid-1970s have been attributed to water withdrawal for irrigation (Nehlsen et al. 1991). In the Columbia River basin, winter stocks are threatened by habitat degradation, main stem passage problems, and interactions with hatchery fish (Nehlson et al. 1991). Declines in several winter populations in the Puget Sound area of Washington have resulted from habitat degradation (e.g., water quality problems, siltation, and sedimentation); predation by sea lions has been reported as a problem for the Lake Washington population (Nehlsen et al. 1991). Whirling disease has caused population declines in some areas. The disease is caused by a protozoan pathogen (inadvertently introduced from Europe) and involves tubifex worms as an alternate host. Brown trout (Salmo trutta) are unaffected by the protozoan and serve as a reservoir.