The rainbow trout (Oncorhynchus mykiss) is a species of trout native to cold-water tributaries of the Pacific Ocean in North America and Asia. The steelhead (sometimes called steelhead trout) is an anadromous (sea-run) form of the coastal rainbow trout or Columbia River redband trout that usually returns to freshwater to spawn after living two to three years in the ocean.
Adult freshwater stream rainbow trout average between , while lake-dwelling and anadromous forms may reach . Coloration varies widely based on subspecies, forms, and habitat. Adult fish are distinguished by a broad reddish stripe along the lateral line, from gills to the tail, which is most vivid in breeding males.
Wild-caught and hatchery-reared forms of the species have been transplanted and introduced for food or sport in at least 45 countries and every continent except Antarctica. Introductions to locations outside their native range in the United States, Southern Europe, Australia, New Zealand, and South America have damaged native fish species. Introduced populations may affect native species by preying on them, out-competing them, transmitting contagious diseases (such as whirling disease), or hybridizing with closely related species and subspecies. The rainbow trout is included in the list of the top 100 globally invasive species. Other introductions into waters previously devoid of fish or with severely depleted stocks of native fish have created sport fisheries, such as the Great Lakes and Wyoming's Firehole River.
Some local populations of specific subspecies, or in the case of steelhead, distinct population segments, are listed as either threatened or endangered under the Endangered Species Act. The steelhead is the official state fish of Washington.
Taxonomy The scientific name of the rainbow trout is . The species was originally named by German naturalist and taxonomist Johann Julius Walbaum in 1792 based on type specimens from the Kamchatka Peninsula in Siberia. Walbaum's original species name, mykiss, was derived from the local Kamchatkan name used for the fish, mykizha. The name of the genus is from the Greek ὄγκος (ónkos) "lump, bend, hook" and ῥύγχος (rhúnkhos) "snout", in reference to the hooked jaws of males in the mating season (the "kype").
Sir John Richardson, a Scottish naturalist, named a specimen of this species in 1836 to honor Meredith Gairdner, a Hudson's Bay Company surgeon at Fort Vancouver on the Columbia River who provided Richardson with specimens. In 1855, William P. Gibbons, the curator of Geology and Mineralogy at the California Academy of Sciences, found a population and named it (Latin: rainbow), later corrected to . These names faded once it was determined that Walbaum's description of type specimens was conspecific and therefore had precedence. In 1989, morphological and genetic studies indicated that trout of the Pacific Basin were genetically closer to Pacific salmon (Oncorhynchus species) than to the genus Salmo (brown trout , Atlantic salmon and relatives) of the Atlantic Basin. Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat, and other Pacific Basin trout into the genus Oncorhynchus. Walbaum's name had precedence, so the species name became the scientific name of the rainbow trout. The previous species names irideus and gairdneri were adopted as subspecies names for the coastal rainbow and Columbia River redband trout, respectively. Anadromous forms of the coastal rainbow trout or Columbia River redband trout are commonly known as steelhead.
Subspecies Subspecies of are listed below as described by fisheries biologist Robert J. Behnke (2002). Geographical group Common nameScientific nameRange ImageType subspecies Kamchatkan rainbow trout (Walbaum, 1792) Western Pacific: the Kamchatka Peninsula, and has been recorded from the Commander Islands east of Kamchatka, and sporadically in the Sea of Okhotsk, as far south as the mouth of the Amur River Coastal forms (irideus group) Coastal rainbow trout (Gibbons, 1855) Pacific Ocean tributaries from Aleutian Islands in Alaska south to Southern California. Anadromous forms are known as steelhead, freshwater forms as rainbow trout. Ocean (a.k.a. "steelhead") and fresh water forms of coastal rainbow troutO. m. irideus Beardslee trout (a genetically unique lake-dwelling variety of the coastal rainbow trout) (Jordan, 1896) Isolated in Lake Crescent, Washington Redband forms (gairdneri group) Columbia River redband trout (Richardson, 1836) Found in the Columbia River and its tributaries in Montana, Washington and Idaho. Anadromous forms are known as redband steelhead. Columbia River redband troutO. m. gairdneri Athabasca rainbow trout, considered by Behnke as a form of , but considered a separate subspecies by biologist L. M. Carl of the Ontario Ministry of Resources, Aquatic Ecosystems Research Section and associates from work published in 1994. Distributed throughout the headwaters of the Athabasca River system in Alberta McCloud River redband trout (Jordan, 1894) Native to the McCloud River, upstream of Middle Falls, and its tributaries in Northern California, south of Mount Shasta. Sheepheaven Creek redband trout Native to Sheepheaven Creek, Siskiyou County, California. Sheepheaven Creek redband were transplanted into Swamp Creek in 1972 and 1974 and into Trout Creek in 1977. Great Basin redband trout (Girard, 1859) Native in southeastern Oregon and parts of California and Nevada on the periphery of the Great Basin. Eagle Lake trout (Snyder, 1917) Endemic to Eagle Lake in Lassen County, California. Kamloops rainbow trout (Jordan, 1892) Native to several large British Columbia lakes, particularly Kamloops Lake and Kootenay Lake. Known for its very large size. Kern River forms (golden trout, aguabonita group) Golden trout (Jordan, 1892) Native to Golden Trout Creek (tributary to the Kern River), Volcano Creek (tributary to Golden Trout Creek), and the South Fork Kern River. Kern River golden troutO. m. aguabonita Kern River rainbow trout (Jordan, 1894) Endemic to the Kern River and tributaries in Tulare County, California. Its current range is drastically reduced from its historic range. Remnant populations live in the Kern River above Durrwood Creek, in upper Ninemile, Rattlesnake, and Osa creeks, and possibly in upper Peppermint Creek. Little Kern golden trout (Evermann, 1906) Endemic to about of the Little Kern River and tributaries of Tulare County, California. Their current range is restricted to five headwater streams in the Kern River basin (Wet Meadows, Deadman, Soda Spring, Willow, Sheep, and Fish creeks) plus an introduced population in Coyote Creek, a tributary of the Kern River.Mexican forms (nelsoni group) Mexican rainbow trout*Rio Yaqui, Rio Mayo and Guzman trout*Rio San Lorenzo and Arroyo la Sidra trout*Rio del Presidio trout (Evermann, 1908)Sometimes referred to as Nelson's trout, occurs in three distinct geographic groups. The taxonomy of these trout is subject to ongoing research, and there may be significant diversity of forms in this group.Mutated forms Golden rainbow troutorPalomino troutorlightning trout So-called golden rainbow trout or palomino trout are bred from a single mutated color variant of that originated in a West Virginia fish hatchery in 1955. They are also known as banana trout. It is stocked outside of West Virginia but West Virginia still stocks 50,000 palominos a year. The golden rainbow trout is predominantly yellowish, lacking the typical green field and black spots, but retaining the diffuse red stripe. The palomino trout is a mixture of golden and common rainbow trout, resulting in an intermediate color. The golden rainbow trout is not the same subspecies as the naturally occurring , the Kern River golden trout of California. Golden rainbow trout
Description Resident freshwater rainbow trout adults average between in riverine environments, while lake-dwelling, and anadromous forms may reach . Coloration varies widely between regions and subspecies. Adult freshwater forms are generally blue-green or olive green with heavy black spotting over the length of the body. Adult fish have a broad reddish stripe along the lateral line, from gills to the tail, which is most pronounced in breeding males. The caudal fin is squarish and only mildly forked. Lake-dwelling and anadromous forms are usually more silvery in color with the reddish stripe almost completely gone. Juvenile rainbow trout display parr marks (dark vertical bars) typical of most salmonid juveniles. In some redband and golden trout forms, parr marks are typically retained into adulthood. Some coastal rainbow trout and Columbia River redband trout populations and cutbow hybrids may also display reddish or pink throat markings similar to cutthroat trout. In many regions, hatchery-bred trout can be distinguished from native trout via fin clips. Fin clipping the adipose fin is a management tool used to identify hatchery-reared fish.
Life cycle Rainbow trout, including steelhead forms, generally spawn in early to late spring (January to June in the Northern Hemisphere and September to November in the Southern Hemisphere) when water temperatures reach at least . The maximum recorded lifespan for a rainbow trout is 11 years.
Freshwater life cycle
Freshwater resident rainbow trout usually inhabit and spawn in small to moderately large, well-oxygenated, shallow rivers with gravel bottoms. They are native to the alluvial or freestone streams that are typical tributaries of the Pacific basin but introduced rainbow trout have established wild, self-sustaining populations in other river types such as bedrock and spring creeks. Lake-resident rainbow trout are usually found in moderately deep, cool lakes with adequate shallows and vegetation to support the production of sufficient food sources. Lake populations generally require access to gravelly-bottomed streams to be self-sustaining.
Spawning sites are usually a bed of fine gravel in a riffle above a pool. A female trout clears a redd in the gravel by turning on her side and beating her tail up and down. Female rainbow trout usually produce 2000 to 3000 eggs per kilogram of weight. During spawning, the eggs fall into spaces between the gravel, and immediately the female begins digging at the upstream edge of the nest, covering the eggs with the displaced gravel. As eggs are released by the female, a male moves alongside and deposits milt (sperm) over the eggs to fertilize them. The eggs usually hatch in about four to seven weeks although the time of hatching varies greatly with region and habitat. Newly hatched trout are called sac fry or alevin. In approximately two weeks, the yolk sac is completely consumed, and fry commence feeding mainly on zooplankton. The growth rate of rainbow trout varies with area, habitat, life history, and quality and quantity of food. As fry grow, they begin to develop "parr" marks or dark vertical bars on their sides. In this juvenile stage, immature trout are often called "parr" because of the marks. These small juvenile trout are sometimes called 'fingerlings' because they are approximately the size of a human finger. In streams where rainbow trout are stocked for sport fishing, but no natural reproduction occurs, some of the stocked trout may survive and grow or "carryover" for several seasons before they are caught or perish.
Steelhead life cycle
The oceangoing (anadromous) form, including those returning for spawning, are known as steelhead in Canada and the U.S. In Tasmania they are commercially propagated in sea cages and are known as ocean trout, although they are the same species.