The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea-run rainbow trout (anadromous) usually returning to freshwater to spawn after two to three years at sea; rainbow trout and steelhead trout are the same species. The fish are often called salmon trout. Several other fish in the salmonid family are called trout; some are anadromous like salmon, whereas others are resident in freshwater only.
The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica. In some locations, such as Southern Europe, Australia and South America, they have negatively impacted upland native fish species, either by eating them, outcompeting them, transmitting contagious diseases, (such as whirling disease transmitted by Tubifex) or hybridization with closely related species and subspecies that are native to western North America (see Salmo marmoratus and Salmothymus obtusirostris salonitana).
The species was originally named by Johann Julius Walbaum in 1792 based on type specimens from Kamchatka. Richardson named a specimen of this species Salmo gairdneri in 1836, and in 1855, W. P. Gibbons found a population and named it Salmo iridia, later corrected to Salmo irideus, these names faded, however, once it was determined that Walbaum's type description was conspecific and therefore had precedence (see e.g. Behnke, 1966). More recently, DNA studies showed rainbow trout are genetically closer to Pacific salmon (Onchorhynchus species) than to brown trout (Salmo trutta) or Atlantic salmon (Salmo salar), so the genus was changed.
Unlike the species' former name's epithet iridia (Latin: rainbow), the specific epithet mykiss derives from the local Kamchatkan name 'mykizha'; all of Walbaum's species names were based on Kamchatkan local names.
The oceangoing (anadromous) form (including those returning for spawning) are known as steelheads, (Canada and the United States) or ocean trout (Australia), although they are the same species.
Like salmon, steelheads are anadromous: they return to their original hatching ground to spawn. Similar to Atlantic salmon, but unlike their Pacific Oncorhynchus salmonid kin, steelheads are iteroparous (able to spawn several times, each time separated by months) and make several spawning trips between fresh and salt water. The steelhead smolts (immature or young fish) remain in the river for about a year before heading to sea, whereas salmon typically return to the seas as smolts. Different steelhead populations migrate upriver at different times of the year. "Summer-run steelheads" migrate between May and October, before their reproductive organs are fully mature. They mature in freshwater before spawning in the spring. Most Columbia River steelheads are "summer-run". "Winter-run steelheads" mature fully in the ocean before migrating, between November and April, and spawn shortly after returning. The maximum recorded life-span for a rainbow trout is 11 years.
Rainbow trout are predators with a varied diet, and will eat nearly anything they can grab. Their image as selective eaters is only a legend. Rainbows are not quite as piscivorous or aggressive as brown trout or lake trout (char). Young rainbows survive on insects, fish eggs, and smaller fish (up to 1/3 of their length), along with crayfish and other crustaceans. As they grow, though, the proportion of fish increases in most all populations. Some lake-dwelling lines may become planktonic feeders. While in flowing waters populated with salmonids, trout eat varied fish eggs, including salmon and cutthroat trout, as well as the eggs of other rainbow trout, alevin, fry, smolt and even leftover carcasses.
Length and weight
As rainbow trout grow, they lengthen and increase in mass. The relationship between length and mass is not linear. The relationship between total length (L, in inches) and total mass (W, in pounds) for steelhead and nearly all other fish can be expressed by an equation of the form:
The factor b is close to 3.0 for all species, and c is a constant that varies among species. For lentic rainbow trout, b = 2.990 and c = 0.000426, and for lotic rainbow trout, b = 3.024 and c = 0.000370. The largest recorded rainbow trout can measure 120 cm (47 in) long and weigh 24 kg (53 lb).
The relationship described in this section suggests a 13 in (33 cm) lentic rainbow trout weighs about 1.0 lb (0.45 kg), while an 18 in (46 cm) lentic rainbow trout weighs about 2.5 lb (1.1 kg).
Threats and conservation
Steelhead trout populations have declined due to human and natural causes. Steelheads (Oncorhynchus mykiss) historically occurred around the North Pacific Ocean from northwestern Mexico in North America to eastern Russia in Asia.
Two West Coast Evolutionarily Significant Units (ESUs) are endangered under the Federal Endangered Species Act (Southern California and Upper Columbia River) and eight ESUs are threatened. The U.S. National Marine Fisheries Service has a detailed description of threats. Southern California (south of Point Conception) ESU steelheads have been affected by habitat loss due to dams, confinement of streams in concrete channels, water pollution, groundwater pumping, urban heat island effects, and other byproducts of urbanization.
Several studies have shown almost all California coastal steelheads are of native origin, despite over a century of hatchery stocking. Genetic analysis shows that South Central California Coast (SCCC) distinct population segment (DPS) and Southern California (SC) DPS from Malibu Creek north, and including the San Gabriel River, Santa Ana River and San Mateo Creek, are not hatchery strains. However, steelheads from Topanga Creek and the Sweetwater River were partly, and from San Juan Creek completely, of hatchery origin. Genetic analysis has also shown the steelheads in the streams of the Santa Clara County and Monterey Bay basins are not of hatchery origin, including the Coyote Creek, Guadalupe River, Pajaro River, Permanente Creek, Stevens Creek, San Francisquito Creek, San Lorenzo River, and San Tomas Aquino Creek basins. Natural waterfalls and two major dams have isolated Russian River anadromous steelheads from its freshwater rainbow trout form above the impassable barriers; however, a recent genetic study of fin samples collected from steelheads at 20 different sites both above and below passage barriers in the watershed found that despite the fact that 30 million hatchery trout were stocked in the river from 1911 to 1925, the steelheads remain of native and not hatchery stock.
The rainbow trout is susceptible to enteric redmouth disease. A considerable amount of research has been conducted on redmouth disease, given its serious implications for rainbow trout farmers. The disease does not affect humans.
The U.S. National Marine Fisheries Service has identified 15 populations, called distinct population segments (DPSs), in Washington, Oregon and California. Eleven of these DPSs are listed under the U.S. Endangered Species Act (ESA). One DPS on the Oregon Coast is designated a U.S. Species of Concern. Species of Concern are those species that lack sufficient data to determine whether to list the species under the ESA.
In 2010, the Oregon Department of Fish and Wildlife hatchery expects to more than double its take over 2009. The 2009 population grew 60% over 2008. Hatchery-taken fish will spawn tens of thousands of juvenile "smolts" that will be released to swim downstream and mature in the Pacific.
In March 2010, the Los Angeles Times reported the New Zealand mud snail had infested watersheds in the Santa Monica Mountains, complicating efforts to improve stream-water quality for the steelheads. According to the article, the snails have expanded "from the first confirmed sample in Medea Creek in Agoura Hills to nearly 30 other stream sites in four years." Researchers at the Santa Monica Bay Restoration Commission believe the snails' expansion may have been expedited after the mollusks traveled from stream to stream on the gear of contractors and volunteers.
Hatcheries have also been demonstrated to present a risk to wild steelhead populations. Releases of conventionally reared hatchery steelheads pose ecological risks to pre-existing wild steelhead populations. Hatchery steelheads are typically larger than the wild forms, and can displace wild-form juveniles from optimal habitats. Dominance of hatchery steelheads for optimal microhabitats within streams may reduce wild steelhead survival as a result of reduced foraging opportunity and increased rates of predation. 
Relation to humans
Rainbow trout and steelhead are both highly desired food and sportfish. A number of angling methods are common. Rainbow trout are a popular target for fly fishers. Spinners, spoons, and small crankbaits can also be used productively, either casting or trolling. Rainbow trout can also be caught on live bait; nightcrawlers, trout worms, and minnows are popular and effective choices. The IGFA recognizes the world record for rainbow trout was caught on Saskatchewan's Lake Diefenbaker by Sean Konrad on September 5, 2009. The fish weighed 48 lb, 0 oz (21.77 kg). Many anglers consider the Rainbow trout the hardest fighting trout species, as this fish is known for leaping when hooked and putting a powerful fight.
Hatcheries and farms
The first rainbow trout hatchery was established on San Leandro Creek, a tributary of San Francisco Bay, in 1870, with trout production beginning in 1871. The hatchery was stocked with the locally native rainbow trout, and likely steelheads. The fish raised in this hatchery were shipped to hatcheries out of state for the first time in 1875, to Caledonia, New York[disambiguation needed] and then in 1876 to Northville, Michigan. In 1877, another rainbow trout hatchery was established on Campbell Creek, a McCloud River tributary. However, the McCloud River stock indiscriminately mixed rainbow trout with Redband trout (Oncorhynchus mykiss stonei).
They are farmed in many countries throughout the world. Since the 1950s, commercial production has grown exponentially, particularly in Europe and recently in Chile. Worldwide, in 2007, 604,695 tonnes (595,145 long tons; 666,562 short tons) of farmed salmon trout were harvested with a value of US$ 2.589 billion. The largest producer is Chile. In Chile and Norway, ocean cage production of steelheads has expanded to supply export markets. Inland production of rainbow trout to supply domestic markets has increased in countries such as Italy, France, Germany, Denmark and Spain. Other significant producing countries include the USA, Iran, Germany and the United Kingdom.
Golden rainbow trout are bred from a single mutated color variant of Oncorhynchus mykiss. Golden rainbow trout are predominantly yellowish, lacking the typical green field and black spots, but retaining the diffuse red stripe. The palomino trout is a mix of golden and common rainbow trout, resulting in an intermediate color. The golden rainbow trout should not be confused with the naturally occurring golden trout.
Rainbow trout is popular in Western cuisine, and is caught wild and farmed. It has tender flesh and a mild, somewhat nutty flavor. Farmed trout and trout taken from certain lakes have a pronounced earthy flavor which some people find unappealing. Wild rainbow trout that eat scuds (freshwater shrimp), insects such as flies, and crayfish are the most appealing. Farmed trout and some populations of wild trout , especially anadromous steelhead , have red/orange flesh as a result of high astaxanthin levels in their diets. Astaxanthin is a powerful antioxidant that may be from a natural source or synthetically produced. The resulting pink flesh is sometimes marketed under names such as Ruby Red or Carolina Red.
The flavour of the meat is related to the size of the fish , fish larger than 1kg having more flavour than small trout just as beef is more flavouful than veal. Steelhead meat is pink like that of salmon, and is more flavorful than the light-colored meat of rainbow trout.
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A few populations are recognized as subspecies:
- Kamchatkan rainbow trout, Oncorhynchus mykiss mykiss (Walbaum, 1792).
- Columbia River redband trout, Oncorhynchus mykiss gairdnerii (Richardson, 1836).
- Coastal rainbow trout (steelhead trout), Oncorhynchus mykiss irideus (Gibbons, 1855).
- Beardslee trout, isolated in Lake Crescent (Washington), Oncorhynchus mykiss irideus var. beardsleei (not a true subspecies, but a lake-dwelling variety of coastal rainbow trout) (Jordan, 1896).
- Great Basin red trout, Oncorhynchus mykiss newberrii (Girard, 1859).
- Golden trout, Oncorhynchus mykiss aguabonita (Jordan, 1892).
- Kamloops rainbow trout, Oncorhynchus mykiss kamloops (Jordan, 1892).
- Kern River rainbow trout, Oncorhynchus mykiss aguabonita var. gilberti (Jordan, 1894).
- Sacramento golden trout, Oncorhynchus mykiss aguabonita var. stonei (Jordan, 1894).
- Little Kern golden trout, Oncorhynchus mykiss aguabonita var. whitei (Evermann, 1906).
- Baja California rainbow trout, Nelson's trout, or San Pedro Martir trout, Oncorhynchus mykiss nelsoni (Evermann, 1908).
- Eagle Lake trout, Oncorhynchus mykiss aquilarum (Snyder, 1917).
- McCloud River redband, Oncorhynchus mykiss stonei
- Sheepheaven Creek redband, Oncorhynchus mykiss spp.
- Southern California steelhead DPS, Oncorhynchus mykiss
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