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The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The fish is sometimes called a 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, (like Whirling disease transmitted by Tubifex) or hybridization with closely-related species and subspecies that are native to western North America.[1][2]

Oncorhynchus mykiss

Oncorhynchus mykiss

Life cycleEdit

Drawing of fish with open mouth, bent body and stones in background

Illustration of a rainbow trout

Like salmon, steelhead are anadromous: they return to their original hatching ground to spawn. Similar to Atlantic salmon, but unlike their Pacific Oncorhynchus salmonid kin, steelhead 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 steelhead" migrate between May and October, before their reproductive organs are fully mature. They mature in freshwater before spawning in the spring. Most Columbia River steelhead are "summer-run". "Winter-run steelhead" 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 a selective eater 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, 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, cutthroat trout, as well as the eggs of other rainbow trout, alevin, fry, smolt and even left-over carcasses.

Threats and conservationEdit

Steelhead trout populations have declined due to human and natural causes. Steelhead (Oncorhynchus mykiss) historically occurred around the North Pacific Ocean from northwestern Mexico in North America to eastern Russia in Asia.[3]

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.[4] The U.S. National Marine Fisheries Service has a detailed description of threats. Southern California (south of Point Conception) ESU steelhead 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.

The rainbow trout is susceptible to enteric redmouth disease. There has been considerable research conducted on redmouth disease, given its serious implications for rainbow trout farmers. The disease does not affect humans.[5]

Rainbow trout, and subspecies thereof, are currently EPA approved indicator species for acute fresh water toxicity testing.[6]

In 2010, the Oregon Department of Fish & 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.[7]

In March 2010, the Los Angeles Times reported that the New Zealand mud snail had infested watersheds in the Santa Monica Mountains, complicating efforts to improve stream-water quality for the steelhead. 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 that the snails' expansion may have been expedited after the mollusks traveled from stream to stream on the gear of contractors and volunteers.[8]

Hatcheries have also been demonstrated to present a risk to wild steelhead populations. Releases of conventionally reared hatchery steelhead pose ecological risks to preexisting wild steelhead populations. Hatchery steelhead are typically larger than the wild form and can displace wild form juveniles from optimal habitat. Dominance of hatchery steelhead for optimal microhabitats within streams may reduce wild steelhead survival as a result of reduced foraging opportunity and increased rates of predation. [9]


Relation to humansEdit


Rainbow Trout

Rainbow trout

Rainbow trout and steelhead are both highly desired food and sportfish. A number of angling methods are common.

Hatcheries and farmsEdit


Rainbow trout, cleaned and iced, in a fish market in Western Australia

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 steelhead. The fish raised in this hatchery were shipped to hatcheries out of state for the first time in 1875, to Caledonia, New York 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).[10]

They are farmed in many countries throughout the world. Since the 1950s commercial production has grown exponentially,[11] particularly in Europe and recently in Chile. Worldwide, in 2007, 604695 t of farmed salmon trout were harvested with a value of 2.589 billion US dollars.[12] The largest producer is Chile. In Chile and Norway, ocean cage production of steelhead 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.[12]

Cultivated varietiesEdit

Golden rainbow trout are bred from a single mutated color variant of Oncorhynchus mykiss.[13] Golden rainbow trout are predominantly yellowish, lacking the typical green field and black spots, but retaining the diffuse red stripe.[13][14] 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.

As foodEdit

Photo of fried steelhead filet on plate

Rainbow trout and potatoes

Rainbow trout is popular in Western cuisine and is caught wild and farmed. It has tender flesh and a mild, somewhat nutty flavor. However, farmed trout and those taken from certain lakes have a pronounced earthy flavor which many people find unappealing; many shoppers therefore ascertain the source of the fish before buying. Wild rainbow trout that eat scuds (freshwater shrimp), insects such as flies, and crayfish are the most appealing. Dark red/orange meat indicates that it is either an anadromous steelhead or a farmed Rainbow trout given a supplemental diet with a high astaxanthin content. The resulting pink flesh is marketed under monikers like Ruby Red or Carolina Red.

Steelhead meat is pink like that of salmon, and is more flavorful than the light-colored meat of rainbow trout.[15]

See alsoEdit


  1. Salmo marmoratus
  2. Salmothymus obtusirostris salonitana
  3. "Population genetic structure and ancestry of Oncorhynchus mykiss populations above and below dams in south-central California" by Anthony J. Clemento, Eric C. Anderson, David Boughton, Derek Girman, John Carlos Garza in: Conservation Genetics, 2009, pages=1321–1336. PDF
  4. Map showing endangered species status of west coast steelhead
  5. LSC - Fish Disease Leaflet 82
  6. EPA Whole Effluent Toxicity
  7. Wall Street Journal: Fish Boom Makes Splash in Oregon, January 21, 2010
  8. Hard-to-kill snails infest Santa Monica Mountain watersheds Even Formula 409 has proven ineffective at destroying the New Zealand mudsnail, an asexually reproducing invasive species that poses a threat to steelhead restoration efforts and native creatures.
  9. McMichael, G. A., T. N. Pearsons, and S. A. Leider. 1999. Behavioral interactions among hatchery-reared steelhead smolts and wild Oncorhynchus mykiss in natural streams. North American Journal of Fisheries Management 19: 948-956
  10. "About Trout: The Best of Robert J. Behnke from Trout Magazine Authors: Robert J. Behnke, Ted Williams (2007) ISBN 9781599212036 Google Books
  11. Cowx, I.G. Cultured Aquatic Species Information Programme Oncorhynchus mykiss Walbaum, 1792 (Salmonidae) Rainbow Trout Food and Agricultural Organisation of the United Nations Fisheries snd Aquaculture Department (online), Rome, Updated 15 June 2005, Retrieved 27 September 2010
  12. 12.0 12.1 FAO: Species Fact Sheets: Oncorhynchus mykiss (Walbaum, 1792) Rome. Accessed 9 May 2009.
  13. 13.0 13.1 Golden Rainbow Trout. Pennsylvania Fish & Boat Commission FAQ.
  14. Golden Rainbow Trout. Photo.
  15. Your Christmas Steelhead


  • Scott and Crossman (1985) Freshwater Fishes of Canada. Bulletin 184. Fisheries Research Board of Canada. Page 189. ISBN 0-660-10239-0

External linksEdit