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      "Oncorhynchus mykiss (Photo: Eric Engbretson / U.S. Fish and Wildlife Service via Wikimedia Commons)
" - Click for full size   "Oncorhynchus mykiss (Photo: U.S. Fish and Wildlife Service via Wikimedia Commons)
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    Taxonomic name: Oncorhynchus mykiss (Walbaum, 1792)
    Synonyms: Fario gairdneri (Richardson, 1836), Onchorrhychus mykiss (Walbaum, 1792), Oncorhynchus kamloops (Jordan, 1892), Oncorhynchus mykiss nelsoni (Evermann, 1908), Parasalmo mykiss (Walbaum, 1792), Salmo gairdneri irideus (Gibbons, 1855), Salmo gairdneri shasta (Jordan, 1894), Salmo gairdneri (Richardson, 1836), Salmo gairdnerii gairdnerii (Richardson, 1836), Salmo gairdnerii irideus (Gibbons, 1855), Salmo gairdnerii (Richardson, 1836), Salmo gilberti (Jordan, 1894), Salmo iridea (Gibbons, 1855), Salmo irideus argentatus (Bajkov, 1927), Salmo irideus (Gibbons, 1855), Salmo kamloops whitehousei (Dymond, 1931), Salmo kamloops (Jordan, 1892), Salmo masoni (Suckley, 1860), Salmo mykiss (Walbaum, 1792), Salmo mykiss (Walbaum, 1792) , Salmo nelsoni (Evermann, 1908), Salmo purpuratus (Pallas, 1814), Salmo rivularis kamloops (Jordan, 1892), Salmo rivularis (Ayres, 1855), Salmo stellatus (Girard, 1856), Salmo truncatus (Suckley, 1859)
    Common names: Alabalik (Turkish), Alabalik türü (Turkish), Amerikaniki Pestrofa (Greek), Aure (Norwegian), Baiser (English-Newfoundland, Canada), Baja California rainbow trout (English-Mexico), Brown trout (English-Nepal), Coast angel trout (English), Coast rainbow trout (English-Canada), Coast range trout (English-United States), Dagova pastarva (Bulgarian), Forel rajduzhna (Ukrainian), Forelle (German), Hardhead (English-United States), Kamchatka steelhead (English-Russian Federation), Kamchatka trout (English-Russian Federation), Kamloops (English-United States), Kamloops trout (English-Canada), Kirjolohi (Finnish-Finland), K'wsech (Salish-British Columbia, Canada), Lord-fish (English-Newfoundland, Canada), Masu (Japanese-Japan), Nijimasu (Japanese), Orret (Norwegian), Pastrva (Serbian-Yugoslavia), Pestropha (Greek), pstrag teczowy (Poland), Pstrag teczowy (Polish), Pstruh duhový (Czech), Pstruh dúhový (Slovak), rainbow trout (English), Rainbow trout (English-Alaska, United States), Redband (English-United States), redband trout (English), Regenbogenforelle (German), Regenbogenforelle (German-Germany), Regenboogforel (Dutch), Regnbåge (Swedish), Regnbågslax (Swedish), Regnbogasilungur (Icelandic), Regnbueørred (Danish), Regnbueørret (Norwegian), Salmon trout (English-United States), Salmones del Pacífico (Spanish), Silver trout (English), Stahlkopfforelle (German), Stålhovedørred (Danish), Steelhead (English-United States), steelhead trout, Steelhead trout (English), Summer salmon (English-United Kingdom), Sxew'k'em (Salish-British Columbia, Canada), Trofta ylberi (Albanian), Trofte ylberi (Albanian), Trota (Italian), Trota iridea (Italian), Trucha (Spanish), trucha arco iris (Dominican Republic), Trucha arco iris (Spanish), Trucha arcoiris (Spanish-Mexico), truite arc-en-ciel (French), Truta (Portuguese), Truta-arco-iris (Portuguese), Urriöi (Icelandic)
    Organism type: fish
    Oncorhynchus mykiss (rainbow trout) are one of the most widely introduced fish species in the world. Native to western North America, from Alaska to the Baja Peninsula, Oncorhynchus mykiss have been introduced to numerous countries for sport and commercial aquaculture. Oncorhynchus mykiss is highly valued as a sportfish, with regular stocking occurring in many locations where wild populations cannot support the pressure from anglers. Concerns have been raised about the effects of introduced trout in some areas, as they may affect native fish and invertebrates through predation and competition.
    Description
    Rainbow trout are a deep-bodied, compressed species of fish, with extremely large sea-run individuals growing to 1220mm and 16.3kg. The general body shape is typical for a trout, with a moderately large head and a mouth that extends back behind the eyes. Rainbow trout have highly variable colouration. Rainbow trout that live in lakes have a very silvery appearance, usually with a dark olive-green colour on the back. Occasionally the back is a deep steely blue, mostly in Rainbow trout that live well offshore in deep lakes or in small fish that have not yet spawned. Numerous spots are present on the back and extend about two-thirds of the way to the lateral line down the sides. The sides are silvery and largely free of spots, while the belly and ventral surface of the head are whitish. Eyes are an olive to bronze colour. Sometimes a soft, metallic-pink colour is present along the sides of the body and the head.

    When rainbow trout leave lakes to spawn, their colours become more intense. The pinkish stripe that is present on the sides of lake fish becomes a rich crimson colour, the fins become a stronger red colour, and there is sometimes a red slash in the folds below the lower jaw. The belly and the lower sides turn a smoky grey and spots on the sides and upper fins become bolder and more clearly delineated. Rainbow trout parr (juveniles) have an olive-green colour on the back and silvery olive high on the sides. There are 8-13 oval-shaped marks along the sides, which may also have smaller dark spots along them. Rosy-yellow markings occur along the lateral lines between the oval marks. As the parr grows, adult colouration is attained (McDowall, 1990)
    Similar Species
    Salmo trutta

    More
    Occurs in:
    lakes, water courses
    Habitat description
    Rainbow trout are primarily a freshwater fish, although sea-run populations, often known as steelhead, exist in some areas. It is unclear whether this migration to sea water is genetic or simply opportunistic, but it appears that any population of rainbow trout is capable of migrating to or surviving in the sea if the need arises (FishBase, 2003)
    Well-oxygenated, clean fresh water, with a temperature of around 12°C is preferred, although a range from 10°C to 24°C is tolerated (FishBase, 2003). Rainbow trout tend to thrive better in lakes than in streams or rivers, although large fish are often present in remote headwaters (McDowall, 1990).
    General impacts
    Impacts include hybridisation, disease transmission, predation and competition with native species.
    In the United States, the introduction of rainbow trout into areas outside of their native range has caused problems due to their ability to hybridise with native salmonid species, affecting their genetic integrity. Some species, such as the Alvord cutthroat (Oncorhynchus clarki subsp1.) have become virtually extinct because of this. Other species known to be affected by hybridisation include the Lahontan cutthroat trout (O. clarki henshawi); golden trout (O. aquabonita); redband trout (O. mykiss subsp.); Gila trout (see O. gilae in IUCN Red List of threatened Species) and Arizona trout (see O. apache in IUCN Red List of Threatened Species) (Fuller, 2000).
    Whirling disease is a condition caused by a protozoan (Myxobolus cerebralis) that causes dysfunction in the nervous system of salmonids, and may result in curvature of the vertebral column. This results in fish losing the ability to maintain a proper orientation, causing them to swim in a spiral motion (McDowall, 1990). The stocking of hatchery-reared trout into the wild has caused outbreaks of this disease in the United States, threatening wild fish populations (Fuller, 2000).
    In many countries introduced rainbow trout have been reported to have negative effects on native fish, amphibians and invertebrates. In New Zealand it is suspected that rainbow trout affect native fish species through direct predation and competition for feeding areas (McDowall, 1990), while in the United States there is evidence that the same is happening to fish such as the humpback chub (see Gila cypha in IUCN Red List of Threatened Species), suckers, and squawfish (Fuller, 2000).
    Uses
    Rainbow trout are highly valued as both a sport and table fish. They are the basis of many sport fisheries and are highly sought-after by anglers. In many locations rainbow trout are raised in hatcheries, then liberated into rivers and streams for recreational anglers to catch. They have also been introduced to many countries as an aquaculture species for commercial purposes (FishBase, 2003).
    Geographical range
    Native range: Eastern Pacific, from Alaska to Baja, California and Mexico.
    Known introduced range: Introduced worldwide. Considered one of the most widely introduced fishes. Restricted to areas above 1200m in tropical localities (FishBase, 2003).
    Introduction pathways to new locations
    Acclimatisation societies: Introduced worldwide as a sportfish by acclimatisation societies.
    Aquaculture: Introduced to many locations as an aquaculture species.
    Landscape/fauna "improvement": Introduced to many locations to 'improve' the native fish fauna for anglers.


    Local dispersal methods
    Acclimatisation societies (local): Introduced as a sportfish by acclimatisation societies. Some areas have regular restocking pogrammes in place.
    Aquaculture (local): Introduced to many locations as an aquaculture species.
    Natural dispersal (local): Trout are able to colonise areas where they were not originally released.
    Management information
    Preventative measures: The use of potentially invasive alien species for aquaculture and their accidental release/or escape can have negative impacts on native biodiversity and ecosystems. Hewitt et al, (2006) Alien Species in Aquaculture: Considerations for responsible use aims to first provide decision makers and managers with information on the existing international and regional regulations that address the use of alien species in aquaculture, either directly or indirectly; and three examples of national responses to this issue (Australia, New Zealand and Chile). The publication also provides recommendations for a ‘simple’ set of guidelines and principles for developing countries that can be applied at a regional or domestic level for the responsible management of Alien Species use in aquaculture development. These guidelines focus primarily on marine systems, however may equally be applied to freshwater.

    Copp et al, (2005) Risk identification and assessment of non-native freshwater fishes presents a conceptual risk assessment approach for freshwater fish species that addresses the first two elements (hazard identification, hazard assessment) of the UK environmental risk strategy. The paper presents a few worked examples of assessments on species to facilitate discussion. The electronic Decision-support tools- Invasive-species identification tool kits that includes a freshwater and marine fish invasives scoring kit are made available on the Cefas (Centre for Environment, Fisheries & Aquaculture Science) page for free download (subject to Crown Copyright (2007-2008)).

    Chemical: Antimycin, an antibiotic, is used to kill rainbow trout in the Great Smoky Mountains National Park in the US, in an effort to protect native brook trout (ESPN, 2003).

    Nutrition
    Adult rainbow trout eat insects (both aquatic and terrestrial), crustaceans, molluscs, fish eggs, and small fish. Young trout feed predominantly on zooplankton (Cadwallader & Backhouse, 1983 in Fishbase, 2003).
    Reproduction
    Sexual. Fertilisation is external, with the female trout excavating a hollow in streambed gravel for the eggs to be laid in. Between 700 and 4000 oarnge-red eggs are laid per spawning event. The male then fertilises the eggs and they are covered with a layer of gravel. This 'nest' is known as a redd (McDowall, 1990; FishBase, 2003).
    Lifecycle stages
    Lake fish usually spawn in lake tributaries, where the young trout feed and grow before migrating downstream after about a year. Growing to maturity in the lake takes around 2 to 4 years, at which time they migrate back to the tributaries to spawn. Most fish will return to the tributary in which they hatched (McDowall, 1990).
    Some lake populations may spawn in lake-shore gravels rather than travel into tributaries (ibid)
    This species has been nominated as among 100 of the "World's Worst" invaders
    Compiled by: IUCN SSC Invasive Species Specialist Group
    Updates with support from the Overseas Territories Environmental Programme (OTEP) project XOT603, a joint project with the Cayman Islands Government - Department of Environment
    Last Modified: Monday, 4 October 2010


ISSG Landcare Research NBII IUCN University of Auckland