Identification key end: Fish: Oncorhynchus mykiss

You have a Oncorhynchus mykiss


Species names: Oncorhynchus mykiss (Walbaum, 1792), Salmonidae

Synonyms: Fario gairdneri Richardson, 1836, Oncorhyncus kamloops Jordan, 1892, Parasalmo penshinensis Pallas, 1814, Salmo gairdneri Rchardson, 1836, Salmo gilberti Jordan, 1894, Salmo irideus Gibbons, 1855, Salmo kamloops Jordan, 1892, Salmo masoni Suckley, 1860, Salmo mykiss Walbaum,1792, Salmo nelsoni Evermann, 1908, Salmo penshinensis Pallas, 1814, Salmo purpuratus Pallas, 1814, Salmo rivularis Ayres, 1855, Salmo stellatus Girard, 1856, Salmo truncates Suckley, 1859 

Common names: Rainbow trout, Steelhead trout (GB), pstruh duhový (CZ), Regenbogenforelle, Stahlkopfforelle (DE), regnbueørred (DK), vikerforell (EE), Kirjolohi (FI), Regnbogasilungur (IS), Varavǐksnes forele (LV), Vaivorykštinus upetakis (LT).Regnbueaure, Regnbueørret (NO), Pstrag teçzowy (PL), Regnbåge, Radoozhnaya forel (RU), Regnbågelax (SE).



Oncorhynchus mykiss has a streamlined body with adipose fin. They are deep-bodied and sideways compressed. Average length of Oncorhynchus mykiss is 30-45 cm and the anadromous form steelhead 50-75 cm. Extremely large sea-run individuals can be more than 120 cm in length and above 25 kg in mass. The general body shape is typical for a trout, with a moderately large head and a mouth that extends back behind the eyes. The colours vary with habitat, size and sexual condition.



Photo: Oncorhynchus mykiss in nuptial dress. The surface is shiny because the scales are embedded in the skin (as extra armour) during spawning, photo by Bror Jonsson.

Oncorhynchus mykiss 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 O. mykiss that live well offshore in deep lakes or in small fish that have not yet spawned. Cheeks and opercula are pink, the body sides marked with a vague, pink blush to rose red band and a large number of rather small black spots, mostly restricted above the lateral line or scattered over the whole side. The belly and ventral surface of the head are whitish. Eyes have an olive to bronze colour. Sometimes a soft, metallic-pink colour is present along the sides of the body and the head (Scott and Crossman 1973).

When Oncorhynchus mykiss leave lakes to spawn in running waters, their colours become more intense. The pinkish stripe that is present on the sides of lake fish becomes a rich crimson colour, the fins attain 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. Oncorhynchus mykiss 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 (Scott and Crossman 1973).


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Native distribution: The native range is the Eastern Pacific Ocean and the freshwater, mainly west of the Rocky Mountains, from northwest Mexico (including extreme northern Baja, California), to the Kuskokwim River, Alaska. It is probably native in the drainages of the Peace and Athabasca rivers east of the Rocky Mountains (MacCrimmon 1971).

Introduced distribution: The first record of Oncorhynchus mykiss in Europe was in 1880 in the Czech Republic The first attempt to introduce Oncorhynchus mykiss into the British Isles was made in 1884 when eyed eggs were shipped from United States (MacCrimmon 1971). In 1887 a good breeding stock was established. Oncorhynchus mykiss were sent to Germany from Michigan in 1882 and dispersed to various hatcheries. From Germany it was spread to the Austrian-Hungarian Monarchy in 1884, Sweden in 1892, Denmark in 1894, Estonia in 1896 and to Finland in 1898. The first introductions of O. mykiss into Poland were probably between 1882 and 1889 through the effort of a German scientist. The species was imported to Norway and Iceland from Denmark in 1902 and 1951, respectively.

Vector: Oncorhynchus mykiss is introduced via aquaculture, and the populations are sustained by continuous releases and escapes from hatcheries and farms. Successful reproduction in nature is rare (Rask et al. 2000)


Oncorhynchus mykiss are primarily a freshwater fish, although sea-run populations, often known as steelhead, exist in some areas. Any population of O. mykiss is, however, capable of migrating to or surviving in the sea.
The species prefers well-oxygenated, clean fresh water. Optimal temperature for growth is 17 °C, preferred temperature for spawning is around 12°C, and it tolerates temperature in the range of 0 °C to 25 °C. O. mykiss tend to thrive better in lakes than in streams or rivers, although large fish are often present in remote headwaters (McDowall 1990).

Reproduction: The reproduction of Oncorhynchus mykiss is sexual. Fertilisation is external, with the female trout excavating a hollow in streambed gravel for the eggs to be laid in winter, spring or early summer. Between 700 and 4000 orange-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 red. The fecundity is approximately 2000 eggs per 1 kg somatic mass of females. Egg diameter is ca. 5 mm (Scott and Crossman 1973).

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. Some lake populations may spawn in lake-shore gravels rather than travel into tributaries. Maximum recorded age is 11 years. In Norway, there appears to be no specific homing behaviour of released hatchery fish, contrasting observations in its original area (Jonsson et al. 1993a). The reason for this is unknown.



Impacts include hybridisation, disease transmission, predation and competition with native species. Furthermore the massive use of the species in aquaculture may in some areas (e.g. in Denmark) lead to pollution with surplus food as well as to problems with the free passage of native species in the affected rivers (Jensen and Rasmussen 1988).

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 Oncorhynchus mykiss into the wild has caused outbreaks of this disease in the United States, threatening wild fish populations (Gilbert and Granath 2003). This disease may be also one of the reasons why O. mykiss have difficulty establish self-sustained populations in Europe, although it is believed that other causes may also cause the problem. In Poland, the species is not self-reproducing although whirling disease has not been observed during the last 50 years.

Oncorhynchus mykiss feed on zoobenthos, zooplankton and fish, and in many countries introduced O. mykiss have been reported to have negative effects on native fish, amphibians and invertebrates. In New Zealand it is suspected that Oncorhynchus mykiss 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. Little is known about negative effects on the native European fauna.

Oncorhynchus mykiss often spawn in the spawning areas of brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) in spring when their spawn (eggs or alevins) are in the gravel nests (Jonsson et al. 1993a). When Oncorhynchus mykiss spawn they may destroy the eggs of the native trout and salmon.

Genetic effects: In the United States, the introduction of O. mykiss 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) have become virtually extinct because of this. Species also known to be affected by hybridization include the Lahontan cutthroat trout (O. clarki henshawi); golden trout (O. aquabonita); 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).

There seems to be little genetic effect (other than through ecological competition) on European salmonids because of a general difference in spawning time. There are fall spawning strains in fish farms in Europe which give viable triploid crosses with brook trout.

There seems to be little genetic effect (other than through ecological competition) on European salmonids because of a general difference in spawning time. There are fall spawning strains in fish farms in Europe which give viable triploid crosses with brook trout.


Since successful spawning is very rare outside its original range, there is usually no need for a special management of the species. However, the building of inland hatcheries and fish ponds and the production and release of Oncorhynchus mykiss is restricted, not the least because of the disease threat to wild fish caused by infected Oncorhynchus mykiss escaping from such facilities. It can carry parasites such as sea lice(Lepeophtheirus salmonis) as well as Gyrodactylus salaris, which are lethal to Atlantic salmon.

Literature on Oncorhynchus mykiss

 Baruš V. and Oliva O. 1995. Fauna of the Czech and Slovak Republics. Volume 28/2. Fishes - Osteichthyes. Academia, Praha. (in Czech with English summary).

Gilbert, M.A. and Granath, W.O. 2003. Whirling disease of salmonid fish: life cycle, biology, and disease. Journal of Parasitology 89: 658-667.

Hindar, Kl, Fleming, I.A. Jonsson, N., Breistein, J., Sægrov, H., Karlsbakk, E., Gammelsæter, M. and Dønnum, B.O. 1996. Regnbueørret i Norge: forekomst, reproduksjon og etablering. – NINA Oppdragsmelding 454, 1-32.

IUCN list of Threatened Species:

Jensen F. and Rasmussen, G. 1988. Regnbueørred (rainbow trout). Natur og Museum 27(4) pp 31.

Jonsson, N, Jonsson, B., Hansen, L.P. and Aass, P. 1993a. Coastal movement and growth of domesticated rainbow trout (Oncorhynchus mykiss ((Walbaum)) in Norway. Ecology of Freshwater Fishes 2, 152-159.

Jonsson, N., Jonsson, B., Hansen, L.P. and Aass, P. 1993b. Potential for sea ranching rainbow trout Oncorhynchus mykiss (Walbaum); evidence from trials in two Norwegian fjords. - Aquaculture and Fisheries Management 24: 653-661.

Larsen, K. L. 1983. Selvreproducerende regnbueørreder i danske vandløb. – Sportsfiskeren 58(2): 4-5.

Larsen, K.L. 1984. Populationsdynamiske undersøgelser over ørred (Salmo trutta L.) og regnbueørred (Salmo gairdneri Rich.) i tilløb til Hald Sø. Specialerapport. Aarhus Universitet.

MacCrimmon, H.R. 1971. World Distribution of rainbow trout (Salmo gairdneri). - Journal of Fisheries Research Board of Canada 28, 663-704.

McDowall, R.M. 1990. New Zealand freshwater fishes: a natural history and guide. Heinemann Reed, Auckland, 533 pp.

Nehring, S. 2006. Invasionen in limnischen und marinen Gewässern: Situation, Besonderheiten, Probleme, Lösungen. Naturschutz und Biologische Vielfalt, in press.

Rask, M., Appelberg, M., Hesthagen, T., Tammi, J., Beier, U. and Lappalainen, A. 2000. Fish Status Survey of Nordic Lakes – species composition, distribution, effects of environmental changes. TemaNord 2000:508.

Rasmussen, G. 2012. Regnbueørred. Carl, H. & Møller, P. R. (red.). Atlas over danske ferskvandsfisk. Statens Naturhistoriske Museum. 413-428.

Scott, W.B. and Crossman, E.J. 1973. Freshwater fishes of Canada. Fisheries Research Board of Canada Bulletin 184: 1-966.