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The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Oceanmarker in Asia and North America as well as much of the central, western, eastern, and especially the northern portions of the United States. The ocean going (anadromous) form (including those returning for spawning) are known as steelhead, or ocean trout (Australia and United States). The species has been introduced for food or sport to at least 45 countries, and every continent except Antarcticamarker. In some of these locations, such as Southern Europe, Australia and South America, they have had very serious negative impacts on upland native fish species, either by eating them, outcompeting them, transmitting contagious diseases, or hybridization with closely related species and subspecies that are native to western North America.

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, however these names became deprecated 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 ( ), the specific epithet mykiss derives from the local Kamchatkan name 'mykizha'; all of Walbaum's species names were based on Kamchatkan local names.

Life cycle

Illustration of a rainbow trout
Like salmon, steelhead are anadromous: they return to their original hatching ground to spawn. Steelhead rejuvenate after spawning, so they may return to the oceans to start the anadromous cycle once again. The steelhead smolts (immature or young fish) usually remain in the river for about a year before heading to sea, whereas salmon typically return to the seas as smolts. Different populations of steelheads 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. "Winter-run steelhead" mature fully in the ocean before migrating, between November and April, and spawn shortly after returning. Similar to Atlantic salmon, but unlike their Pacific Oncorhynchus kin, steelhead are iteroparous and may make several spawning trips between fresh and salt water. The maximum recorded life-span for a rainbow trout is 11 years. Salmon is often sold as a replacement because they taste similar.

Diet

Rainbow trout are predators with a varied diet, and will eat nearly anything they can grab, in contrast to the legendary, selective image people often have of the fish's dietary habits. Rainbows are not quite as piscivorous or aggressive as the brown trout or lake trout . When young, insects make up a large portion of the diet, as well as fish eggs, smaller fish (up to 1/3 of their length), along with crayfish and other crustaceans make up the remainder. 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 salmon, trout will eat varied fish eggs, to include salmon, cutthroat trout, as well as the eggs of other rainbow trout, alvein, fry, smolt and even salmon carcasses.

Length and Weight

As rainbow trout grow longer, they increase in weight. The relationship between length and weight is not linear. The relationship between total length (L, in inches) and total weight (W, in pounds) for nearly all species of fish can be expressed by an equation of the form:
W = cL^b\!\,


Invariably, 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 relationship described in this section suggests that a 13-inch lentic rainbow trout will weigh about 1.0 pound, while an 18-inch lentic rainbow trout will weigh about 2.5 pounds.

Fisheries

Rainbow trout


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

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 of farmed salmon trout were harvested with a value of 2.589 billion USD dollars. The largest producer is Chile. In Chile and Norway, the ocean cage production of steelhead has expanded to supply export markets. Inland production of rainbow trout to supply domestic markets has increased strongly in countries such as Italy, France, Germany, Denmark and Spain. Other significant producing countries include the USA, Iran, Germany and the UK.

There are tribal commercial fisheries for steelhead in the Puget Sound, the Washington Coast and in the Columbia River.

Threats and conservation

Steelhead trout have declined due to a number of human and natural causes. The U.S. National Marine Fisheries Service has a detailed description of threats. Steelhead that spawn in Southern California streams (south of Point Conceptionmarker) 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 U.S. National Marine Fisheries Service has identified 15 populations, called Distinct Population Segments(DPSs), of steelhead trout in Washingtonmarker, Oregonmarker 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 about which the National Marine Fisheries Service has some concerns regarding status and threats, but for which insufficient information is available to indicate a need to list the species under the ESA.

The rainbow trout is susceptible to enteric redmouth disease. There has been considerable research conducted on redmouth disease, as its implications for rainbow trout farmers are significant. The disease does not affect humans.

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

Steelhead

File:Lake Washington Ship Canal Fish Ladder pamphlet - ocean phase Steelhead.jpg|Male ocean phase steelheadFile:Lake Washington Ship Canal Fish Ladder pamphlet - male freshwater phase Steelhead.jpg|Male spawning phase steelheadFile:Jumping Salmon.jpg|Steelhead attempting to jump some rapidsImage:SteelheadRainbowTrout.jpg|Steelhead with clear spot pattern on fins and body

As food

Rainbow trout and steelhead are popular in Western cuisine and are both caught wild and farmed for food. 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 make it a point to ascertain the source of the fish before buying. Rainbow trout are raised in many countries throughout the world. Rainbow trout that are wild have a diet of 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.

Medicine

The sperm of rainbow trout contains protamine as does that of salmon and some other species of similar fish. Protamine sulphate is an "antidote" to the anticoagulant heparin. Originally protamine was isolated from fish sperm, but is now produced in the laboratory. Back when the pilgrims came to America native Americans used the skin of the rainbow trout as medicine.

Subspecies

A few populations are recognized as subspecies:

Cultivated varieties

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.

See also



Notes

References

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


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