Rainbow Trout (Oncorhynchus mykiss)

The rainbow trout is one of the most widely distributed freshwater fish and the one member of the Salmonidae family that presently has global distribution. Endemic to western North America, it was reclassified from the trout genus Salmo to the Pacific salmon genus Oncorhynchus (it was formerly identified as Salmo gairdneri) and occurs in both freshwater resident and anadromous, or sea-run, races (see: Steelhead).

One landlocked variety of rainbow trout from the interior of British Columbia is called the Kamloops trout, a genetically large strain called Gerrard trout exists in British Columbia’s Kootenay Lake and its Lardeau River tributary, and there are many other variations (as well as hatchery-created hybrids) of rainbows known.

Identification

The rainbow trout possesses the typical elongated and streamlined salmonid form, although body shape and coloration vary widely and reflect habitat, age, sex, and degree of maturity. The body shape may range from slender to thick. The back may shade from blue-green to olive. There is a reddish-pink band along each side about the midline that may range from faint to radiant. The lower sides are usually silver, fading to pure white beneath.


The rainbow has numerous prominent black spots that may cover the entire body or may be more abundant near the tail. The spots characteristically extend onto the dorsal fin, the adipose fin, and the tail. Those on the tail radiate outward in an even, orderly pattern. Spots may be present on any of the lower fins. Rainbow trout are positively identified by the 8 to 12 rays in the anal fin, a mouth that does not extend past the back of the eye, and the lack of teeth at the base of the tongue.

Coloration varies greatly with size, habitat, and spawning periods. Stream dwellers and spawners usually show the darkest and most vivid colors and markings. River or stream residents normally display the most intense pink stripe coloration and the heaviest spotting, followed by rainbows from lake and lake-stream systems.

Size/Age

In general, stream-dwelling rainbows commonly weigh a pound or so, whereas fish from larger rivers and lakes commonly weigh between 2 and 4 pounds. Rainbows that have migrated to a large inland lake, such as one of the Great Lakes, may attain double-digit weights, although most weigh 7 to 10 pounds, and sea-run fish likewise become heavyweights.

The largest nonanadromous rainbow trout in North America presently come from Alaska and British Columbia waters. World records are kept for all varieties of rainbow trout as one species, meaning that the anadromous form dominates the record books, including specimens from 20 to more than 30 pounds. They can live for 11 years but typically have a 4- to 6-year life span.

Life history/Behavior

Most varieties of rainbow trout spawn in the spring in small tributaries of rivers or in inlets or outlets of lakes. Spawning frequency ranges from annually to once every 3 years. Rainbow trout usually return to the streams where they hatched.

During the late winter or the early spring, when water temperatures are on the rise, maturing adult rainbows usually seek out shallow gravel riffles in their stream or a suitable clear-water stream that enters their lake. Spawning takes place from the late winter or the early spring through the early summer.

The female uses her tail to prepare a nest (redd) 4 to 12 inches deep and 10 to 15 inches in diameter. Eggs are deposited in the redd, fertilized by a male, and covered with gravel. Hatching normally occurs from a few weeks to 4 months after spawning, depending on the water temperature.

Small trout assemble in groups and seek shelter along the stream margins or protected lakeshore, feeding on crustaceans, plant material, and aquatic insects and their larvae. They rear in similar habitats for the first 2 or 3 years, then move into the larger water of lakes and streams and turn more to a diet of fish, salmon carcasses, eggs, and even small mammals.

Food

Rainbows feed on a variety of food, mainly insects, crustaceans, snails, leeches, and other fish, if available.

Other Names

steelhead, rainbow, ’bow, redsides, Kamloops, redband trout, Eagle Lake trout, Kern River trout, Shasta trout, San Gorgonio trout, Nelson trout, Whitney trout, silver trout; Danish: regnbueørred; Finnish: kirjolohi; French: truite-arc-en-ciel; German: regenbogenforelle; Italian: trota iridea; Japanese: nijimasu; Russian: forel raduzhnaya; Spanish: trucha arco iris; Swedish: regnbåge; Turkish: alabalik türü.

Distribution

The rainbow trout is native to the West Coast of North America from southern Alaska to Durango, Mexico, and inland as far as central Alberta in Canada and Idaho and Nevada in the United States.

It has been extensively introduced across the lower Canadian provinces, throughout the Great Lakes region and the northeastern United States to the Atlantic coast and south through the Appalachians to northern Georgia and Alabama, in the western United States easterly to western Texas, and sporadically in the central United States south of the Great Lakes.

Habitat

Although rainbows do well in large lakes with cool, deep waters, they prefer moderately flowing streams with abundant cover and deep pools. In most streams they are found in stretches of swift-flowing water, at the edge of strong currents, and at the head of rapids or strong riffles. They prefer water temperatures of 55° to 64°F but can tolerate water to 70°F.

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