Fish Facts: Rainbow Trout (Oncorhynchus mykiss)

Alaska is home to some of the largest native rainbows in the world.
Photo courtesy Crystal Creek Lodge

Glorified as spectacular game fish, vilified as an invasive species, and ridiculed as cookie-cutter hatchery clones, rainbow trout are reflections of man’s relationship to nature.

The most widely cultivated trout species in the world, rainbow trout (Oncorhynchus mykiss) are bred for sport and food, and they inhabit lakes and streams on every continent except Antarctica. For many anglers, their first trout caught on a worm, lure, or fly was a hatchery-bred rainbow stocked by the state just days or weeks earlier. Yet wild rainbows, within their native range or in a foreign land, are revered for their tenacious fight, which often includes multiple leaps. The fact that they are considered less wary and easier to catch than brown trout has also helped to endear rainbows to anglers. But anyone who has sight-fished to big rainbows in a Montana spring creek or crystal-clear New Zealand river knows that the species can be maddeningly selective and easy to spook.

What’s in a Name?
Europeans first encountered rainbow trout on Russia’s Kamchatka Peninsula, and based on these specimens, German naturalist Johann Julius Walbaum named the species Oncorhynchus mykiss in 1792. Oncorhynchus means “hook nosed,” describing the kype of spawning males, and mykiss is derived from the Kamchatkan word for the fish, mykizha. Some 44 years later, Scottish biologist Sir John Richardson named a trout found in the Columbia River Salmo gairdneri—after Meredith Gairdner, who had provided the specimen—and this was the most commonly used Latin name in angling literature for the next century and a half. Then, in 1989, genetic studies determined that rainbows and cutthroat trout were much more closely related to Pacific salmon (genus Oncorhynchus) than to brown trout and Atlantic salmon (genus Salmo). Thus, Walbaum’s original name was restored.

This beautiful fish was caught in the Spanish Pyrenees, far outside the fish’s original range.
Photo by Sandy Hays

World Domination
In the spring of 1875, rainbow trout eggs made the journey by train from California to New York—the first time the species had been shipped out of its native range. Because they are so easy to raise in hatcheries, can survive in a wide variety of environments, and offer such great sport, rainbow trout were over the next century shipped to all four corners of the globe. The Global Invasive Species Database lists 87 countries where O. mykiss can be found. The reason that rainbows are on that database is that introduction of the species has caused declines in native fishes—through hybridization, predation, and competition—as well as spreading whirling disease. Anyone interested in the history of O. mykiss in the U.S. and the world should read An Entirely Synthetic Fish, by Anders Halverson.

Range and Species History
The native range of the rainbow trout is a fairly narrow band along the western coast of North America, from the southern tributaries of Alaska’s Kuskokwim River in the north to the mountains of northern Mexico in the south. On the other side of the Pacific, the range includes the Kamchatka Peninsula and surrounding waters. Subspecies of O. mykiss  in North America include steelhead (O. m. irideus), Columbia River redband trout (O. m. gairneri), Great Basin redband trout (O. m. gibbsi), three Kern River subspecies, and several Mexican subspecies. You can now catch rainbows in such exotic locations as the South Island of New Zealand, the Highlands of Lesotho in southern Africa, the mountains of Costa Rica in Central America, and the Himalayas of Bhutan in Asia. They are also found across Europe and South America.

The light-brown shading represents the original range of the species in North America.
Photo via Wikipedia

Although they were once thought to be a different species, steelhead (or coastal rainbow trout) are now considered the same species as rainbows, but with a different life history. The propensity for migrating to the ocean or a large lake is not a choice, however, but is hard-wired into the fish. Studies of rivers where both anadromous and resident rainbows co-exist have found that the offspring of anadromous fish become anadromous, while those of resident fish stay put. Why some members of the species evolved to migrate, while others didn’t, remains a mystery.

Rainbow trout can thrive in a wide variety of waters, from coastal estuaries to freestone streams to coldwater lakes, but generally they prefer clear, cold water and complex habitat—a mix of riffles, runs, and pools. Because even resident fish tend to migrate widely within a stream system, from the lower river to headwaters and back, they can find suitable habitat in almost any conditions. Rainbows do not spawn in lakes, so self-sustaining populations require tributaries with gravelly bottoms for natural reproduction. Where they share a system with other migratory species, such as Pacific salmon, rainbows will often follow spawning fish upstream to feat on the eggs and/or flesh.

Rainbows spawn in late winter or spring, depending on latitude, usually between March and May. Offspring will stay in the spawning habitat for one or two years before migrating downstream to a larger stream or lake.

Steelhead are just rainbows that have an anadromous life history.
Photo courtesy Montana Angler

Flies and Tactics
Depending on where they live, rainbow trout can grow to enormous proportions, but the fish that is recognized as the world record by the International Game Fish Association is the subject of much debate. The 48-pound rainbow caught from Canada’s Lake Diefenbaker was a triploid, a genetically modified fish that had escaped from a nearby fish farm. Because triploids are sterile, they grow faster and larger than others of the same species.

Rainbow trout are less pisciverous than brown trout, so they are more likely to rise to a dry fly or take a nymph. Even as they grow larger and begin to focus more on food that swims, they continue to feed heavily on bugs. Rainbows will hold in faster currents than browns will, including riffles and heavy pocket water.