Rainbows were originally found in the Western rivers of the United States, specifically those West of the Sierra Nevadas, as well as tributaries of the Snake River in Idaho. This geographic isolation in the United States allowed the species to evolve separately from their cousin the Cutthroat, some of which were literally just on the other side of the ridge line. As well, the big brothers in this family, the Steelhead have a range that extends to the Kamchatka territory of Russia. In fact the species epithet of the Rainbow trout, mykiss, is derived from the Kamchatkan word for “fish”-mykizha. While the Rainbow is an American fish, we owe its scientific name to the fact that this fish has the perseverance to fight against its environment and make it across an ocean.
The genus oncorhynchus describes it as a member of the pacific trout group, which also includes the Cutthroat Trout. The native range of Rainbow Trout includes the states of California, Oregon, Washington, Washington, Nevada, and Idaho. Various Rainbow subspecies have been introduced to Eastern and Western water systems that do not have native populations. And it is this introduction in waters that have native populations of Cutthroats that is displacing that native fish, either by outright competition or by hybridization.
Due to the various subspecies of Rainbow it is difficult to give accurate descriptions of each, however, there are some characteristics that each fly-fisherman should be aware of. Rainbows can vary in size from 6-40 inches in length and in weight from mere ounces to 20 pounds. All of these size variations are due to the various environments that the Rainbow is found in. Stream fish are of course smaller than those found in larger rivers and stillwaters.
Rainbow spawning takes place anywhere from February to early July, depending on geographic placement of the subspecies. Water temperature is key to the spawn, it will begin when water temps reach between 43*-46*. Rainbow spawn in small to moderately large streams and rivers that are clear, well oxygenated, and have a pebble or fine gravel bottom. The female will construct a redd in the gravel and then lay anywhere between 2,000-4,000 eggs. After birth, the juvenile Rainbows take about 3-5 years to mature.
While the native range of the Rainbow was along the Pacific Coast of the United States and Canada, it has been introduced into every State of the Union. Even California has a small population and Steelhead have been introduced into the Great Lakes region, giving those fishermen there the opportunity to challenge these heavyweight fighters. Regardless of their home, Rainbows prefer to live in cold, clear, well-oxygenated, shallow rivers with gravel bottoms, or cold, moderately deep lakes. Stream-side vegetation that reduces erosion is great Cutthroat habitat.
Generally, the Rainbow Trout is looked upon as being a healthy fish, and one that is not as in danger of extinction as the Cutthroat, however, there are ecological concerns for the Rainbow Trout population. First, native ranges of this trout are being lost due to human growth, pollution, and farming. Especially the Steelhead, it has been listed as either endangered, threatened, or listed as a species of concern depending on the governmental entity that is listing them. Another concern is Whirling Disease, of which the Rainbow is highly susceptible. This single disease, or more truthfully the parasite that causes it, can lead to the decline and even complete elimination of populations of fish. Hybridization with the Cutthroat, while a major concern for the Cutthroat populations, is still a concern for the Rainbows. Slowly, both fish are mating to their mutual extinction. Ironically, however, these hybridized "Cutbows," seem to have a natural resistance to whirling disease, and unlike other hybridized species, they are fertile.
What does this mean for the fly-fisherman. First, fish for Rainbows as you would any other trout species in the United States. Use nymphs, wet flies, dries, streamers, whatever you have in your trout arsenal and you'll more than likely catch a Bow if you're fishing in waters that hold them. What we need to do is make sure that we're fishing in a manner that is in the best interest of our quarry. If practicing Catch and Release, make sure everything that you are doing is going to minimize loss and maximize healthy return.
There is nothing like catching a wild rainbow from your favorite stream. Where the Cutthroat is ready to take a dry fly on the surface, the Rainbow is more than happy to put a tug on your line and don't be surprised when that fish you expected to be 20 inches turns into something closer to 12. As always, good luck and guid luck!