The River Rainbow Trout

rainbow trout

A cock river rainbow approaching breeding condition. The coloration of each individual trout changes through the year. Reds, violets, greens and yellows turn a richer shade as the breeding season draws close.
Field study/pencil and wash

The river rainbow trout, so prized for its glittering iridescent sheen, shares most of its home waters with its sea-run relative, the steelhead, and both are classified Salmo gairdneri. In most waters, the steelheads dominate the resident rainbows both in size and number. However, in some rivers around Crescent Lake in Washington State and the Kamloops Lakes of British Columbia, and in the Apache, Gila and McCloud Rivers of California, the migratory form has been lost because the trout of these waters are prevented from running to sea by physical barriers, such as dams.

The natural range of the wild rainbow trout extends from the Kuskokwim River region of Alaska to the Baja California Peninsula and the coastal rivers of Mexico. Within this region, many of the rivers draining into the Pacific still hold a head of resident wild rainbow trout, including the MacKenzie River of Oregon, the McCIoud and Kern of California, Henry's Fork of the Snake River system in Idaho, the Babine River of British Columbia and the Kenai-Russia River system of Alaska. However, during .the last 100 years, rainbow trout have been introduced into countless other waters throughout the world. Today, many rivers close to the original range in the USA and Canada have stocks of rainbow trout; but these are not native wild trout stocks.

In rivers that have both resident river rainbows and steelhead trout, it can be difficult to distinguish between the two. When the adult steelhead returns to the river from its feeding sojourn in the Pacific it is steel-grey or silvery in colour, but a flash of green, red and blue soon appears and the steelheads merge with the river rainbow trout. Most anglers solve the problem by calling the large trout of such a river steelheads, while the smaller fish are designated rainbows. This is probably fair enough, for no river can produce the quantity of food available in the ocean, and few if any river rainbows reach the average size of steelheads.

River rainbow trout spawn in the late winter or spring, at the same time as the migratory steelheads. We do not know whether they ever interbreed. Certainly they do not in the rivers of Washington State and British Columbia, which produce the biggest steelheads. There the gravel, chosen by the steelheads when they cut their redds is too large for smaller rainbow trout, and often the flow in the reaches of the river used by the spawning steelheads is too strong for the smaller trout to mate. In other rivers they possibly do interbreed, for rainbow trout and steelheads have been reported on the same spawning gravel.

River rainbow trout are vigorous feeders. Unlike the river brown trout and cutthroat trout, which spend much of the non-breeding season in one lie and wait for the flow to bring food to them, the river rainbow will go in search of food. When there is little being carried by the stream, the trout will slowly cruise around a big pool or tack upstream, devouring anything that is edible. On one tiny feeder stream of the Alaskan Kenai, a feeding rainbow trout was observed covering almost a mile, over a period of two hours.

The river rainbow also takes a much wider range of foods. After the fry stage, the brown trout and cutthroat ignore tiny morsels like algae and the smallest insect larvae and pupae. Rainbow trout will take these throughout their lives, and they also gather a much greater proportion of their food from the river bed: stonefly and mayfly nymphs and caddis larvae. A short bout of feeding, either on bottom foods or on flies at the water surface, will satiate the brown and cutthroat trout, but not the rainbow.

As a result, river rainbows grow much faster than other trout in similarly productive waters. In rich English chalkstreams, brown trout reach between 12 and 14 inches in length only at three years of age. In the most productive rivers of Idaho and Montana, rainbow trout exceed these sizes at two years. One rainbow, caught from Jackson Hole, Wyoming, in 1976 scaled 6 lbs. at just four years of age: yet this fish had been feeding mainly on the alga Cladophora and the tiniest of caddis larvae and stone fly nymphs.

With fast growth, rainbows reach sexual maturity earlier than cutthroats and brown trout. Rainbows tend to spawn first when they reach about 12-16 inches in length, often at the end of their second year. Brown trout and cutthroats may not mature until their third or fourth year. However, few wild rainbow trout survive more than two or three spawnings: their-maximum life expectancy is about four or five years. River brown and cutthroat trout can reach eight, nine or even ten years.