Oncorhynchus mkyiss (Walbaum, 1792): rainbow trout

Biological Description

Body elongate, moderately compressed, and rather deep; scales cycloid 120-160 in the lateral line. Head short; snout rounded; eye moderate; mouth terminal; small sharp teeth on jaws and tongue, poorly developed on vomer and absent from basibranchial plate between lower ends of gill arches. Dorsal fin rays 11; anal fin rays 11; caudal fin slightly forked. Color variable with habitat, normally bluish to olive green on back side; side lighter and silvery with a reddish horizontal band; belly white or silvery; irregular black spots on the back, side, and head; dorsal, adipose, and caudal fins also spotted; the pelvic and anal fins usually with a narrow white border, most notable near the tip.

Rainbow trout and steelhead are distinguished from cutthroat trout by the lack of the red-orange "slash marks" on the undersides of the lower jaw and the basibranchial (hyoid) teeth behind the tongue that are usually present in cutthroat. The name "rainbow" comes from the reddish stripe which is often, but not always, present along the side. Steelhead are uniformly silvery until they darken toward spawning time.

Life History

Sexual maturity is reached in two or three years, but under favorable hatchery conditions, they often spawn at one year of age. Females produce approximately 1,000 eggs per pound of body weight. Spawning takes place in early spring from March through June, however hatchery strains have been developed that will spawn any month of the year. They are basically stream spawners and usually search out the small tributaries where gravel riffles are abundant. Spawning by winter-run steelhead occurs in early spring within a month or two after the fish arrive in the home stream. Summer-run fish also spawn in the spring, but are in fresh water for a longer period of time (up to six months). Spawning behavior is similar to that of salmon. The female digs a redd, successively digging, spawning, and resting as she moves upstream. The redd covers up to 6.5 square yards of bottom. The eggs are covered with several inches to a foot of gravel. Eggs spawned first are usually covered with the most gravel. Like cutthroat, there is a fairly heavy post-spawning mortality of adult females. In steelhead it approaches 100 percent; those that survive usually return to the sea in a short time. The eggs hatch in four to seven weeks, depending on water temperature. Although up to 95 percent of the eggs are fertilized, only about 65 to 85 percent survive the embryonic stage. The principal loss of developing eggs is probably due to suffocation by silt. After hatching, the young alevins remain in the peripheral waters of pools until they become large enough to maintain themselves in the current of riffles. The growth of rainbow trout varies greatly, depending upon conditions such as water temperature, water chemistry, and food supply. Water chemistry is affected by the geological characteristics of the area. Generally, limestone or sandstone in a drainage will produce more fertile trout water than granite or basalt. The growth of steelhead increases dramatically when they move into salt water.

Steelhead juveniles begin to migrate to sea when one year old, but most stay in fresh water for two years, and a few stay three years. They occupy riffle areas in summer and pools during the other seasons. Generally they are associated with the stream bottom. Seaward migration depends on such factors as fish size and time of year. Somewhere in their range, juvenile steelhead will be migrating to sea during each month of the year. The stocking location, time of stocking, and size of hatchery fish all have a bearing on survival of adults. In a California study, age at seaward migration of hatchery-reared juvenile steelhead had a profound effect on the survival to adulthood: age one at migration, 2.5 percent survival; age two, 6 percent; and age three, 18 percent. Probably the larger wild fish also have better survival rates than the smaller ones.

Virtually nothing is known about steelhead migrations or habits in the oceans. During surveys for salmon, Washington and Oregon steelhead have been taken scattered westward as far as 160° W, northward to the Gulf of Alaska, and westward along the Aleutians to 175° W.

Geographic Distribution

The rainbow trout is native to the Pacific Coast from Alaska to northern Mexico. Steelhead, the anadromous form of this species, originally was native to the Snake River and its tributaries upstream to Shoshone Falls. Dam construction for irrigation and power has now reduced its range to that portion of the Snake River below Hells Canyon Dam and the Salmon and Clearwater drainages. Human activities have eliminated the anadromous form south of San Francisco. There is considerable residualism among steelhead and these fish remain as residents in fresh water.

Habits and Habitats

Rainbow trout prefer cool water, less than 70° F, with plenty of oxygen, although they can survive from 32° F up to 80° F. In lakes where surface waters warm above 70° F in the summer, trout will move to deeper, cooler water if the oxygen content is sufficient there The rainbow is tolerant of a wide range of salinities. Steelhead commence life in streams pure as rain water and after a year or two migrate into sea water. The migratory urge is hereditary. Some stocks have few or no individuals that will migrate to sea.

Steelhead entering Idaho waters are of two races and are classed as A and B Group steelhead. The A Group enter both the Clearwater and the Salmon River systems and are quite generally widespread within each system. The B Group steelhead have a more restricted range. They are confined primarily to the North and Middle Forks of the Salmon River. The average size range of A Group steelhead is 4-8 pounds and B Group steelhead is 12 to 20 pounds. A Group steelhead enter the Columbia River earlier than the B Group. Both groups spend two or three years in fresh water and one to three years in the ocean. The life span of rainbow is fairly short and few fish live beyond five or six years of age.


Rainbow trout and juvenile steelhead feed primarily on foods that associated with the bottom; examples are aquatic insects (Diptera, mayflies, stoneflies, and beetle larvae), amphipods, aquatic worms, and fish eggs. Occasionally they eat small fish. The diet changes seasonally, depending on fluctuations in the availability of food items. In the ocean, steelhead eat crustaceans such as amphipods as well as squid, herring, and other fishes.

Idaho Conservation Status

 Idaho Native or Import


Through fish cultural practices, the rainbow has been widely distributed in many lakes and streams throughout the state of Idaho and other states where there are suitable lakes and streams. The form known as kamloops was introduced into Lake Pend Oreille from British Columbia in 1941. That introduction proved highly successful, probably due in part to the large population of kokanee in the lake.


The rainbow trout is the most important game fish in Idaho; the steelhead is one of the most highly regarded game fishes in the Pacific Northwest. It responds well to fish cultural practices and large numbers are planted as fingerlings and catchable-size fish each year. It responds well to all types of fishing tackle and it is the fly fisherman's delight as it takes a fly without indecision, fights hard at the surface, and often makes several leaps. It is the major species of fish raised commercially and Idaho is the largest commercial trout producer in the United States. Each year millions of pounds are marketed as fresh frozen fish. A lesser amount is sold for stocking in catch-out ponds in other states.

The state record fish for each strain is: resident rainbow: Hayden Lake, 1947, 19 pounds; kamloops: Pend Oreille Lake, 1947, 37 pounds; and steelhead, Clearwater River near Lewiston, 1973, 30 pounds.


Simpson and Wallace 1982.Wydoski and Whitney 1979. Image Copyright Joseph Tomelleri. Used by permission.