Oncorhynchus mykiss
rainbow trout
(Also: redband trout)

Oncorhynchus mykiss are only native to the Pacific Coast of North America, extending from Alaska down to the border between California and Mexico. However, they have been introduced throughout the United States. and in every continent except for Antarctica for game fishing purposes. There are two forms: freshwater resident and anadromous. The resident form is commonly called rainbow trout while the anadromous form is called steelhead. (Delaney, 2005; NOAA Fisheries Office of Protected Resources, 1998)

Freshwater, brackish, or marine waters of temperate zones. The anadromous form, called steelhead, spawn and complete their early development in freshwater mountain streams, then migrate to spend their adult life in the ocean. In freshwater, they prefer cool water but have been known to tolerate water temperatures up to 24°C (native climates have water temperatures around 12°C in the summer). Productive streams have a good mixture of riffles and pools and overhanging vegetation for shade. Most importantly, they require gravel beds to lay their eggs, and therefore, are sensitive to sedimentation and channel scouring. Juvenile trout prefer protective cover and low velocity water and have been known to be swept away and killed in water that is too fast. Since they are native to the western U.S., then tend to be found in coastal streams and rivers which naturally have reduced flow in summer months. (Behnke, 1992; Gall and Crandell, 1992; Ohio Department of Natural Resources and Division of Wildlife, 2005)

Physical description varies widely with sex, age, and habitat. In general, they are streamlined, with 8 to 12 spines in the anal fin and lack teeth at the base of the tongue (unlike their close relatives, Oncorhynchus clarkii). The undersides tend to be silvery with a pinkish red stripe along the upper-middle part of the body, though this stripe can vary from dark to light. Resident rainbows and spawning steelhead tend to be lighter with more pronounced pink stripes, while ocean-going steelhead are darker and silvery to blend into their ocean environment. Most have black spots above the lateral line, and resident rainbows tend to have more intense spotting, well below the lateral line. Juvenile fish have 8 to 13 parr marks on their sides and become silvery as they mature. (Delaney, 2005; Gall and Crandell, 1992; Klontz, 1991; Van Hulle, 2005)

Oncorhynchus mykiss larvae go through a series of morphological changes to prepare for life in the sea, and spend their adult life there for 2 to 3 years before migrating upstream to spawn in their natal stream. ("The Life Histories of the Steelhead Rainbow Trout and Silver Salmon", 1954; Thrower, Hard, and Joyce, 2004)

Oncorhynchus mykiss individuals live for 6 to 8 years in the wild, possibly up to 11 years. (Gall and Crandell, 1992; Michigan Department of Natural Resources, 2005)

Steelhead and rainbow trout are solitary fish, leaving the group of juveniles once they have hatched from eggs. As adults, they compete with all kinds of trout and salmon for food and habitat. The largest trout tend to get the best habitat. Adult steelhead have a remarkable homing instinct and consistently return to their natal stream to spawn. Steelhead have been known to migrate thousands of kilometers between the ocean and their natal stream to spawn. Migration ranges have been severely cut due to excessive damming of most western rivers and streams. ("The Life Histories of the Steelhead Rainbow Trout and Silver Salmon", 1954; Alexander, 1991; Behnke, 1992)

Home Range

Resident rainbow trout maintain small territories but also disperse from areas with higher population densities in order to find food. ("The Life Histories of the Steelhead Rainbow Trout and Silver Salmon", 1954; Behnke, 1992)

There is little communication between rainbow trout and steelhead. Once the fry emerge from the gravel, they become hostile to each other and compete for habitat. Larger fish usually win out the best habitat and food sources, and there is a size hierarchy within aquatic systems among all trout species. Potential mates communicate before spawning with visual cues. Oncorhynchus mykiss individuals are visual predators, relying on a keen sense of vision to detect prey. Trout species use both chemical cues and detection of the earth's magnetic fields to navigate to and from natal streams and on ocean journeys. (Grubb, 2003)

Rainbow trout and steelhead are insectivorous and piscivorous. Resident rainbow trout tend to eat more fish than steelhead. Both species primarily feed on invertebrate larvae drifting in mid-water to conserve energy that would be expended if they were foraging for food in the substrate. Young rainbow trout and steelhead eat insect larvae, crustaceans, other aquatic invertebrates, and algae. (Behnke, 1992; Delaney, 2005; Klontz, 1991; Michigan Department of Natural Resources, 2005; Smith, 1991; Van Hulle, 2005)

In the Great Lakes, sea lampreys are the most common predators of all salmonid species, including rainbow trout. Other predators in both native and introduced habitats include: larger trout, fish-eating birds like great blue herons (Ardea herodias), mergansers (Mergus), and kingfishers (Ceryle), and mammals including mink (Neovison vison and Mustela lutreola), raccoons (Procyon lotor), river otters (Lontra), grizzly bears (Ursus arctos), American black bears (Ursus americanus), humans, and larger marine mammals who feed on migrating steelhead. Rainbow trout tend to stick to the sides of streams and rivers where shading is prevalent, the water is less swift, and protection is greatest. Trout species are vigilant and capable of rapid swimming to escape predation. (Michigan Department of Natural Resources, 2005; Smith, 1991)

Rainbow trout and steelhead are important predators in their native habitats, they also serve as important sources of food for larger predators. (Smith, 1991)

Rainbow trout have been introduced throughout the world, negatively impacting species of native freshwater fishes and, therefore, native fisheries.

These fish are one of the most popular game fishes around the world, leading to nearly global introduction. They are introduced to stimulate local angling and associated recreational economies. However, where they are introduced, they can outcompete native trout species. (Michigan Department of Natural Resources, 2005; NOAA Fisheries Office of Protected Resources, 1998; Ohio Department of Natural Resources and Division of Wildlife, 2005)

Steelhead are endangered in Washington and California, and threatened in California, Oregon, Washington, and Idaho. Most of their decline has resulted from impacts to habitat and shrinking of spawning routes due to dams and other diversions. Siltation, caused by forestry practices, and erosion, caused by urban and agricultural development, has also impacted spawning beds. (Behnke, 1992; Delaney, 2005; NOAA Fisheries Office of Protected Resources, 1998; Van Hulle, 2005)