BioKIDS home

Kids' Inquiry of Diverse Species

Coast rainbow trout

Oncorhynchus mykiss

What do they look like?

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)

  • Sexual Dimorphism
  • male larger
  • Range mass
    25.4 (high) kg
    55.95 (high) lb
  • Average mass
    4 kg
    8.81 lb
  • Range length
    120 (high) cm
    47.24 (high) in
  • Range basal metabolic rate
    0.6 to 75 cm3.O2/g/hr
  • Average basal metabolic rate
    55 cm3.O2/g/hr

Where do they live?

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; "Oregon Coast Steelhead Evolutionary Significant Unit", 1998)

What kind of habitat do they need?

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; "Life History Notes: Rainbow Trout", 2005)

  • Range elevation
    0 to 3000 m
    0.00 to 9842.52 ft
  • Range depth
    10 to 200 m
    32.81 to 656.17 ft

How do they grow?

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, et al., 2004)

How do they reproduce?

Female fish find suitable nest sites while their male mate guards the site from other interested males and predators. The female digs the nest (called a redd) with her anal fin and then descends upon it to position her vent and anal fin into the deepest part of the redd. The male joins her in a parallel position so that their vents are opposite each other. The male and female open their mouths, arch their backs, and deposit the eggs and milt (fish sperm) at the same time. The eggs are enveloped in a cloud of milt and are fertilized. Only a few seconds elapse from the time the female drops into the redd and fertilization occurs. The female then covers the nest with gravel and repeats the process again a few times until she has deposited all of her eggs. ("The Life Histories of the Steelhead Rainbow Trout and Silver Salmon", 1954)

Adult rainbow trout and steelhead lay their eggs in a series of nests in gravel. Collectively, the nests are called a redd. When they hatch, the hatchlings are still attached to, and survive on their yok sac. They remain in the protective gravel for about 2 to 3 weeks when they have shed their yolk sacs and are fit enough to survive in the open water. Juvenile fish tend to stick to shallow and side areas of the streams where there is protective cover and slow-moving currents. The remain in their native streams for 1 to 3 years while they grow fit enough to spawn or migrate to the ocean, in the case of steelheads. ("The Life Histories of the Steelhead Rainbow Trout and Silver Salmon", 1954; Behnke, 1992; Delaney, 2005; Thrower, et al., 2004)

  • How often does reproduction occur?
    Rainbow trout breed every three to five years. Though steelhead are one of the only salmonids able to spawn twice in a lifetime, the return rate is very low, about 10-20%
  • Breeding season
    Spawning occurs from March to July, depending on temperature and other climatic variables. Winter steelhead in California start spawning as early as January.
  • Range number of offspring
    200 to 8000
  • Average number of offspring
  • Range time to hatching
    3 to 16 weeks
  • Range time to independence
    one to three years
  • Range age at sexual or reproductive maturity (female)
    3 to 11 years
  • Range age at sexual or reproductive maturity (male)
    3 to 11 years

Female rainbow trout and steelehead simply lay their eggs in a gravel bed and leave the young hatchlings to mature on their own. Male steelhead frequently breed with multiple female partners, possibly because more females than males die during the breeding period. (Delaney, 2005)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • protecting
      • female
  • inherits maternal/paternal territory

How long do they live?

Oncorhynchus mykiss individuals live for 6 to 8 years in the wild, possibly up to 11 years. (Gall and Crandell, 1992; "Steelhead: Oncorhynchus Mykiss", 2005)

  • Range lifespan
    Status: wild
    11 (high) years
  • Typical lifespan
    Status: wild
    6 to 8 years

How do they behave?

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)

  • Range territory size
    10 to 5000 km^2

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)

How do they communicate with each other?

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)

What do they eat?

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; "Steelhead: Oncorhynchus Mykiss", 2005; Smith, 1991; Van Hulle, 2005)

  • Animal Foods
  • fish
  • insects
  • aquatic or marine worms
  • aquatic crustaceans
  • Plant Foods
  • algae

What eats them and how do they avoid being eaten?

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. ("Steelhead: Oncorhynchus Mykiss", 2005; Smith, 1991)

What roles do they have in the ecosystem?

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

Commensal or parasitic species (or larger taxonomic groups) that use this species as a host

Do they cause problems?

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

How do they interact with us?

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. ("Steelhead: Oncorhynchus Mykiss", 2005; "Oregon Coast Steelhead Evolutionary Significant Unit", 1998; "Life History Notes: Rainbow Trout", 2005)

Are they endangered?

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; "Oregon Coast Steelhead Evolutionary Significant Unit", 1998; Van Hulle, 2005)


Tanya Dewey (editor), Animal Diversity Web.

Katherine Ridolfi (author), University of Michigan-Ann Arbor, Kevin Wehrly (editor, instructor), University of Michigan-Ann Arbor.


Ohio Department of Natural Resources, Division of Wildlife. 2005. "Life History Notes: Rainbow Trout" (On-line). Accessed October 09, 2005 at

NOAA Fisheries Office of Protected Resources. 1998. "Oregon Coast Steelhead Evolutionary Significant Unit" (On-line). Accessed October 09, 2005 at

Michigan Department of Natural Resources. 2005. "Steelhead: Oncorhynchus Mykiss" (On-line). Accessed October 07, 2005 at,1607,7-153-10364_18958-45692--,00.html.

California Department of Fish and Game. The Life Histories of the Steelhead Rainbow Trout and Silver Salmon. Bulletin No. 98. Sacramento, CA: California Department of Fish and Game. 1954. Accessed October 10, 2005 at

Alexander, G. 1991. Trout as Prey. Pp. 112-117 in J Schnell, J Stolz, eds. Trout: The Wildlife Series. Harrisburg, PA: Stackpole Books.

Behnke, R. 1992. Native Trout of Western North America. Bethesda, MD: American Fisheries Society.

Delaney, K. 2005. "Rainbow Trout: Wildlife Notebook Series" (On-line). Accessed October 09, 2005 at

Gall, G., P. Crandell. 1992. "Oncorhynchus mykiss Rainbow Trout" (On-line). Fishbase. Accessed October 07, 2005 at

Grubb, T. 2003. The Mind of the Trout: A Cognitive Ecology for Biologists and Anglers. Madison, WI: The University of Wisconsin Press.

Klontz, G. 1991. "UC Davis California Aquaculture" (On-line pdf). Manual for Rainbow Trout Production on the Family-Owned Farm. Accessed October 20, 2005 at

Smith, R. 1991. Rainbow Trout (Oncorhynchus mykiss). Pp. 304-323 in J Stoltz, J Schnell, eds. Trout: The Wildlife Series. Harrisburg, PA: Stackpole Books.

Thrower, F., J. Hard, J. Joyce. 2004. Genetic architecture of growth and early life-history transitions in anadromous and derived freshwater populations of steelhead. Journal of Fish Biology, 65: 286-307.

Van Hulle, F. 2005. "Steelhead Trout: Wildlife Notebook Series" (On-line). Accessed October 10, 2005 at

University of Michigan Museum of ZoologyNational Science Foundation

BioKIDS home  |  Questions?  |  Animal Diversity Web  |  Cybertracker Tools

Ridolfi, K. 2006. "Oncorhynchus mykiss" (On-line), Animal Diversity Web. Accessed April 13, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
Copyright © 2002-2024, The Regents of the University of Michigan. All rights reserved.

University of Michigan