Rainbow trout

What it is

This fish is a member of the salmon family, characterised by a broad reddish stripe running along its side from gills to tail. The colour is most vivid in breeding males. Resident adults in rivers are typically blue-green or olive with heavy black spots, while those in lakes or the sea appear more silvery with a fainter stripe. The tail fin is squarish and only slightly forked. Juveniles display dark vertical bars known as parr marks. The scientific name refers to the hooked jaws, or kype, that males develop during mating season.

Where it lives

Native waters include the Pacific basin, stretching from the Kamchatka Peninsula in Russia, across the Aleutian Islands and Alaska, down the coasts of British Columbia and the United States to northern Mexico. Some populations extend east into Canadian river systems draining toward the Arctic Ocean. Since the late nineteenth century, humans have transplanted this species to suitable rivers and lakes worldwide. It now exists on every continent except Antarctica, often establishing self-sustaining wild populations outside its original home range.

How it lives

These predators consume a varied diet, including aquatic insect larvae, adult terrestrial insects, crustaceans, and algae. As they grow, they eat more fish, sometimes taking prey up to one-third of their own length. They also scavenge carcasses and have been observed eating small snakes, rodents, or birds. Ocean-going forms feed on squid, amphipods, and other fish. High metabolic activity during rapid growth can cause oxidative stress, affecting mitochondrial function in the heart and brain tissues as the animal matures.

Breeding and life cycle

Spawning occurs in spring when water temperatures reach at least 6 °C. Females clear nests in gravel beds by beating their tails, laying thousands of tiny eggs which males fertilise with milt. The female covers the eggs with displaced gravel. Hatching takes four to seven weeks. Newly hatched fry feed on zooplankton after consuming their yolk sacs. Anadromous forms, known as steelhead, undergo physiological changes called smoltification to survive in saltwater. They may spawn multiple times, returning to their natal streams.

Fish and people

Anglers prize this fish for sport, and it is widely farmed for food. Hatchery-reared individuals are often marked by clipping the adipose fin to distinguish them from wild stocks. Introductions have created new fisheries in places like the Great Lakes but have also harmed native species through competition, predation, and disease transmission such as whirling disease. It is listed among the top hundred globally invasive species. In Washington state, the steelhead form holds the title of official state fish.

Status and threats

While the species as a whole is widespread, specific local populations and distinct segments of steelhead are listed as threatened or endangered under national legislation. Conservation concerns focus on the impact of introduced populations on native fish stocks. Hybridisation with closely related trout species and the spread of contagious diseases pose additional risks to indigenous biodiversity in regions where this trout has been artificially established.

Questions people ask about Rainbow trout

Good reading and sources

• Trout as Wild as All Outdoors, Almost (Published 1998)The New York Times, Article
• 48-Pound Trout: World Record or Genetic Cheat?Wired, Article
• Trout farming in land-locked Lesotho is an export successBBC, Article
• Very fishy: China rules rainbow trout can be sold as salmonThe Guardian, Article

• Rainbow trout on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data