Scientific Name: Oncorhynchus mykiss (Previous names include Salmo gairdneri and Salmo irideus. There are several subspecies.)
Description: Rainbow trout/steelhead are ray-finned fishes in the salmon family, and they are one of the top sport fish in North America. Rainbow trout and steelhead are the same species, but they have different lifestyles. Steelhead are anadromous, meaning they spend part of their lives in the sea, while rainbow trout spend their lives mostly or entirely in freshwater. Because of their different lifestyles rainbow trout and steelhead are different in appearance, most noticeably in size and color. Rainbow trout derive their name from their beautiful, multi-hued coloration. Their bodies are blue, green or yellowish, shading to silvery white on the underside, with a horizontal pink-red stripe running from the gills to the tail and black spots along their backs. Steelhead are generally more streamlined in shape and silvery or brassy in color as adults, earning them the name steelhead.
Size: Adult rainbow trout/steelhead range in size. They can reach 45 inches in length, but are usually much smaller. They can weigh over 50 pounds, but a more typical weight is 8 pounds. Because steelhead spend 2-3 years in fresh water followed by 2-3 years in the ocean, they are typically larger than rainbow trout, which live all of their lives in fresh or sometimes brackish water.
Diet: Rainbow trout/steelhead have a varied diet and will feed on just about anything, such as zooplankton when they’re young, and as they mature, fish eggs, small fish, crustaceans, mollusks, insects and even mice. In habitats that are dense with aquatic vegetation, rainbow trout often have the opportunity to eat arthropods that fall into the stream.
Predation: Rainbow trout/steelhead are food for many different predators depending on the region and habitat. Their predators include lampreys, fish, birds, bears, river otters, raccoons, and humans. In the sea steelhead are eaten by many species including seals, sea lions and orcas.
Typical Lifespan: Four to six years in the wild. The largest rainbow trout ever caught was estimated to be 11 years old.
Habitat: All rainbow trout/steelhead occupy freshwater streams or lakes at some point in their lives. They use aquatic vegetation, boulders, and wood as protective cover. Steelhead spend the majority of the year in estuaries or open ocean and only return to freshwater to spawn.
Range: Rainbow trout/steelhead are native to North America west of the Rockies, but this popular fish has been introduced in almost every other state and on every continent except Antarctica.
Life History and Reproduction: Rainbow trout/steelhead are specially adapted to several different lifestyles. Some populations will stay in the same stream their whole lives. Others are anadromous, meaning that they spend their lives at sea and come back to the stream of their birth to spawn. When spawning, females dig out a depression called a redd in the gravelly bottom of stream riffle. She deposits her eggs in the depression and the male fertilizes them. They are covered up by the gravel and will remain there to hatch.
- A group of rainbow trout is called a hover.
- Unlike a salmon, which dies after spawning, steelhead trout can spawn, return to the ocean, and migrate back upstream to spawn several times.
- Some offspring of two steelhead can stay in freshwater and be resident trout, and two offspring of resident rainbow trout can create a steelhead.
Conservation status: Because they’ve been so widely introduced, rainbow trout are not at risk of extinction. They are even considered a pest species in some places where they aren’t native. Native populations, though, are threatened by disease, habitat degradation, and fishing. Dam construction has hindered the ability of rainbow trout to migrate upriver, which is necessary for their reproductive cycle. One of the most notable diseases affecting rainbow trout is whirling disease—a parasitic ailment that causes skeletal deformities and death in young, hatchery-raised rainbow trout. Some native populations are federally listed as endangered. One of the main problems with rainbow trout is hybridization with other trout including non-native species.
NOAA Fisheries Office of Protected Resources
Alaska Department of Fish and Game Steelhead/Rainbow Trout Species Profile
National Geographic Society
USDA National Resources Conservation Service – Fish and Wildlife Habitat Management Leaflet
Montana Fish, Wildlife & Parks – “Whirling Disease in the United States”