Rainbow Trout (Oncorhynchus mykiss)

FishSalmonidIntroduced Worldwide

Overview

The rainbow trout (Oncorhynchus mykiss) is a salmonid native to the Pacific drainages of North America and northeastern Asia, named for the broad pink or reddish stripe along its flank and the fine black spotting scattered across its body and fins.

Despite being called a trout, it belongs to the Pacific salmon genus Oncorhynchus rather than to the genus containing the brown trout — a detail that reflects its true relationships.

The species has two life histories in one. Fish that stay in fresh water are 'rainbow trout'; those that migrate to sea and return to spawn are called steelhead, and grow larger and more silvery. They are the same species, differing in life history rather than in identity.

Habitat & Range

Rainbow trout are native to cool, clean, well-oxygenated fresh water on the Pacific side of North America and in parts of northeastern Asia, and steelhead populations use the north Pacific. Through deliberate introduction for angling and aquaculture the species now occurs on every continent except Antarctica, in rivers, streams, lakes and reservoirs where temperatures stay cool enough.

Diet

Rainbow trout are opportunistic carnivores. Diet varies with size and habitat: aquatic insect larvae, adult insects taken at the surface, crustaceans, molluscs, fish eggs and smaller fish. Larger individuals take proportionally more fish. Sea-run steelhead feed heavily in the ocean on fish and crustaceans, which is why they grow so much larger.

Behavior

Rainbow trout spawn in spring in gravel-bottomed streams, where the female excavates a depression called a redd, deposits eggs, and covers them with gravel. Unlike most Pacific salmon, which die after spawning, rainbow trout and steelhead can survive to spawn again. Stream-resident fish hold feeding positions in current where drifting food passes, defending those positions from other trout.

Human Interaction & Conservation

Rainbow trout are one of the most widely introduced fish in the world, moved deliberately for sport fishing and farmed extensively in aquaculture. That success has costs: introduced populations can outcompete or hybridise with native fish and alter stream communities, and rainbow trout are treated as an invasive species in a number of countries. In parts of their native range certain steelhead populations are of serious conservation concern even while the species globally is abundant. This profile is educational and offers no fishing or aquaculture guidance. Verify current status at the IUCN Red List and NOAA Fisheries.