What's the difference between steelhead and rainbow trout?
Rainbow trout have two basic life histories with different names: steelhead (anadromous) and rainbow trout (resident). The relationship between steelhead and rainbows is beautifully flexible and complex, making the species one of most successful salmonids in the world, but creating headaches for managers.
Anadromous steelhead regularly produce to resident trout offspring. Resident trout populations regularly produce anadromous smolts. In populations where both forms occur, they regularly interbreed. This varied life history has a strong adaptive genetic basis.
This diversity of life histories appears to be particularly important in California, where both fresh water and ocean environments can undergo dramatic multi-year fluctuations in habitat quality. For example, if survival is low during down-stream migration or in the ocean for an extended period, then resident trout will have an adaptive advantage. If ocean conditions are good and promote high survival and growth, then offspring of migratory steelhead females will predominate in fresh water. Resident trout also thrive above natural barriers to anadromy (e.g., waterfalls) while steelhead can re-colonize streams where resident fish have been eliminated by natural causes (e.g. volcanic eruptions, catastrophic wildfire). In watersheds with both steelhead and wild resident Rainbow trout, the fish are part of a single, complex gene pool, which allows them to adapt to river systems that are highly variable.
Recognizing their diverse domestic and wild origins, the ‘Coastal Rainbow trout’ is not really a discrete genetic entity. Coastal Rainbow trout are resident Rainbows that mostly inhabit watersheds upstream of natural and man-made barriers without access to the ocean, although robust populations of resident trout are now found in tailwater habitats below dams throughout the state.
Photo by Barbara Wampole