The rainbow trout and the steelhead are the same species of fish; the difference between the two names is the two distinct lives that they live. The name rainbow trout is used for the rainbow trout that does not leave the stream to go to the ocean. They spend their entire life in the lakes, rivers and stream. The name steelhead is for the rainbow trout which migrates to the ocean where it spend most of its adult life before return to the same steam in which they were raised making them an anadromous rainbow or more commonly called a steelhead.

Anadromous steelhead and resident rainbow trout come from the same evolutionary lines. There is a close genetic and biological relationship between these two forms. The anadromous form “Steelhead” can convert to resident populations “Rainbow Trout” when drought events or damming of rivers blocks their access to the ocean. And the same is true of the resident rainbow trout population which can become anadromous if they should find access to the ocean. It is common to have both life history patterns occurring in the same stream. It is possible for, resident and anadromous parents to produce offspring of both varieties.

While know one knows for sure it is widely speculated that there is a food related trigger which will determines whether a particular fish emigrates to the ocean or remains in the stream. Some who have studied this believe that if there is abundant food in the stream and a fish grow at a rapid rate, they will remain in the stream. However if their food resource is limited and their growth is slowed, they will have an inclination to immigrate to the ocean if possible.

This ability of the steelhead and rainbow trout makes the species more adaptable to the changing ecological conditions. In the southern limits of steelhead distribution this is has played an important role for the steelhead due to unstable, variable climatic and hydrographic conditions that disrupt their normal migration.