There are two primary forms of this species: the "rainbow trout," which is smaller, darker and lives in streams and rivers; and the "steelhead," a larger, silvery form that lives in large bodies of water such as lakes or the ocean. The rainbow trout is another Pacific coast species that has been intentionally introduced to all of the Great Lakes with reasonable success.
The most obvious identifying characteristic on the rainbow trout--the rosy band running horizontally down the fish's side--is rarely visible on individuals found in the Great Lakes. Lake-run rainbow trouts, called steelheads, tend to be entirely silver or bluish-silver. Spawning or recently spawned fish may be quite dark. Breeding males have an extended, hooked jaw.
The best way to identify the Great Lakes rainbow trout is the spotting--the tail and adipose fins are distinctly and profusely spotted in black or brown. However, rainbow trouts do not have the reddish spots shown by the brook and brown trouts.