Oncorhynchus mykiss is among the most important game fishes in North America. These common fish are found in cold headwaters, creeks, and small to large rivers, as well as lakes. They are anadromous in coastal streams, moving upstream from the ocean to spawn (unlike salmon, adults usually survive spawning and may breed again). (Page and Burr 1991)
The different forms of Oncorhynchus mykiss are known by different common names. The sea-run Rainbow Trout are known as Steelhead; interior populations are sometimes known as Redband Trout. These fish are highly variable in color, but have small, irregular black spots on the back and most fins. There are radiating rows of black spots on the caudal (tail) fin and a pink to red stripe on the side. Stream and spawning fish have intense dark colors whereas lake fish are light and silvery. The upper jaw reaches barely behind the eye in young and female individuals, but well behind the eye in large males. Sea-run individuals (Steelhead) are silvery and largely lack the pink stripe on the side; they typically develop a more pointed head and grow much larger than Rainbow Trout. (Page and Burr 1991)
Rainbow Trout are native to the Pacific Slope of North America from Alaska and northwestern Canada to Baja California. They have been widely introduced in cold waters elsewhere in North America and the rest of the world. (Page and Burr 1991)
In contrast to the Rainbow Trout, the Golden Trout (O. aguabonita) has a red belly and cheek, a gold lower side, and large black spots on the dorsal and caudal fins (but in some areas where they co-occur, these two species hybridize). The Cutthroat Trout (O. clarki) has an orange or red "cutthroat" mark on the underside of the lower jaw and small teeth on the floor of the mouth between the gill arches (no such teeth are present in Rainbow Trout). Salvelinus species have light spots on a dark background. Salmon have 13 or more anal rays (8 to 12 in Rainbow Trout). (Page and Burr 1991)
For detailed information on the biology and status of this species, including conservation issues, see this resource from the NOAA Fisheries Office of Protected Resources.