The steelhead trout is a species of fish, closely related to the Pacific salmon, that occurs naturally in the Northern Pacific Ocean and adjacent streams and rivers. The US federal government has listed the steelhead as an endangered species, its numbers having been decimated primarily because of habitat destruction.
The steelhead, being a bony, salmon-like fish, is of class Osteichthyes, order Salmoniformes, and family Salmonidae. For a long time the steelhead was believed to be closely related to the Atlantic salmon (genus Salmo) and had therefore been assigned the name Salmo gairdneri, but in 1998 new fossil evidence showed that the steelhead was actually more closely related to Pacific salmon (genus Oncorhynchus). At around the same time biologists determined that the steelhead was the same species as another fish in Kamchatka, formerly Salmo mykiss. Since S. mykiss was described before S. gairdneri, the former species name took precedence and the steelhead became Oncorhynchus mykiss.
The name steelhead is somewhat misleading, as it refers to only some individuals of the species. The sleek, oceangoing, silver fish we call steelhead are actually the same species as the fat, freshwater, multicolored rainbow trout. All steelhead are hatched as rainbow trout fry. At 1-2 years of age, the trout destined to become steelhead lose their rainbow coloring and begin to migrate downstream towards the ocean, while others of the same parents keep their rainbow coloring and remain in freshwater for their entire lives. (Fish which exhibit this migratory behavior are known as anadromous fish.) Adult steelhead remain in the ocean for at least one winter to feed, and may return to the stream of their birth to spawn in any spring following their first winter at sea. Upon arrival at their spawning grounds, the steelhead lose the silvery color essential for survival at sea and regain their rainbow coloring, which is believed to be helpful for attracting a mate. Unlike most anadramous salmon, which die after laying their eggs, steelhead then regain their silvery color and migrate back to the sea after spawning. They will return to the stream of their birth to spawn several times over the course of their lives.
Like many other species, the survival of O. mykiss is threatened by habitat fragmentation and destruction. Steelhead in streams prefer to feed in fast-moving, turbulent water, and lay their eggs in gravelly pools free of fine sediment. Migrating steelhead, obviously, need to be able to make their way upstream unimpeded from the ocean to their breeding areas. But dams and other impediments to the free flow of streams block access for spawning fish, and allow sediment to fill formerly clean pools upstream. Some efforts to remove dams that block steelhead access are under way; time will tell if these efforts will be successful.
McEwan, D. and Jackson, T. (1996) Steelhead Restoration and Management Plan for California. Available at http://www.dfg.ca.gov/nafwb/pubs/swshplan.pdf
Alaska Department of Fish and Game "Steelhead" webpage: http://www.adfg.state.ak.us/pubs/notebook/fish/steelhd.php
Bureau of Reclamation, Lower Colorado Region, Boulder City, NV. (1995) Rindge Dam Removal Study. Pages 1-4. Available at http://www.healthebay.org/assets/pdfdocs/mcwstudies/DFG%201995.pdf
Abramson, M. and Grimmer, M. (2005) Fish Migration Barrier Severity and Steelhead Habitat Quality in the Malibu Creek Watershed. Available at http://www.healthebay.org/assets/pdfdocs/streamteam/SteelheadBarrierandHabitat062305.pdf
Froese, R. and D. Pauly. Editors. (2005) FishBase.
World Wide Web electronic publication.