Historically, steelheads have been differentiated from other species of Pacific salmon; such as distinction is commonly made even today. When they were first described, all native species of North American trout (including steelhead) were placed in the genus Salmo. However, biologists have recently concluded that steelhead have a greater affinity with Pacific salmon (genus Oncorhynchus) than with trout. Therefore, the term “Pacific salmon” includes the steelhead species.

Steelhead exhibit one of the most complex suites of life history traits of any salmon species. Steelhead may exhibit anadromy (meaning they migrate from fresh water to the ocean, and then return to spawn in fresh water) or non-anadromy (meaning they reside their entire life in fresh water). Resident forms are usually referred to as “rainbow” or “redband” trout, while anadromous life forms are termed “steelhead.” Few detailed studies have been conducted regarding the relationship between resident and anadromous O mykiss as a result; the relationship between these two life history forms is poorly understood.

Two major genetic groups or “subspecies” of steelhead occur on the west coast of the United States: a coastal group and an inland group, separated approximately in the Fraser and Columbia River Basins by the Cascade crest. Behnke (1992) proposed to classify the coastal subspecies as O. m. irideus and the inland subspecies as O. m. gairdneri. The anadromous and nonanadromous forms of O. mykiss are grouped together for each “subspecies.” Both coastal and inland steelhead occurs in Washington and Oregon. California is thought to have only coastal steelhead while Idaho has only inland steelhead.

Steelheads typically migrate to marine waters after spending two years in fresh water. In the marine environment they typically rear for 1 to 3 years prior to returning to their natal stream to spawn primarily as three-and four years old. Unlike other Pacific salmon, steelheads are iteroparous, meaning that they are capable of spawning more than once before they die. However, most steelhead survives to spawn only once; they majority of multiple spawners are females. Steelhead spawning season typically stretches from December through June. Steelhead eggs may incubate in “redds” (nesting gravels) for 1.5 to 4 (depending on water temperature) months before hatching as “alevins” (a larval life state dependent on food stored in a yold sac). Following yolk sac absorption, alevins emerge from the gravel as young juveniles or “fry” and begin actively feeding. Juveniles rear in fresh water from 1 to 4 years (usually 2 years), and then migrate to the ocean as “smolts.”

Biologically, steelhead can be divided into two reproductive ecotypes, based on their state of sexual maturity at the time of river entry and the duration of their spawning migration. These two ecotypes are termed “stream maturing” and “ocean maturing.” Stream maturing steelheads enter fresh water in a sexually immature condition and require several months to mature and spawn. Ocean maturing steelheads enter fresh water with well-developed gonads and spawn shortly after river entry. These two reproductive ecotypes are more commonly referred to by their season of freshwater entry (e.g., summer and winter steelhead, respectively).

Historically, steelheads were distributed throughout the North Pacific Ocean from Russia to the northern Baja Peninsula. Presently, the species’ distribution extends from the Kamchatka Peninsula in Asia, east and south along the Pacific coast of North America to the Santa Margarita River in southern California (Figure 1). Historically, steelhead likely inhabited most coastal streams in Washington, Oregon, and California as well as many other States.