Steelhead (Oncorhynchus mykiss) - KFFS

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Species Studied

Steelhead (Oncorhynchus mykiss)

Adult steelhead

Adult steelhead. Credit: Chris Adams, California Department of Fish and Game. (Public domain.)

Steelhead (Oncorhynchus mykiss) are anadromous rainbow trout, a migratory life history form of this common trout that spends some portion of its life in the ocean. In many respects steelhead are similar to Pacific salmonids such as Chinook and coho salmon, to which they are closely related. Spawning adult steelhead are not as large as salmon but can grow to nearly two feet in length and reach weights of 10 pounds. One significant difference between steelhead and salmon is that steelhead are iteroparous, meaning that they do not die after spawning. Steelhead can spawn multiple times, but it is common for fish to skip years between spawning attempts and individuals rarely spawn more than a few times before they die. Like salmon, steelhead display a variety of life histories. Juveniles can spend one to three years in freshwater habitats (usually two) and adults usually remain in the ocean for one or two years before returning to spawn. Steelhead populations include two run types. Winter-run fish mature in the ocean and spawn soon after arriving at their freshwater spawning areas in late winter or early spring. In contrast, summer-run fish migrate into freshwater to complete their maturation before spawning, which typically occurs farther upstream of and prior to spawning of winter-run fish. Some populations of steelhead even include small jack males that return to spawn after as little as a few months in the ocean.

Juvenile steelhead

Juvenile steelhead. Credit: Chris Adams, California Department of Fish and Game. (Public domain.)

Steelhead are common and widely distributed throughout the lower Klamath River Basin. Similar to Chinook salmon, evidence suggests that spawning steelhead historically migrated into tributaries of Upper Klamath Lake in the Upper Klamath Basin. Klamath steelhead populations include both winter-run and summer-run fish, and both runs are substantially reduced from historical levels. Winter-run fish are the most abundant form, and they migrate upstream between September and March. The summer-run population is far less abundant and migrates upstream mostly in June and July. Peak spawning for fish of both run types occurs in March and April. Most juvenile steelhead in the Klamath River watershed smolt and migrate to the ocean in April and May. Steelhead in the Klamath River Basin are not federally listed under the Endangered Species Act, but are recognized by NOAA NMFS as part of the Klamath Mountains Province Evolutionarily Significant Unit. Populations in this ESU are rather unique in that substantial portions of the populations are comprised of a life history form known as “half-pounders,” fish that migrate to the ocean for the summer and then return to streams to overwinter in freshwater.