Steelhead, or occasionally steelhead trout, is the anadromous form of the coastal rainbow trout (Oncorhynchus mykiss irideus) or Columbia River redband trout (O. m. gairdneri, also called redband steelhead). Steelhead are native to cold-water tributaries of the Pacific basin in Northeast Asia and North America. Like other sea-run (anadromous) trout and salmon, steelhead spawn in freshwater, smolts migrate to the ocean to forage for several years and adults return to their natal streams to spawn. Steelhead are iteroparous, although their survival rate is approximately only 10–20%.
Description
The freshwater form of the steelhead is the rainbow trout (Oncorhynchus mykiss). The difference between these forms of the species is that steelhead migrate to the ocean and return to freshwater tributaries to spawn, whereas non-anadromous rainbow trout do not leave freshwater. Steelhead are also larger and less colorful than rainbow trout. Steelhead can weigh up to 55 lb (25 kg) and can reach 45 in (110 cm) in length, but are typically much smaller at around 8 lb (3.6 kg). In the wild, they usually live four to six years and spawn multiple times. The body of the steelhead trout is silvery and streamlined with a rounder head. This silver color and round head is what gives the steelhead its name. There are black dots that are more concentrated on the back of the fish and become sparser closer to the lateral line of the fish. Steelhead also develop a pink horizontal stripe. When steelhead return to freshwater to spawn, their color begins to more closely resemble that of a normal rainbow trout. The longer a steelhead spends in freshwater, the darker their green body colorations and horizontal pink stripe become. When a steelhead exits the river and returns to the ocean again, it regains its silvery appearance while in saltwater. The fish develop a larger and stronger tail relative to a normal rainbow trout to help aid in swimming in ocean currents and swimming up strong river currents. A number of distinct population segments of steelhead are endangered or threatened across the United States, with their decline largely attributed to causes such as changing ocean conditions, overharvest, hatcheries, habitat destruction, and the blocking of waterways by the construction of dams. Human interaction has had considerable consequences on reducing the population of steelhead trout.
Characteristics Steelhead currently support or have historically supported fisheries across Northern California, and thus they hold "economic as well as cultural value" (Moyle, Israel, & Purdy, 2008). Contrary to popular belief, the California Steelhead is a coastal variation of the same species of fish as the Rainbow Trout, Oncorhynchus mykiss; "rainbow trout are the 'landlocked' variant, and remain in freshwater throughout their life" (King County, 2016). An ocean-going variety, the Steelhead, or Oncorhynchus mykiss irideus has developed unique adaptive characteristics that distinguish this fish from its common counterpart and have allowed a wider distribution of this species across the Pacific Northwest, including the coastal regions of California. In fact, "coastal rainbow trout is the most widely-distributed native trout form" (CA.gov). Among other necessary biological utilities fulfilled by this species, Steelhead Trout "are also strong indicators of the condition of California's streams; large self-sustaining populations of native salmon and trout are found where streams are in reasonably good condition" (Moyle, Israel, & Purdy, 2008). A chrome underside and olive-grey back reminiscent to that of a steel beam are attributed as the main defining physical characteristic for which the Steelhead is distinguished and earns its namesake.
The Steelhead is an ocean-going predatory fish with a typical lifespan of four to six years. Like the common rainbow trout, Steelhead predominantly feast on zooplankton when they are young and then transition to eating fish, some rodents, mollusks, and crustaceans. When these fish enter the open ocean, however, their diet typically consists of squid, crustaceans, and small fish including anchovies, herring, and sardines, though the capacity at which this dietary intake occurs is highly contingent upon the geographical region where Steelhead choose to migrate (Salmon Fishing Now). "Rainbow trout and steelhead represent two divergent ecotypes that are genetically identical but separated by life history strategies" (Heath 2001); the unique biological diversity of the Steelhead Trout from that of the Rainbow Trout is determined through a capacity to migrate to the open ocean. Homing behavior is a unique adaptation of Steelhead Trout that allows the fish to migrate to and from freshwater and saltwater bodies on multiple occasions. Through the process of imprinting, Steelhead Trout deposit a chemical indicator "that is specific to their natal stream" (NRC 1996).
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