The rainbow trout (Oncorhynchus mykiss) is a species of trout native to cold-water tributaries of the Pacific Ocean in North America and Asia. The steelhead (sometimes called steelhead trout) is an anadromous (sea-run) form of the coastal rainbow trout (O. m. irideus) or Columbia River redband trout (O. m. gairdneri) that usually returns to freshwater to spawn after living two to three years in the ocean. Adult freshwater stream rainbow trout average between 0.5 and 2.5 kilograms (1 and 5 lb), while lake-dwelling and anadromous forms may reach 9 kg (20 lb). Coloration varies widely based on subspecies, forms, and habitat. Adult fish are distinguished by a broad reddish stripe along the lateral line, from gills to the tail, which is most vivid in breeding males. Wild-caught and hatchery-reared forms of the species have been transplanted and introduced for food or sport in at least 45 countries and every continent except Antarctica. Introductions to locations outside their native range in the United States, Southern Europe, Australia, New Zealand, and South America have damaged native fish species. Introduced populations may affect native species by preying on them, out-competing them, transmitting contagious diseases (such as whirling disease), or hybridizing with closely related species and subspecies. The rainbow trout is included in the list of the top 100 globally invasive species. Other introductions into waters previously devoid of fish or with severely depleted stocks of native fish have created sport fisheries, such as the Great Lakes and Wyoming's Firehole River. Some local populations of specific subspecies, or in the case of steelhead, distinct population segments, are listed as either threatened or endangered under the Endangered Species Act. The steelhead is the official state fish of Washington.
Taxonomy The scientific name of the rainbow trout is Oncorhynchus mykiss. The species was originally named by German naturalist and taxonomist Johann Julius Walbaum in 1792 based on type specimens from the Kamchatka Peninsula in Siberia. Walbaum's original species name, mykiss, was derived from the local Kamchatkan name used for the fish, mykizha. The name of the genus is from the Greek ὄγκος (ónkos) "lump, bend, hook" and ῥύγχος (rhúnkhos) "snout", in reference to the hooked jaws of males in the mating season (the "kype"). Sir John Richardson, a Scottish naturalist, named a specimen of this species Salmo gairdneri in 1836 to honor Meredith Gairdner, a Hudson's Bay Company surgeon at Fort Vancouver on the Columbia River who provided Richardson with specimens. In 1855, William P. Gibbons, the curator of Geology and Mineralogy at the California Academy of Sciences, found a population and named it Salmo iridia (Latin: rainbow), later corrected to Salmo irideus. These names faded once it was determined that Walbaum's description of type specimens was conspecific and therefore had precedence. In 1989, morphological and genetic studies indicated that trout of the Pacific Basin were genetically closer to Pacific salmon (Oncorhynchus species) than to the genus Salmo (brown trout (Salmo trutta), Atlantic salmon (Salmo salar) and relatives) of the Atlantic Basin. Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat, and other Pacific Basin trout into the genus Oncorhynchus. Walbaum's name had precedence, so the species name Oncorhynchus mykiss became the scientific name of the rainbow trout. The previous species names irideus and gairdneri were adopted as subspecies names for the coastal rainbow and Columbia River redband trout, respectively. Anadromous forms of the coastal rainbow trout (O. m. irideus) or Columbia River redband trout (O. m. gairdneri) are commonly known as steelhead.
Subspecies Subspecies of Oncorhynchus mykiss are listed below as described by fisheries biologist Robert J. Behnke (2002).
Description Resident freshwater rainbow trout adults average between 0.5 and 2.5 kg (1 and 5 lb) in riverine environments, while lake-dwelling, and anadromous forms may reach 9 kg (20 lb). Coloration varies widely between regions and subspecies. Adult freshwater forms are generally blue-green or olive green with heavy black spotting over the length of the body. Adult fish have a broad reddish stripe along the lateral line, from gills to the tail, which is most pronounced in breeding males. The caudal fin is squarish and only mildly forked. Lake-dwelling and anadromous forms are usually more silvery in color with the reddish stripe almost completely gone. Juvenile rainbow trout display parr marks (dark vertical bars) typical of most salmonid juveniles. In some redband and golden trout forms, parr marks are typically retained into adulthood. Some coastal rainbow trout (O. m. irideus) and Columbia River redband trout (O. m. gairdneri) populations and cutbow hybrids may also display reddish or pink throat markings similar to cutthroat trout. In many regions, hatchery-bred trout can be distinguished from native trout via fin clips. Fin clipping the adipose fin is a management tool used to identify hatchery-reared fish.
Life cycle Rainbow trout, including steelhead forms, generally spawn in early to late spring (January to June in the Northern Hemisphere and September to November in the Southern Hemisphere) when water temperatures reach at least 6 to 7 °C (42 to 44 °F). The maximum recorded lifespan for a rainbow trout is 11 years.
Freshwater life cycle
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