<Li> Salmo mykiss Walbaum, 1792 Parasalmo mykiss (Walbaum, 1792) Salmo purpuratus Pallas, 1814 Salmo penshinensis Pallas, 1814 Parasalmo penshinensis (Pallas, 1814) Salmo gairdnerii Richardson, 1836 Fario gairdneri (Richardson, 1836) Oncorhynchus gairdnerii (Richardson, 1836) Salmo gairdnerii gairdnerii Richardson, 1836 Salmo rivularis Ayres, 1855 Salmo iridea Gibbons, 1855 Salmo gairdnerii irideus Gibbons, 1855 Salmo irideus Gibbons, 1855 Trutta iridea (Gibbons, 1855) Salmo truncatus Suckley, 1859 Salmo masoni Suckley, 1860 Oncorhynchus kamloops Jordan, 1892 Salmo kamloops (Jordan, 1892) Salmo rivularis kamloops (Jordan, 1892) Salmo gairdneri shasta Jordan, 1894 Salmo gilberti Jordan, 1894 Salmo nelsoni Evermann, 1908 </Li> <P> The rainbow trout (Oncorhynchus mykiss) is a trout and species of salmonid native to cold - water tributaries of the Pacific Ocean in Asia and North America . 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 fresh water to spawn after living two to three years in the ocean . Freshwater forms that have been introduced into the Great Lakes and migrate into tributaries to spawn are also called steelhead . </P> <P> Adult freshwater stream rainbow trout average between 1 and 5 lb (0.5 and 2.3 kg), while lake - dwelling and anadromous forms may reach 20 lb (9 kg). 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 . </P> <P> Wild - caught and hatchery - reared forms of this 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 (U.S.), 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, thus reducing genetic purity . The rainbow trout is included in the list of the top 100 globally invasive species . Nonetheless, other introductions into waters previously devoid of any fish species or with severely depleted stocks of native fish have created sport fisheries such as the Great Lakes and Wyoming's Firehole River . </P>

What is the average size of a rainbow trout
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