The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout (anadromous) usually returning to freshwater to spawn after 2 to 3 years at sea. The fish is sometimes called a salmon trout.[1] Several other fish in the salmonid family are called trout, some are anadromous like salmon, whereas others are resident in freshwater only.[2]

The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica. In some locations, such as Southern Europe, Australia and South America, they have negatively impacted upland native fish species, either by eating them, outcompeting them, transmitting contagious diseases, (like Whirling disease transmitted by Tubifex) or hybridization with closely-related species and subspecies that are native to western North America.[3][4]

Taxonomy
The species was originally named by Johann Julius Walbaum in 1792 based on type specimens from Kamchatka. Richardson named a specimen of this species Salmo gairdneri in 1836, and in 1855, W. P. Gibbons found a population and named it Salmo iridia, later corrected to Salmo irideus, however these names faded once it was determined that Walbaum's type description was conspecific and therefore had precedence (see e.g. Behnke, 1966).[5] More recently, DNA studies showed rainbow trout are genetically closer to Pacific salmon (Onchorhynchus species) than to brown trout (Salmo trutta) or Atlantic Salmon (Salmo salar), so the genus was changed.

Unlike the species' former name's epithet iridia (Latin: rainbow), the specific epithet mykiss derives from the local Kamchatkan name 'mykizha'; all of Walbaum's species names were based on Kamchatkan local names.

The ocean going (anadromous) form (including those returning for spawning) are known as steelhead, (Canada and the United States) or ocean trout (Australia), although they are the same species.