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. Several other fish in the salmonid family are called trout, some are anadromous like salmon, whereas others are resident in freshwater only
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.
Rainbow trout are predators with a varied diet, and will eat nearly anything they can grab. Their image as a selective eater is only a legend. Rainbows are not quite as piscivorous or aggressive as brown trout or lake trout (char). Young rainbows survive on insects, fish eggs, smaller fish (up to 1/3 of their length), along with crayfish and other crustaceans. As they grow, though, the proportion of fish increases in most all populations. Some lake dwelling lines may become planktonic feeders. While in flowing waters populated with salmon, trout eat varied fish eggs, including salmon, cutthroat trout, as well as the eggs of other rainbow trout, alevin, fry, smout and even salmon carcasses.
Like salmon, steelhead are anadromous: they return to their original hatching ground to spawn. Similar to Atlantic salmon, but unlike their Pacific Oncorhynchus salmonid kin, steelhead are iteroparous (breed many times, each time separated by months) and make several spawning trips between fresh and salt water. The steelhead smolt (immature or young fish) remain in the river for about a year before heading to sea, whereas salmon typically return to the seas as smolts. Different steelhead populations migrate upriver at different times of the year. "Summer-run steelhead" migrate between May and October, before their reproductive organs are fully mature. They mature in freshwater before spawning in the spring. Most Columbia River steelhead are "summer-run". "Winter-run steelhead" mature fully in the ocean before migrating, between November and April, and spawn shortly after returning. The maximum recorded life-span for a rainbow trout is 11 years.