Fresh-water species of fish
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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. Freshwater forms that have been introduced into the Great Lakes and migrate into tributaries to spawn are also called steelhead.
Adult freshwater stream rainbow trout average between , while lake-dwelling and anadromous forms may reach . 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 "" (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 "Salmo"s – brown trout ("Salmo trutta") or Atlantic salmon ("Salmo salar") 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 in riverine environments, while lake-dwelling, and anadromous forms may reach . 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 . The maximum recorded lifespan for a rainbow trout is 11 years.
Freshwater life cycle.
Freshwater resident rainbow trout usually inhabit and spawn in small to moderately large, well-oxygenated, shallow rivers with gravel bottoms. They are native to the alluvial or freestone streams that are typical tributaries of the Pacific basin but introduced rainbow trout have established wild, self-sustaining populations in other river types such as bedrock and spring creeks. Lake-resident rainbow trout are usually found in moderately deep, cool lakes with adequate shallows and vegetation to support the production of sufficient food sources. Lake populations generally require access to gravelly-bottomed streams to be self-sustaining.
Spawning sites are usually a bed of fine gravel in a riffle above a pool. A female trout clears a redd in the gravel by turning on her side and beating her tail up and down. Female rainbow trout usually produce 2000 to 3000 eggs per kilogram of weight. During spawning, the eggs fall into spaces between the gravel, and immediately the female begins digging at the upstream edge of the nest, covering the eggs with the displaced gravel. As eggs are released by the female, a male moves alongside and deposits milt (sperm) over the eggs to fertilize them. The eggs usually hatch in about four to seven weeks although the time of hatching varies greatly with region and habitat. Newly hatched trout are called sac fry or alevin. In approximately two weeks, the yolk sac is completely consumed, and fry commence feeding mainly on zooplankton. The growth rate of rainbow trout varies with area, habitat, life history, and quality and quantity of food. As fry grow, they begin to develop "parr" marks or dark vertical bars on their sides. In this juvenile stage, immature trout are often called "parr" because of the marks. These small juvenile trout are sometimes called 'fingerlings' because they are approximately the size of a human finger. In streams where rainbow trout are stocked for sport fishing, but no natural reproduction occurs, some of the stocked trout may survive and grow or "carryover" for several seasons before they are caught or perish.
Steelhead life cycle.
The oceangoing (anadromous) form, including those returning for spawning, are known as steelhead in Canada and the U.S. In Tasmania they are commercially propagated in sea cages and are known as ocean trout, although they are the same species.
Like salmon, steelhead return to their original hatching grounds to spawn. Similar to Atlantic salmon, but unlike their Pacific "Oncorhynchus" salmonid kin, steelhead are iteroparous (able to spawn several times, each time separated by months) and make several spawning trips between fresh and salt water, although fewer than 10 percent of native spawning adults survive from one spawning to another. The survival rate for introduced populations in the Great Lakes is as high as 70 percent. As young steelhead transition from freshwater to saltwater, a process called "smoltification" occurs where the trout undergoes physiological changes to allow it to survive in seawater. There are genetic differences between freshwater and steelhead populations that may account for the smoltification in steelhead.
Juvenile steelhead may remain in the river for one to three years before smolting and migrating to sea. Individual steelhead populations leave the ocean and migrate into their freshwater spawning tributaries at different times of the year. Two general forms exist—"summer-run steelhead" and "winter-run steelhead". Summer-run fish leave the ocean between May and October before their reproductive organs are fully mature. They mature in freshwater while en route to spawning grounds where they spawn in the spring. Summer-run fish generally spawn in longer, more inland rivers such as the Columbia River. Winter-run fish are ready to spawn when they leave the ocean, typically between November and April, and spawn shortly after returning to fresh water. Winter-run fish generally spawn in shorter, coastal rivers typically found along the Olympic Peninsula and British Columbia coastline, and summer-run fish are found in some shorter, coastal streams. Once steelhead enter riverine systems and reach suitable spawning grounds, they spawn just like resident freshwater rainbow trout.
Growth and oxidative stress on mitochondria.
During periods of rapid growth and aging, trout display high levels of metabolic activity. High metabolic activity has been correlated with increased levels of oxidative stress and decreased machinery repair in rainbow trout. During high oxidative stress, the mitochondria are the most important organelle contributing to tissue damage because of their role in metabolism and production of reactive oxygen species. In a study done by Almaida-Pagàn et al., researchers identified changes to the heart and brain mitochondrial membrane phospholipid composition in rainbow trout due to the differing levels of oxidative stress each organ faced during a high-stress time such as rapid growth and development. Stock rainbow trout of ages 1, 2, and 4 years had their heart and brain mitochondria isolated and analyzed for fatty acid composition. The tissues showed an overall similar percentage of total phospholipids but differed in the types and proportions of phospholipids. With age, the heart showed more unsaturated phospholipids, which are more susceptible to peroxidation, and thus, damage. The brain mitochondria of rainbow trout show decreased levels of docosahexaenoic acid and a lower peroxidation index, suggesting a lower susceptibility to damage by oxidative stress and a different reaction to growth compared to heart mitochondria. During the trout's development into an adult, a time of intense growth, the mitochondrial membrane composition and fluidity changes, which can cause defects in the electron transport chain. These defects combined with altered machinery repair and reactive oxygen species may cause more detrimental effects on the mitochondria in the fish as it matures. 
Feeding.
Rainbow trout are predators with a varied diet and will eat nearly anything they can capture. They are not as piscivorous or aggressive as brown trout or chars. Rainbow trout, including juvenile steelhead in fresh water, routinely feed on larval, pupal, and adult forms of aquatic insects (typically caddisflies, stoneflies, mayflies and aquatic diptera). They also eat fish eggs and adult forms of terrestrial insects (typically ants, beetles, grasshoppers, and crickets) that fall into the water, as well as algae. Other prey includes small fish up to one-third of their length, crayfish, shrimp, and other crustaceans. As rainbow trout grow, the proportion of fish consumed increases in most populations. Some lake-dwelling forms may become planktonic feeders. In rivers and streams populated with other salmonid species, rainbow trout eat varied fish eggs, including those of salmon, brown and cutthroat trout, mountain whitefish, and the eggs of other rainbow trout. Rainbows also consume decomposing flesh from the carcasses of other fish. They have even been observed to have consumed hatchling snakes, such as the barred grass snake ("Natrix helvetica"). Adult steelhead in the ocean feed primarily on other fish, squid and amphipods.
Range.
The native range of "Oncorhynchus mykiss" is in the coastal waters and tributary streams of the Pacific basin, from the Kamchatka Peninsula in Russia, east along the Aleutian Islands, throughout southwest Alaska, the Pacific coast of British Columbia and southeast Alaska, and south along the west coast of the U.S. to northern Mexico. It is claimed that the Mexican forms of "Oncorhynchus mykiss" represent the southernmost native range of any trout or salmon ("Salmonidae"), though the Formosan landlocked salmon ("O. masou formosanus") in Asia inhabits a similar latitude. The range of coastal rainbow trout ("O. m. irideus") extends north from the Pacific basin into tributaries of the Bering Sea in northwest Alaska, while forms of the Columbia River redband trout ("O. m. gairdneri") extend east