Rainbow trout
From Wikipedia, the free encyclopedia

Order: Salmoniformes
Family: Salmonidae

[Photo] Rainbow Trout (Oncorhynchus mykiss). Date May 04 2004. Author Engbretson, Eric / U.S. Fish and Wildlife Service. License: public domain.

The rainbow trout (Oncorhynchus mykiss), also called the redband trout, is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America as well as much of the central, western, eastern, and especially the northern portions of the United States. The ocean going (anadromous) form (including those returning for spawning) are known as steelhead, or ocean trout (Australia). The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica. In some of these locations, such as Australia and South America, they have had very serious negative impacts on upland native fish species, either by eating them, outcompeting them or transmitting contagious diseases. In some cases, they have been responsible for the extinction of native fish populations.

The species was originally named by Johann Julius Walbaum in 1792. In 1855, W. P. Gibbons found a population and named it Salmo iridia, however this name became deprecated once it was determined that this was a population of the already named species. More recently, DNA studies showed Rainbow Trout are genetically closer to Pacific salmon (Onchorhynchus species) than Brown trout (Salmo trutta) or Atlantic Salmon (Salmo salar), so the genus was changed.

Physical characteristics
The Rainbow trout are unusual in that there are two forms which sometimes share the same habitat. The anadromous form called "Steelhead" migrate to the ocean, though they must return to fresh water to reproduce.

The freshwater form is called "Rainbow Trout", based on the broad red band along their sides. Steelhead are exactly the same species as Rainbow Trout. However, the difference is anadromy. After going to sea, their color changes, including loss of the red band. They stay at sea for 1-4 years, and return to fresh water to spawn. Rainbows stay in fresh water their whole lives.

It is a common misconception that "redband trout" and "rainbow trout" are the same fish. While closely related, Redbands are a different subspecies, somewhat intermediate between Rainbow and Cutthroat trout(Oncorhynchus clarki). Redbands have larger spots and darker colors than Rainbows and sometimes have Cutthroat-like slash marks, but not the Cutthroat's longer maxilla or hyoid (basibranchial) teeth. Redbands include anadromous Steelhead forms such as the "redsides" of the Columbia River system above the Columbia River Gorge and similar fish from the Fraser River system above Fraser Canyon and the most inland parts of the Sacramento River, Klamath River and others flowing into the Pacific.

Redbands are also found in Endorheic basins partly or wholly isolated since the Pleistocene including Goose Lake (Oregon-California) which only intermittently overflows into the Pit River; the Kern River which has lost its connection to the San Joaquin River; and other basins that have lost connections to the Columbia-Snake, Sacramento or Klamath systems since the Pleistocene. However basins isolated much longer tend to have endemic varieties of Cutthroat trout.

Steelheads also tend to be more silvery than the freshwater Rainbow form. Lake and stream dwellers tend to be lighter as well. River dwelling Rainbow Trout are generally darker and more brilliant. In all habitats, Rainbow's backs varies from blue or green to a yellow-green or brown. They always have dark spots on their heads, backs, bellies and across their dorsal fins and cadula fins, a famed attribute of true trout. (Rainbow Trout are considered to be the best example of true trout known at this time) Rainbows also have a red or pink streak that runs from the gill cover to the caudal fin, inspiring their name. Steelhead usually lack the pink stripe and have chrome-colored sides.

Unlike other Pacific Salmon, Rainbow Trout and Steelhead do not necessarily die after spawning (they may spawn as many as four times). All Pacific Salmon, including male Steelhead, undergo changes when spawning or migrating to spawn. They change color, head and mouth shape. Steelhead populations and rainbow trout populations can and do interbreed at different levels. It is possible for the offspring of rainbow trout to become anadromous and for the offspring of steelhead to develop a resident life history.

Rainbows and Steelheads occur in well-oxygenated lakes and streams where the temperature normally doesn't rise above 12°C in summer. Behind the dorsal fin, rainbows have an adipose fin that is commonly clipped from fish raised in a hatchery. Clipping the adipose fin helps distinguish hatchery fish from wild fish when they are taken by anglers. This allows enforcement of regulations to protect wild rainbows. (*Mark Selective Fishing information from Washington Department of Fish and Wildlife)

Rainbows range from 12 to 36 inches in length. Steelhead grow longer, ranging from 50 to 122 cm (20 to 48 inches) in length. Steelhead range in weight from 2.5 kg to 10 kg (5.5 - 22 pounds). The world record Rainbow was a 43.6 pound caught from the shore at Lake Diefenbaker, Saskatchewan in June 2007.

Most steelhead populations in Oregon, Washington, and California have been listed under the U.S. Endangered Species Act as either threatened or endangered species. This decision has been controversial, however, particularly among the community of anglers who fish for them, since the freshwater form is typically not considered to be endangered, while being technically the same species. However, Steelhead face different mortality pressures not faced by resident Rainbows. These mortality sources often involve their migration, ocean survival, and harvest.

Steelhead that spawn in Southern California streams (south of Point Conception) have been particularly decimated by habitat loss due to dams, confinement of streams in concrete channels, water pollution, groundwater pumping, Urban heat island effects, and other byproducts of urbanization. This sub-population is believed to have adapted to higher water temperatures and to natal streams not being suitable for spawning every year, depending on weather variability and other factors. It has been named "Southern Steelhead" and is the focus of major restoration efforts.

Lifecycle
Like salmon, steelhead are anadromous: they return to their original hatching ground to spawn. Different populations of steelheads 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. "Winter-run steelhead" mature fully in the ocean before migrating, between November and April, and spawn shortly after returning. Similar to Atlantic salmon, but unlike their Pacific Oncorhynchus kin, steelhead are iteroparous and may make several spawning trips between fresh and salt water.

Diet
Rainbow Trout have a varied diet. They are predators, eating any smaller fish from nearly the time they are born. Insects make up a large portion of the diet, along with crayfish and other crustaceans, some lake dwelling species may become planktonic feeders. Trout of all ages will eat nearly anything they can grab, in contrast with the legendary, selective nature the fish often gets. They are near the top of the food chain in most freshwater environments. However they are lower on the rung of other freshwater predators such as pike, muskie, lake trout, and chinook salmon. Rainbows will take fish up to and over 1/3 of their length and larger. However they are not quite as piscivorous or aggressive as the Brown Trout or Lake Trout, which is actually a char. The rule of thumb is that Rainbows consume more fish and fewer insects as they grow, but insects continue to be a part of the diet in most all populations.

As food
Rainbow trout and steelhead are popular in Western cuisine and are both wild caught and farmed for food. It has tender flesh and a mild, somewhat nutty flavor. However, farmed trout and those taken from certain lakes have a pronounced earthy flavor which many people find unappealing; many shoppers therefore make it a point to ascertain the source of the fish before buying. Buhl, Idaho, United States is the world's largest producer of farmed rainbow trout. Rainbow trout are raised in many countries throughout the world. Rainbow trout that are wild and have a diet of scuds (freshwater shrimp) and crawdads are the most appealing, with orange pink flesh.

Steelhead are farmed, primarily in British Columbia and in Chile. Steelheads' meat is pink that of a salmon, and is more flavorful than the light-colored meat of a Rainbow Trout.

Fisheries
Rainbow Trout and the anadromous form, steelhead, are both highly desired sport fish. There are some tribal commercial fisheries for steelhead in the Puget Sound, the Washington Coast and in the Columbia River. Most rainbow trout and steelhead harvest in the United States is supported by hatchery production.

The rainbow trout is also especially susceptible to enteric redmouth disease caused by the pathogen Yersinia ruckeri. There has been considerable research conducted on redmouth disease, as its implications for rainbow trout farmers are significant. The disease does not affect humans.

Subspecies
A few populations are recognized as subspecies:

Beardslee trout, Salmo gairdneri beardsleei, isolated in Lake Crescent in Washington.
California golden trout, Oncorhynchus aguabonita aguabonita (Jordan, 1892).
Columbia River redband trout, Oncorhynchus mykiss gairdnerii (Richardson, 1836).
Kern golden trout, Oncorhynchus aguabonita gilberti (Jordan, 1894).
Coastal rainbow trout, Oncorhynchus mykiss irideus (Gibbons, 1955).
Kamchatkan rainbow trout, Oncorhynchus mykiss mykiss (Walbaum, 1792).
Baja California rainbow trout, Nelson's trout, or San Pedro Martir trout, Oncorhynchus mykiss nelsoni (Evermann, 1908).
Great Basin redband trout, Oncorhynchus mykiss newberrii (Girard 1859)
Sacramento golden trout, Oncorhynchus aguabonita stonei (Jordan, 1894).
Little Kern golden trout, Oncorhynchus aguabonita whitei (Evermann, 1906).
Eagle Lake rainbow trout, Oncorhynchus mykiss aquilarum

http://en.wikipedia.org/wiki/Rainbow_trout

The text in this page is based on the copyrighted Wikipedia article shown in above URL. It is used under the GNU Free Documentation License. You may redistribute it, verbatim or modified, providing that you comply with the terms of the GFDL.