Rainbow trout

Rainbow trout
Oncorhynchus mykiss.jpg
Conservation status
Secure
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Salmoniformes
Family: Salmonidae
Genus: Oncorhynchus
Species: O. mykiss
Binomial name
Oncorhynchus mykiss
Walbaum, 1792
Subspecies

See text

The rainbow trout (Oncorhynchus mykiss) 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 Southern Europe, Australia and South America, they have had very serious negative impacts on upland native fish species, either by eating them, outcompeting them, transmitting contagious diseases, or hybridization with closely related species and subspecies that are native to western North America.[1][2]

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 became deprecated once it was determined that Walbaum's type description was conspecific and therefore had precedence (see e.g. Behnke, 1966).[3] 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.

Contents

Physical characteristics

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

The freshwater form is called "rainbow trout"[4], 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.

Steelhead 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 caudal fins, a famed attribute of true trout. 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.

Rainbow trout

Rainbows and steelhead occur in well-oxygenated lakes and streams where the temperature normally doesn't rise above 12°C in summer. Behind the dorsal fin, rainbow trout 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.[5] 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. [6]

Lifecycle

Illustration of a rainbow trout

Like salmon, steelhead are anadromous: they return to their original hatching ground to spawn. Unlike salmon, which die after spawning, steelhead rejuvenate after spawning so they may return to the oceans to start the anadromous cycle once again. The steelhead smolts (immature or young fish) usually remain in the river for about a year before heading to sea, whereas salmon typically return to the seas as smolts. 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. i

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. While in flowing waters consisting of salmon, trout will eat salmon eggs, salmon fry to even salmon carcasses. Trout of all ages will eat nearly anything they can grab, in contrast with the legendary, selective image people often have of the animal's nutrition habits. 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. 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 potatoes

Rainbow trout and steelhead are popular in Western cuisine and are both caught wild 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. Rainbow trout are raised in many countries throughout the world. Rainbow trout that are wild and have a diet of scuds (freshwater shrimp) and crayfish are the most appealing, with orange pink flesh.

Steelhead are farmed, primarily in British Columbia and in Chile. Steelhead meat is pink like that of salmon, and is more flavorful than the light-colored meat of rainbow trout.[7]

In some places, if fished and cleaned immediately, the meat will have a sweet and clean flavor rather than nutty or earthy, especially if it is a native rainbow trout.

Fisheries

Rainbow trout and 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.[8]

Threats and Conservation

Steelhead trout have declined due to a number of human and natural causes. The U.S. National Marine Fisheries Service has a detailed description of threats. 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.

Rainbow trout, and subspecies thereof, are currently EPA approved indicator species for acute fresh water aquatic toxicity testing. [9]

Subspecies

A few populations are recognized as subspecies:

Cultivated varieties

Golden rainbow trout and palomino trout are artificially developed color variants of Oncorhynchus mykiss.[10] Golden rainbow trout are predominantly yellowish, lacking the typical green field and black spots, but retaining the diffuse red stripe.[11] They were developed based on one spontaneously lighter animal.[10] The palomino trout is a mix of golden and common rainbow trout, resulting in an intermediate color. The golden rainbow trout should not be confused with the naturally occurring golden trout.

Pictures

See also

References

  1. ↑ Salmo marmoratus
  2. ↑ Salmothymus obtusirostris salonitana
  3. ↑ Robert J. Behnke. 1966. Relationships of the Far Eastern Trout, Salmo mykiss walbaum Copeia, Vol. 1966, No. 2 (Jun. 21, 1966), pp. 346-348
  4. ↑ Trout Science: "Science of Trout"
  5. ↑ Scott and Crossman. 1985. Freshwater Fishes of Canada. Bul.184 Fisheries Research Board of Canada. page 189. ISBN 0-660-10239-0.
  6. ↑ ESPN - 43-pound, world-record rainbow trout caught in Canada
  7. ↑ Your Christmas Steelhead
  8. ↑ LSC - Fish Disease Leaflet 82
  9. ↑ EPA Whole Effluent Toxicity
  10. ↑ 10.0 10.1 Golden Rainbow Trout. Pennsylvania Fish & Boat Commission FAQ.
  11. ↑ Golden Rainbow Trout. Photo.

Scott and Crossman. 1985. Freshwater Fishes of Canada. Bulletin 184. Fisheries Research Board of Canada. Page 189. ISBN 0-660-10239-0

External links