Brown, American Rainbow and Cutthroat are all types of which fish?
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The brown trout (Salmo trutta) is a European species of salmonid fish that has been widely introduced into suitable environments globally. It includes both purely freshwater populations, referred to Salmo trutta morpha fario and S. trutta morpha lacustris, and anadromous forms known as the sea trout, S. trutta morpha trutta. The latter migrates to the oceans for much of its life and returns to fresh water only to spawn.  Sea trout in the UK and Ireland have many regional names, including sewin (Wales), finnock (Scotland), peal (West Country), mort (North West England), and white trout (Ireland).

The lacustrine morph of brown trout is most usually potamodromous, migrating from lakes into rivers or streams to spawn, although evidence indicates stocks spawn on wind-swept shorelines of lakes. S. trutta morpha fario forms stream-resident populations, typically in alpine streams, but sometimes in larger rivers. Anadromous and nonanadromous morphs coexisting in the same river appear genetically  identical.  What determines whether or not they migrate remains unknown.

Taxonomy 

The scientific name of the brown trout is Salmo trutta. The specific epithet trutta derives from the Latin trutta, meaning, literally, "trout".  Behnke (2007) relates that the brown trout was the first species of trout described in the 1758 edition of Systema Naturae by Swedish zoologist Carl Linnaeus.  Systema Naturae established the system of binomial nomenclature for animals.  Salmo trutta was used to describe anadromous or sea-run forms of brown trout. Linnaeus also described two other brown trout species in 1758.  Salmo fario was used for riverine forms.  Salmo lacustris was used for lake-dwelling forms. 

Range 

The native range of brown trout extends from northern Norway and White Sea tributaries in Russia in the Arctic Ocean to the Atlas Mountains in North Africa. The western limit of their native range is Iceland in the north Atlantic, while the eastern limit is in Aral Sea tributaries in Afghanistan and Pakistan.

Introduction outside of their natural range

Brown trout have been widely introduced into suitable environments around the world, including North and South America, Australasia, Asia, and South and East Africa. Introduced brown trout have established self-sustaining, wild populations in many introduced countries.   The first introductions were in Australia in 1864 when 300 of 1500 brown trout eggs from the River Itchen survived a four-month voyage from Falmouth, Cornwall, to Melbourne on the sailing ship Norfolk. By 1866, 171 young brown trout were surviving in a Plenty River hatchery in Tasmania. Thirty-eight young trout were released in the river, a tributary of the River Derwent in 1866. By 1868, the Plenty River hosted a self-sustaining population of brown trout which became a brood source for continued introduction of brown trout into Australian and New Zealand rivers.  Successful introductions into the Natal and Cape Provinces of South Africa took place in 1890 and 1892, respectively. By 1909, brown trout were established in the mountains of Kenya. The first introductions into the Himalayas in northern India took place in 1868, and by 1900, brown trout were established in Kashmir and Madras. 

Introduction to Americas 

The first introductions in Canada occurred in 1886 in Newfoundland and continued through 1933. The only Canadian regions without brown trout are the Yukon and Northwest Territories. Introductions into South America began in 1904 in Argentina. Brown trout are now established in Chile, Peru, and the Falklands.  In the 1950s and 1960s, Edgar Albert de la Rue, a French geologist, began the introduction of several species of salmonids on the remote Kerguelen Islands in the southern Indian Ocean. Of the seven species introduced, only brook trout, Salvelinus fontinalis, and brown trout survived to establish wild populations.  Sea-run forms of brown trout exceeding 20 lbs are caught by local anglers on a regular basis.

The first introductions into the U.S. started in 1883 when Fred Mather, a New York pisciculturist and angler, under the authority of the U.S. Fish Commissioner, Spencer Baird, obtained brown trout eggs from a Baron Lucius von Behr, president of the German Fishing Society. The von Behr brown trout came from both mountain streams and large lakes in the Black Forest region of Baden-Württemberg.  The original shipment of "von Behr" brown trout eggs were handled by three hatcheries, one on Long Island, the Cold Spring Hatchery operated by Mather, one in Caledonia, New York operated by pisciculturalist Seth Green, and other hatchery in Northville, Michigan. Additional shipments of "von Behr" brown trout eggs arrived in 1884. In 1885, brown trout eggs from Loch Leven, Scotland, arrived in New York. These "Loch Leven" brown trout were distributed to the same hatcheries. Over the next few years, additional eggs from Scotland, England, and Germany were shipped to U.S. hatcheries. Behnke (2007) believed all life forms of brown trout—anadromous, riverine and lacustine—were imported into the U.S. and intermingled genetically to create what he calls the American generic brown trout and a single subspecies the North European brown trout (S. t. trutta).

In April 1884, the U.S. Fish Commission released 4900 brown trout fry into the Baldwin River, a tributary of the Pere Marquette River in Michigan. This was the first release of brown trout into U.S. waters. Between 1884 and 1890, brown trout were introduced into suitable habitats throughout the U.S.  By 1900, 38 states and two territories had received stocks of brown trout. Their adaptability resulted in most of these introductions establishing wild, self-sustaining populations.

Conservation status 

The fish is not considered to be endangered, although, in some cases, individual stocks are under various degrees of stress mainly through habitat degradation, overfishing, and artificial propagation leading to introgression. Increased frequency of excessively warm water temperatures in high summer causes a reduction in dissolved oxygen levels which can cause "summer kills" of local populations if temperatures remain high for sufficient duration and deeper/cooler or fast, turbulent more oxygenated water is not accessible to the fish. This phenomenon can be further exacerbated by eutrophication of rivers due to pollution—often from the use of agricultural fertilizers within the drainage basin.

Overfishing is a problem where anglers fail to identify and return mature female fish into the lake or stream. Each large female removed can result in thousands fewer eggs released back into the system when the remaining fish spawn.

Another threat is other introduced organisms. For example, in Canada's Bow River, a non-native alga Didymosphenia geminata—common name rock snot (due to appearance)—has resulted in reduced circulation of water amongst the substrate of the river bed in affected areas. This, in turn, can greatly reduce the number of trout eggs which survive to hatch. Over time, this leads to reduction of the population of adult fish in the areas affected by the algae, forming a circle of decline. Rock snot is believed to have spread accidentally on the soles of the footwear of visitors from areas where the alga is native. The wide variety of issues that adversely affect brown trout throughout its range do not exclusively affect brown trout, but affect many or all species within a water body, thus altering the ecosystem in which the trout reside.

In small streams, brown trout are important predators of macroinvertebrates, and declining brown trout populations in these specific areas affect the entire aquatic food web."Climate Change and Freshwater". 11 February 2009 . 

Global climate change is also of concern. S. trutta morpha fario prefers well-oxygenated water in the temperature range of 60 to. S. trutta bones from an archaeological site in Italy, and ancient DNA extracted from some of these bones, indicate that both abundance and genetic diversity increased markedly during the colder Younger Dryas period, and fell during the warmer Bølling-Allerød event. 

Cover or structure is important to trout, and they are more likely to be found near submerged rocks and logs, undercut banks, and overhanging vegetation. Structure provides protection from predators, bright sunlight, and higher water temperatures. Access to deep water for protection in winter freezes, or fast water for protection from low oxygen levels in summer are also ideal. Trout are more often found in heavy and strong currents.

Characteristics 

The brown trout is a medium-sized fish, growing to 20 kg or more and a length of about 100 cm in some localities, although in many smaller rivers, a mature weight of  or less is common. S. t. lacustris reaches an average length of 40 – with a maximum length of 140 cm and about 60 lb.
The spawning behaviour of brown trout is similar to that of the closely related Atlantic salmon. A typical female produces about 2,000 eggs per kg (900 eggs per lb) of body weight at spawning.
On September 11, 2009, a 41.45-lb (18.80-kg) brown trout was caught by Tom Healy in the Manistee River system in Michigan, setting a new state record. As of late December 2009, the fish captured by Healy was confirmed by both the International Game Fish Association and the Fresh Water Fishing Hall of Fame as the new all-tackle world record for the species. This fish now supplants the former world record from the Little Red River in Arkansas.

Brown trout can live 20 years, but as with the Atlantic salmon, a high proportion of males die after spawning, and probably fewer than 20% of anadromous female kelts recover from spawning. The migratory forms grow to significantly larger sizes for their age due to abundant forage fish in the waters where they spend most of their lives. Sea trout are more commonly female in less nutrient-rich rivers. Brown trout are active both by day and by night and are opportunistic feeders. While in fresh water, their diets frequently include invertebrates from the streambed, other fish, frogs, mice, birds, and insects flying near the water's surface. The high dietary reliance upon insect larvae, pupae, nymphs, and adults allows trout to be a favoured target for fly fishing. Sea trout are fished for especially at night using wet flies. Brown trout can be caught with lures such as spoons, spinners, jigs, plugs, plastic worm imitations, and live or dead baitfish.
Freshwater brown trout range in colour from largely silver with relatively few spots and a white belly, to the more well-known brassy brown cast fading to creamy white on the fish's belly, with medium-sized spots surrounded by lighter halos. The more silver forms can be mistaken for rainbow trout. Regional variants include the so-called "Loch Leven" trout, distinguished by larger fins, a slimmer body, and heavy black spotting, but lacking red spots. The continental European strain features a lighter golden cast with some red spotting and fewer dark spots. Notably, both strains can show considerable individual variation from this general description. Early stocking efforts in the United States used fish taken from Scotland and Germany.

Brown trout rarely form hybrids with other species; if they do, they are almost invariably infertile. One such example is the tiger trout, a hybrid with the brook trout.

Diet 

Field studies have demonstrated that brown trout fed on several animal prey species, aquatic invertebrates being the most abundant prey items. However, brown trout also feed on other taxa such as terrestrial invertebrates (e.g. Hymenoptera) or other fish.  Moreover, in brown trout, as in many other fish species, a change in the diet composition normally occurs during the life of the fish,  and piscivorous behaviour is most frequent in large brown trout.  These shifts in the diet during fish lifecycle transitions may be accompanied by a marked reduction in intraspecific competition in the fish population, facilitating the partitioning of resources.  

First feeding of newly emerged fry is very important for brown trout survival in this phase of the lifecycle, and first feeding can occur even prior to emergence.   Fry start to feed before complete yolk absorption and the diet composition of newly emerged brown trout is composed of small prey such as chironomid larvae or baetid nymphs. 

Stocking, farming and non-native brown trout 

The species has been widely introduced for sport fishing into North America, South America, Australia, New Zealand, and many other countries, including Bhutan, where they are the focus of a specialised fly fishery. First planting in the United States occurred April 11, 1884, into the Baldwin River, one mile east of Baldwin, MI.  Brown trout have had serious negative impacts on upland native fish species in some of the countries where they have been introduced, particularly Australia. Because of the trout's importance as a food and game fish, it has been artificially propagated and stocked in many places in its range, and fully natural populations (uncontaminated by allopatric genomes) probably exist only in isolated places, for example in Corsica or in high alpine valleys on the European mainland.

Farming of brown trout has included the production of infertile triploid fish by increasing the water temperature just after fertilisation of eggs, or more reliably, by a process known as pressure shocking. Triploids are favoured by anglers because they grow faster and larger than diploid trout. Proponents of stocking triploids argue, because they are infertile, they can be introduced into an environment that contains wild brown trout without the negative effects of cross-breeding. However, stocking triploids may damage wild stocks in other ways. Triploids certainly compete with diploid fish for food, space, and other resources. They could also be more aggressive than diploid fish and they may disturb spawning behaviour.

Scottish and Irish sea trout populations in recent years have seriously declined, possibly due to infestation by sea lice from salmon farms. 

Angling 

The brown trout has been a popular quarry of European anglers for centuries. It was first mentioned in angling literature as "fish with speckled skins" by Roman author AElian (circa 200 AD) in On the Nature of Animals. This work is credited with describing the first instance of fly fishing for trout, the trout being the brown trout found in Macedonia. The Treatyse of Fysshynge with an Angle (1496) by Dame Juliana Berners, O.S.B is considered a foundational work in the history of recreational fishing, especially fly fishing. One of the most prominent fish described in the work is the brown trout of English rivers and streams:

The renowned The Compleat Angler (1653) by Izaak Walton is replete with advice on "the trout":

Throughout the 17th, 18th, and 19th centuries, angling authors, mostly British, some French, and later American, writing about trout fishing were writing about fishing for brown trout. Once brown trout were introduced into the U.S. in the 1880s, they became a major subject of American angling literature. In 1889, Frederic M. Halford, a British angler, author published Dry-Fly Fishing in Theory and Practice, a seminal work codifying a half century of evolution of fly fishing with floating flies for brown trout. In the late 19th century, American angler and writer Theodore Gordon, often called the "Father of American Dry Fly Fishing" perfected dry-fly techniques for the newly arrived, but difficult-to-catch brown trout in Catskill rivers such as the Beaverkill and Neversink Rivers.  In the early 20th century, British angler and author G.E.M Skues pioneered nymphing techniques for brown trout on English chalk streams. His Minor Tactics of the Chalk Stream (1910) began a revolution in fly fishing techniques for trout.   In 1917, Scottish author Hamish Stuart published the first comprehensive text, The Book of The Sea Trout, specifically addressing angling techniques for the anadromous forms of brown trout. 

Introductions of brown trout into the American West created new angling opportunities, none so successful from an angling perspective as was the introduction of browns into the upper Firehole River in Yellowstone National Park in 1890. One of the earliest accounts of trout fishing in the park is from Mary Trowbridge Townsend's 1897 article in Outing Magazine "A Woman's Trout Fishing in Yellowstone Park" in which she talks about catching the von Behr trout in the river:

Within the US, brown trout introductions have created self-sustaining fisheries throughout the country. Many are considered "world-class" such as in the Great Lakes and in several Arkansas tailwaters.  Outside the US and outside its native range in Europe, introduced brown trout have created "world-class" fisheries in New Zealand,  Patagonia  and the Falklands. The rainbow trout (Oncorhynchus mykiss) is a trout and species of salmonid native to cold-water tributaries of the Pacific Ocean in Asia and North America.  The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coastal rainbow trout  or Columbia River redband trout  that usually returns to fresh water 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 1 and, while lake-dwelling and anadromous forms may reach 20 lbs. 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 this 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 (U.S.), 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, thus reducing genetic purity. Other introductions into waters previously devoid of any fish species or with severely depleted stocks of native fish have created world-class 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 . 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 onkos ("hook") and rynchos ("nose"), 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  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 . 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 Salmos – brown trout  or Atlantic salmon  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  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   or redband trout  are commonly known as steelhead.

Subspecies

Subspecies of  are listed below as described by fisheries biologist Robert J. Behnke (2002).

Description

Resident freshwater rainbow trout adults average between 1 and in riverine environments, while lake-dwelling and anadromous forms may reach 20 lbs.  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  and Columbia River redband trout  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,  

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 42 to. 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 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 4 to 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 is variable 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 sea water.  There are genetic differences between freshwater and steelhead populations that may account for the smoltification in steelheads. 

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 fresh water 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.

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. Other prey include 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 carcasses of other fish.  Adult steelhead in the ocean feed primarily on other fish, squid and amphipods. 

Range

The native range of  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  represent the southernmost native range of any trout or salmon (Salmonidae), though the Formosan landlocked salmon () in Asia inhabits a similar latitude.  The range of coastal rainbow trout  extends north from the Pacific basin into tributaries of the Bering Sea in northwest Alaska, while forms of the Columbia River redband trout  extend east into the upper Mackenzie River and Peace River watersheds in British Columbia and Alberta, Canada, which eventually drain into the Beaufort Sea, part of the Arctic Ocean. Since 1875, the rainbow trout has been widely introduced into suitable lacustrine and riverine environments throughout the United States and around the world. Many of these introductions have established wild, self-sustaining populations. 

Artificial propagation

Since 1870, rainbow trout have been artificially propagated in fish hatcheries to restock streams and to introduce them into non-native waters. The first rainbow trout hatchery was established on San Leandro Creek, a tributary of San Francisco Bay, in 1870, and trout production began in 1871. The hatchery was stocked with the locally native rainbow trout, and likely steelhead of the coastal rainbow trout subspecies . The fish raised in this hatchery were shipped to hatcheries out of state for the first time in 1875, to Caledonia, New York, and then in 1876 to Northville, Michigan. In 1877, another California rainbow trout hatchery, the first federal fish hatchery in the National Fish Hatchery System, was established on Campbell Creek, a McCloud River tributary.  The McCloud River hatchery indiscriminately mixed coastal rainbow trout eggs with the eggs of local McCloud River redband trout . Eggs from the McCloud hatchery were also provided to the San Leandro hatchery, thus making the origin and genetic history of hatchery-bred rainbow trout somewhat diverse and complex.  In the U.S., there are hundreds of hatcheries operated by the U.S. Fish and Wildlife Service and various state agencies and tribal governments propagating rainbow trout for conservation and recreational sport fishing.      Six of ten Canadian provinces have rainbow trout farms, with Ontario leading production. 

Aquaculture

Rainbow trout are commercially farmed in many countries throughout the world. The practice began in the late 19th century,  and since the 1950s commercial production has grown dramatically.  Worldwide, in 2007, 604695 t of farmed rainbow trout were harvested with a value of about US $2.6 billion. The largest producer is Chile. In Chile and Norway, sea cage production of steelhead has expanded to supply export markets. Inland production of rainbow trout to supply domestic markets has increased in countries such as Italy, France, Germany, Denmark and Spain. Other significant trout-producing countries include the U.S., Iran, the United Kingdom, and Lesotho.  While the U.S. rainbow trout industry as a whole is viewed as ecologically responsible, trout raised elsewhere are not necessarily farmed with the same methods.

About three-quarters of U.S. production comes from Idaho, particularly the Snake River area, due in part to the quality and temperature of the water available there. California and Washington also produce significant amounts of farmed trout.  In the east, Pennsylvania, North Carolina and West Virginia have farming operations. Rainbow trout farming is one of the largest finfish aquaculture industries in the U.S. They are raised inland in facilities where raceways or ponds have continuously flowing water with little pollution and a low risk of escape. The U.S. industry is noted for using best management practices. Imports constitute only about 15 percent of farmed rainbows sold in the U.S., and nearly all domestic production is consumed within the country; very little is exported. The U.S. produces about 7 percent of the world's farmed trout. Rainbow trout, especially those raised in farms and hatcheries, are susceptible to enteric redmouth disease. A considerable amount of research has been conducted on redmouth disease, given its serious implications for rainbow trout farming. The disease does not infect humans. 

Conservation

Populations of many rainbow trout subspecies, including anadromous forms (steelhead) of  (coastal rainbow trout) and  (Columbia River redband trout) have declined in their native ranges due to over-harvest, habitat loss, disease, invasive species, pollution and hybridization with other subspecies, and some introduced populations, once healthy, have declined for the same reasons. As a consequence, some rainbow populations, particularly anadromous forms within their native range, have been classified as endangered, threatened or species of special concern by federal or state agencies.  Rainbow trout, and subspecies thereof, are currently a U.S. Environmental Protection Agency-approved indicator species for acute fresh water toxicity testing. 

Many non-profit organizations have formed to protect, conserve and restore native rainbow trout and steelhead populations. Generally, in partnership with various universities, state, federal and tribal agencies and private interests, these organizations sponsor projects to restore habitat, prevent habitat loss and promote awareness of threats to native trout populations. 
Trout Unlimited (TU) is a non-profit organization dedicated to the conservation of North American freshwater streams, rivers, and associated upland habitats for trout, salmon, other aquatic species and people.  A typical TU project is the Circle Creek Fish Passage Project, in which access to a spawning stream is being improved for steelhead and other salmonid species.  The Wild Salmon Center, an international coalition of Russian, Canadian and U.S. scientists, sponsors the Kamchatka Steelhead Project, a 20-year (1994–2014) scientific program to study and conserve the present condition of Kamchatkan steelhead ("mikizha"), a species listed in the Red Data Book of Russia.  Other high-profile organizations involved in rainbow trout conservation include California Trout, which protects wild trout and other salmonids in the waters of California.  The Steelhead Society of British Columbia promotes the wellbeing of wild salmonids in British Columbia.  In 1997, a group of approximately 40 ichthyologists, biologists and naturalists from several U.S. and Mexican institutions formed a collaborative group—Truchas Mexicanas—to study the diversity of Mexican native trout, most of which are considered subspecies of . 

Hybridization and habitat loss

Rainbow trout, primarily hatchery-raised fish of the coastal rainbow trout subspecies  introduced into waters inhabited with cutthroat trout, will breed with cutthroats and produce fertile hybrids called cutbows.  In the case of the westslope cutthroat trout (O. clarki lewisi), hybridization with introduced rainbow and Yellowstone cutthroat trout (O. clarki bouvieri) is threatening the westslope cutthroat trout with genomic extinction.  Such introductions into the ranges of redband trout  have severely reduced the range of pure stocks of these subspecies, making them "species of concern" in their respective ranges. 

Within the range of the Kern River golden trout of Southern California, hatchery-bred rainbows introduced into the Kern River have diluted the genetic purity of the Kern River rainbow trout  and golden trout  through intraspecific breeding.     The Beardslee trout, , a genetically unique lake-dwelling variety of the coastal rainbow trout that is isolated in Lake Crescent (Washington), is threatened by the loss of its only spawning grounds in the Lyre River to siltation and other types of habitat degradation.

Invasive species and disease

;Whirling disease

 is a myxosporean parasite of salmonids (salmon, trout, and their allies) that causes whirling disease in farmed salmon and trout and also in wild fish populations. It was first described in rainbow trout in Germany a century ago, but its range has spread and it has appeared in most of Europe, northern Asia, the U.S., South Africa and other countries. In the 1980s,  was found to require Tubifex tubifex (a kind of segmented worm) to complete its life cycle. The parasite infects its hosts with its cells after piercing them with polar filaments ejected from nematocyst-like capsules.

This parasite was originally a mild pathogen of brown trout in central Europe and other salmonids in northeast Asia, and the spread of the rainbow trout has greatly increased its impact. Having no innate immunity to , rainbow trout are particularly susceptible, and can release so many spores that even more resistant species in the same area, such as , can become overloaded with parasites and incur mortalities of 80 to 90 percent.  Where  has become well-established, it has caused decline or even elimination of whole cohorts of fish.  

The parasite  was first recorded in North America in 1956 in Pennsylvania, but until the 1990s whirling disease was considered a manageable problem only affecting rainbow trout in hatcheries. It eventually became established in natural waters of the Rocky Mountain states (Colorado, Wyoming, Utah, Montana, Idaho, New Mexico), where it is damaging several sport fishing rivers. Some streams in the western U.S. lost 90 percent of their trout.  Whirling disease threatens recreational fishing, which is important for the tourism industry, a key component of the economies of some U.S. western states. For example, in 2005 anglers in Montana spent approximately $196,000,000 in activities directly related to trout fishing in the state.  Some of the salmonids that  infects (bull trout, cutthroat trout, and anadromous forms of rainbow trout—steelhead) are already threatened or endangered, and the parasite could worsen their population decline. 

;New Zealand mud snail

The New Zealand mud snail , once endemic to New Zealand, has spread widely and has become naturalised and an invasive species in many areas including: Australia, Tasmania, Asia (Japan), in the Garmat Ali River in Iraq since 2008 ), Europe (since 1859 in England), and North America (U.S. and Canada: Thunder Bay in Ontario since 2001, British Columbia since July 2007 ), most likely inadvertently during human activity. It can reach concentrations greater than 500,000 per m², endangering the food chain by outcompeting native snails and water insects for food, leading to sharp declines in native populations.  There is evidence North American fishes are unable to digest the tiny but hard shells of the mud snail, and that their presence may result in poor growth outcomes for rainbow trout. 

The mud snail was first detected in the U.S. in Idaho's Snake River in 1987. Since then, the snail has spread to the Madison River, Firehole River, and other watercourses around Yellowstone National Park, and has been discovered throughout the western U.S. The exact means of transmission is unknown, but it is likely that it was introduced in water transferred with live game fish and has been spread by ship ballast or contaminated recreational equipment such as wading gear. 

;Didymo
, commonly known as didymo or rock snot, is a species of diatom that produces nuisance growths in freshwater rivers and streams with consistently cold water temperatures. In New Zealand, invasive didymo can form large mats on the bottom of rivers and streams in late winter. It is not considered a significant human health risk, but it can affect stream habitats and sources of food for fish, including rainbow trout, and make recreational activities unpleasant.  Even though it is native in North America, it is considered a nuisance organism or invasive species. 

;Redmouth disease
Enteric redmouth disease is a bacterial infection of freshwater and marine fish caused by the pathogen .  It is primarily found in rainbow trout and other cultured salmonids. The disease is characterized by subcutaneous hemorrhaging of the mouth, fins, and eyes. It is most commonly seen in fish farms with poor water quality. Redmouth disease was first discovered in Idaho rainbow trout in the 1950s. 

Removal Methods 

Some fisheries are focused on removing rainbow trout in order to reestablish native trout populations.  This can be done by poisoning rivers with chemicals such as  antimycin or rotenone which have been declared safe  in the USA by the Environmental Protection Agency.  Once the chemicals have dissipated native trout are  released into the river.  Another method is to use electrofishing which enable the fish to be caught alive and harvested or re-located.  This technique has been used in the Great Smokey Mountains National Park to rid it of rainbow trout that were introduced in the 1930s and have thrived ever since.  They are hoping to re-establish native brook trout in at least some of the 2100-mile river system.  Neither method of control is 100% effective and are best regarded as methods to change the relative population sizes of fish species.

Steelhead declines

Steelhead populations in parts of its native range have declined due to a variety of human and natural causes. While populations in Alaska and along the British Columbia coast are considered healthy, populations in Kamchatka and some populations along the U.S. west coast are in decline.
The U.S. National Marine Fisheries Service has 15 identified distinct population segments (DPS)s, in Washington, Oregon, and California. Eleven of these DPSs are listed under the U.S. Endangered Species Act, ten as threatened and one as endangered. One DPS on the Oregon Coast is designated a U.S. Species of Concern. 

The Southern California DPS, which was listed as endangered in 2011, has been affected 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.  Steelhead in the Kamchatka Peninsula are threatened by over-harvest, particularly from poaching and potential development, and are listed in the Red Data Book of Russia that documents rare and endangered species. 

Hatchery stocking influence

Several studies have shown that almost all California coastal steelhead are of native origin, despite over a century of hatchery stocking. Genetic analysis shows that the South Central California Coast DPS and Southern California DPS from Malibu Creek north, and including the San Gabriel River, Santa Ana River and San Mateo Creek, are not hatchery strains. Steelhead from Topanga Creek and the Sweetwater River were partly, and those from San Juan Creek completely, of hatchery origin. Genetic analysis has also shown that the steelhead in the streams of the Santa Clara County and Monterey Bay basins are not of hatchery origin, including the Coyote Creek, Guadalupe River, Pajaro River, Permanente Creek, Stevens Creek, San Francisquito Creek, San Lorenzo River, and San Tomas Aquino Creek basins.  Natural waterfalls and two major dams have isolated Russian River steelhead from freshwater rainbow trout forms above the impassable barriers; a 2007 genetic study of fin samples collected from steelhead at 20 different sites both above and below passage barriers in the watershed found that although 30 million hatchery trout were stocked in the river from 1911 to 1925, the steelhead remain of native and not hatchery origin. 

Releases of conventionally reared hatchery steelhead pose ecological risks to wild steelhead populations. Hatchery steelhead are typically larger than the wild forms and can displace wild-form juveniles from optimal habitats. Dominance of hatchery steelhead for optimal microhabitats within streams may reduce wild steelhead survival as a result of reduced foraging opportunity and increased rates of predation.

Uses

Fishing

Rainbow trout and steelhead are highly regarded game fish. Rainbow trout are a popular target for fly fishers, and several angling methods are used. The use of lures presented via spinning, casting or trolling techniques is common. Rainbow trout can also be caught on various live and dead natural baits. The International Game Fish Association recognizes the world record for rainbow trout as a fish caught on Saskatchewan's Lake Diefenbaker by Sean Konrad on September 5, 2009. The fish weighed 48 lb and was a genetically modified hatchery escapee.  Many anglers consider the rainbow trout the hardest-fighting trout species, as this fish is known for leaping when hooked and putting up a powerful struggle.  It is considered one of the top five sport fish in North America and the most important game fish west of the Rocky Mountains.

There are tribal commercial fisheries for steelhead in Puget Sound, the Washington coast and in the Columbia River, but there has been controversy regarding over-harvesting of native stocks. 

The highly desirable sporting qualities and adaptability of the rainbow trout to hatchery rearing and new habitats resulted in it being introduced to many countries around the world by or at the behest of sport fishermen.  Many of these introductions have resulted in environmental and ecological problems, as the introduced rainbow trout disrupt local ecosystems and outcompete or eat indigenous fishes.  Other introductions to support sport angling in waters either devoid of fish or with seriously depleted native stocks have created world-class fisheries such as in the Firehole River in Yellowstone National Park,    and in the Great Lakes.  

As food

Rainbow trout is popular in Western cuisine; both wild-caught and farmed fish are eaten. It has tender flesh and a mild, somewhat nutty flavor. Wild fish has a stronger, gamier taste than farmed fish. While the taste of wild-caught trout is often promoted as superior, it is illegal to sell or market wild-caught rainbow trout, which are legally classified as game fish, in the United States. Thus, rainbow trout and "steelhead" sold in American restaurants is farmed.  Farmed rainbow are considered one of the safest fish to eat and are noted for high levels of vitamin B and a generally appealing flavor.  Seafood Watch ranks farmed rainbow as a "Best Choice" fish for human consumption.

The color and flavor of the flesh depends on the diet and freshness of the trout.  Farmed trout and some populations of wild trout, especially anadromous steelhead, have reddish or orange flesh as a result of high astaxanthin levels in their diets. Astaxanthin is a powerful antioxidant that may be from a natural source or a synthetic trout feed.  Rainbow trout raised to have pinker flesh from a diet high in astaxanthin are sometimes sold in the U.S. with labeling calling them "steelhead".  As wild steelhead are in decline in some parts of their range, farmed rainbow are viewed as a preferred alternative.  In Chile and Norway, rainbow trout farmed in saltwater sea cages are sold labeled as steelhead.

Trout can be cooked as soon as they are cleaned, without scaling, skinning or filleting. If cooked with the skin on, the meat tends to hold together better.  While trout sold commercially in Europe is often prepared and served this way, most trout sold commercially in the U.S. have had heads removed and have been fully or partially deboned and filleted. Medium to heavy bodied white wines, such as chardonnay, sauvignon blanc or pinot gris are typical wine pairings for trout. 

Notes The cutthroat trout  is a fish species of the family Salmonidae native to cold-water tributaries of the Pacific Ocean, Rocky Mountains, and Great Basin in North America. As a member of the genus Oncorhynchus, it is one of the Pacific trout, a group that includes the widely distributed rainbow trout. Cutthroat trout are popular gamefish, especially among anglers who enjoy fly fishing. The common name "cutthroat" refers to the distinctive red coloration on the underside of the lower jaw. The specific name clarkii was given to honor explorer William Clark, coleader of the Lewis and Clark Expedition.

Cutthroat trout usually inhabit and spawn in small to moderately large, clear, well-oxygenated, shallow rivers with gravel bottoms. They also reproduce in clear, cold, moderately deep lakes. They are native to the alluvial or freestone streams that are typical tributaries of the rivers of the Pacific basin, Great Basin and Rocky Mountains. Cutthroat trout spawn in the spring and may inadvertently but naturally hybridize with rainbow trout, producing fertile cutbows. Some populations of the coastal cutthroat trout  are semi-anadromous.

Several subspecies of cutthroat trout are currently listed as threatened in their native ranges due to habitat loss and the introduction of non-native species. Two subspecies,  and , are considered extinct. Cutthroat trout are raised in hatcheries to restore populations in their native range, as well as stock non-native lake environments to support angling. The cutthroat trout type species and several subspecies are the official state fish of seven western U.S. states.

Taxonomy

The scientific name of the cutthroat trout is . Cutthroat trout were the first New World trout encountered by Europeans when in 1541, Spanish explorer Francisco de Coronado recorded seeing trout in the Pecos River near Santa Fe, New Mexico. These were most likely Rio Grande cutthroat trout  The species was first described in the journals of explorer William Clark from specimens obtained during the Lewis and Clark Expedition from the Missouri River near Great Falls, Montana, and these were most likely the westslope cutthroat trout . As one of Lewis and Clark's many missions was to describe the flora and fauna encountered during their expedition, cutthroat trout were given the name  in honor of William Clark. In 1836, the type specimen of  was described by naturalist John Richardson from a tributary of the lower Columbia River, identified as the "Katpootl", which was perhaps the Lewis River as there was a Multnomah village of similar name at the confluence. This type specimen was most likely the coastal cutthroat trout subspecies . Until the 1960s, populations of westslope cutthroat trout and Yellowstone cutthroat trout were lumped into one subspecies; Salmo clarkii lewisii. Biologists later split the group into two subspecies, christening the name westslope cutthroat trout with the lewisii name which honors explorer Meriwether Lewis and renaming the Yellowstone cutthroat trout Salmo bouvierii,  the first name given to the Yellowstone cutthroat trout by David Starr Jordan in 1883 honoring a U.S. Army Captain Bouvier. 

In 1989, morphological and genetic studies indicated trout of the Pacific basin were genetically closer to Pacific salmon (Oncorhynchus species) than to the Salmos–brown trout  or Atlantic salmon  of the Atlantic basin. Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat and other Pacific basin trout into the genus Oncorhynchus.

Subspecies

Behnke in his salmon and trout handbook of 2002 recognized 14 subspecies of cutthroat trout that are each native to a separate geographic area. Not all of them were scientifically described, and different views on the taxonomic identities have been presented in some cases. It has been suggested that the cutthroat trout evolved from a common Oncorhynchus ancestor that migrated along the Pacific coast and into the mountain west primarily via the Columbia and Snake river basins 3-5 million years ago, in the late Pliocene or early Pleistocene epochs. These epochs had repeated glacial and interglacial periods that would have caused repeated fracturing and isolation of cutthroat trout populations, eventually resulting in the different subspecies found today. The 14 subspecies are found in four evolutionary groups—Coastal, Westslope, Yellowstone and Lahontan.

Description

Throughout their native and introduced ranges, cutthroat trout vary widely in size, coloration and habitat selection. Their coloration can range from golden to gray to green on the back. Cutthroat trout can generally be distinguished from rainbow trout by the presence of basibranchial teeth at the base of tongue and a maxillary that extends beyond the posterior edge of the eye.  Depending on subspecies, strain and habitat, most have distinctive red, pink, or orange linear marks along the underside of their mandibles in the lower folds of the gill plates. These markings are responsible for the common name "cutthroat", first given to the trout by outdoor writer Charles Hallock in an 1884 article in The American Angler. These markings are not unique to the species, some coastal rainbow trout  and Columbia River redband trout  populations also display reddish or pink throat markings.

At maturity, different populations and subspecies of cutthroat trout can range from 6 to in length, depending on habitat and food availability. Sea-run forms of coastal cutthroat trout average 2 to. The length and weights of mature inland forms vary widely depending on their particular environment and availability of food. Stream-resident fish are much smaller, , while lacustrine populations have attained weights ranging from 12 to in ideal conditions. The largest cutthroat trout subspecies is the Lahontan cutthroat trout . These fish average 8 to in small streams and 8 to in larger rivers and lakes. In ideal environments, the Lahontan cutthroat trout attains typical weights of . The world record cutthroat trout is a Lahontan at 39 in and 41 lbs.

Lifecycle

Cutthroat trout usually inhabit and spawn in small to moderately large, clear, well-oxygenated, shallow rivers with gravel bottoms. They are native to the alluvial or freestone streams typical of tributaries of the Pacific Basin, Great Basin and Rocky Mountains. They spawn in the spring, as early as February in coastal rivers and as late as July in high mountain lakes and streams. Spawning begins when water temperatures reach 43 to. Cutthroat trout construct a redd in the stream gravel to lay eggs. The female selects the site for and excavates the redd. Females, depending on size, lay between 200 and 4,400 eggs. Eggs are fertilized with milt (sperm) by an attending male. Eggs hatch into alevins or sac fry in about a month and spend two weeks in the gravel while they absorb their yolk sack before emerging. After emergence, fry begin feeding on zooplankton. Juvenile cutthroat trout typically mature in three to five years.  Lake-resident cutthroat trout are usually found in moderately deep, cool lakes with adequate shallows and vegetation for good food production. Lake populations generally require access to gravel-bottomed streams to be self-sustaining, but occasionally spawn on shallow gravel beds with good water circulation.

Cutthroat trout naturally interbreed with the closely related rainbow trout, producing fertile hybrids commonly called "cutbows". This hybrid generally bears similar coloration and overall appearance to the cutthroat trout, usually retaining the characteristic orange-red slash. Cutbow hybrids often pose a taxonomic difficulty when trying to distinguish any given specimen as a rainbow or cutthroat trout. In addition, cutthroat trout may hybridize with  the Gila trout and  the Apache trout in regions where their ranges overlap. 

Ecology

Range

Cutthroat trout are native to western North America and have evolved through geographic isolation into 14 subspecies, each native to a different major drainage basin. Native cutthroat trout species are found along the Pacific Northwest coast from Alaska through British Columbia into northern California, in the Cascade Range, the Great Basin and throughout the Rocky Mountains including southern Alberta. Some coastal populations of the coastal cutthroat trout  are semianadromous, spending a few months in marine environments to feed as adults and returning to fresh water from fall through early spring to feed on insects and spawn. Cutthroat trout have the second-largest historic native range of North American trout; the lake trout  having the largest. Ranges of some subspecies, particularly the westslope cutthroat trout  have been reduced to less than 10 percent of their historic range due to habitat loss and introduction of non-native species.

Although members of Oncorhynchus, the Pacific trout/salmon species, three subspecies—the westslope , the greenback  and Yellowstone cutthroat trout —evolved populations east of the Continental Divide in the upper Missouri River basin, upper Arkansas and Platte River basins and upper Yellowstone River basin, each which drain into the Atlantic basin via the Mississippi River (in 2005, researchers published a report stating that a natural distribution of , the Conchos trout, is also located in an Atlantic basin drainage ). Scientists believe that the climatic and geologic conditions 3 to 5 million years ago allowed cutthroat trout from the Snake River to migrate over the divide into the Yellowstone plateau via Two Ocean Pass. There is also evidence that Yellowstone Lake once drained south into the Snake River drainage.  Evidence suggests that the westslope cutthroat trout was able to establish populations east of the divide via Summit Lake at Marias Pass which at one time connected the Flathead River drainage with the upper Missouri River drainage.  Scientists speculate that there are several mountain passes associated with the headwaters of the Colorado River drainage and Arkansas/Platte River drainages that would have allowed migration of cutthroat trout east of the divide. 

Cutthroat trout have been introduced into non-native waters outside their historic native range, but not to the extent of the rainbow trout . Within the native range of the Yellowstone cutthroat trout, U.S. Fisheries Bureau and National Park Service authorities introduced Yellowstone cutthroat trout into many fishless lakes in Yellowstone National Park. Cutthroat trout were introduced into Lake Michigan tributaries in the 1890s and sporadically in the early 20th century, but never established wild populations. A population of Yellowstone cutthroat trout purportedly has been established in Lake Huron.   Although cutthroat trout are not native to Arizona, they are routinely introduced by the Arizona Game and Fish Department into high mountain lakes in the White Mountains in the northeastern region of that state. 

Habitat

Cutthroat trout require cold, clear, well-oxygenated, shallow rivers with gravel bottoms or cold, moderately deep lakes. Healthy stream-side vegetation that reduces siltation is typical of healthy cutthroat trout habitat and beaver ponds may provide refuge during periods of drought and over winter.  Most populations stay in fresh water throughout their lives and are known as nonmigratory, stream-resident or riverine populations. The coastal cutthroat trout  is the only cutthroat trout subspecies to coevolve through its entire range with the coastal rainbow trout (O. m. irideus). Portions of the westslope cutthroat trout's  range overlap with the Columbia River redband trout , but the majority of its native range is in headwater tributary streams above major waterfalls and other barriers to upstream migration. At least three subspecies are confined to isolated basins in the Great Basin and can tolerate saline or alkaline water. 

Cutthroat trout are opportunistic feeders. Stream-resident cutthroat trout primarily feed on larval, pupal and adult forms of aquatic insects (typically caddisflies, stoneflies, mayflies and aquatic dipterans), and adult forms of terrestrial insects (typically ants, beetles, grasshoppers and crickets) that fall into the water, fish eggs, small fish, along with crayfish, shrimp and other crustaceans. As they grow the proportion of fish consumed increases in most populations. In saltwater estuaries and along beaches, Coastal cutthroat trout feed on small fish such as sculpins, sand lance, salmon fry and herring.  They also consume shrimp, small squid and krill. In fresh water they consume the same diet as stream resident trout—aquatic insects and crustaceans, amphibians, earthworms, small fish and fish eggs.  Within the range of the bull trout  the cutthroat trout is a forage fish for the piscivorous bull trout. 

Artificial propagation

Various subspecies of cutthroat trout are raised in commercial, state and federal hatcheries for introduction into suitable native and non-native riverine and lacustrine environments. In the early 20th century, several hatcheries were established in Yellowstone National Park by the U.S. Bureau of Fisheries. These hatcheries not only produced stocks of the Yellowstone cutthroat trout  for the park, but also took advantage of the great spawning stock of cutthroat trout to supply eggs to hatcheries around the U.S. Between the years 1901 and 1953 a total of 818 million trout eggs were exported from the park to hatcheries throughout the U.S. The Lahontan National Fish Hatchery operated by the U.S. Fish and Wildlife Service exists to restore populations of the Lahontan cutthroat trout  in Pyramid, Walker, Fallen Leaf, June, Marlette, and Gull Lakes and the Truckee River in California and Nevada.  The hatchery produces about 300,000–400,000 Lahontan cutthroat trout fry annually. The Jackson National Fish Hatchery produces around 400,000 Snake River fine-spotted cutthroat trout  annually to support fisheries in Idaho and Wyoming.  The Leadville National Fish Hatchery produces 125,000–200,000 Snake River fine-spotted, greenback cutthroat and rainbow trout annually to support fishing in the Fryingpan and Arkansas River drainages and other Colorado waters.  The Bozeman Fish Technology Center, formerly a cutthroat trout fish hatchery in Bozeman, Montana, plays a major role in the restoration of the greenback  and westslope cutthroat trout  subspecies. 

Population threats

The historic native range of cutthroat trout has been reduced by overfishing, urbanization and habitat loss due to mining, livestock grazing and logging. Population densities have been reduced and in some cases populations have disappeared though competition with non-native brook, brown, lake and rainbow trout, kokanee salmon, lake whitefish and mysis shrimp which were introduced in the late 19th and early 20th centuries. Scientists believe the westslope cutthroat trout will eventually be extirpated from the large lakes in Western Montana due to the trophic cascades resulting from lake trout and mysis shrimp introductions.  The most serious current threats to several subspecies are interspecific breeding with introduced rainbow trout creating hybrid cutbows and intraspecific breeding with other introduced cutthroat trout subspecies. In the Greater Yellowstone Ecosystem, the presence of lake trout  in Yellowstone Lake in Yellowstone National Park has caused a serious decline in . Outbreaks of whirling disease in major spawning tributaries within the native ranges have also caused declines. Most subspecies of cutthroat trout are highly susceptible to whirling disease, although the Snake River fine-spotted cutthroat trout  appears to be resistant to the parasite. 

Interspecific and intraspecific breeding

The most serious impact on the genetic purity of most cutthroat trout subspecies results from interspecific and intraspecific breeding resulting in hybrids that carry the genes of both parents. In inland populations, the introduction of rainbow trout from hatchery stocks have resulted in cutbow hybrids that continue to diminish the genetic purity of many cutthroat trout subspecies. The introduction of hatchery-raised Yellowstone cutthroat trout into native ranges of other cutthroat trout subspecies, particularly the westslope cutthroat trout, has resulted in intraspecific breeding and diminished genetic purity of the westslope subspecies. As such, populations of genetically pure westslope cutthroat trout are very rare and localized in streams above barriers to upstream migrations by introduced species. Fisheries biologist Robert J. Behnke attributes the extinction of the yellowfin cutthroat trout  and Alvord cutthroat trout  subspecies to the introduction of non-native rainbow trout.

Decline of the Yellowstone subspecies

The population at the core of the Yellowstone cutthroat trout's native range, in Yellowstone Lake, declined significantly in the 1960s due to overharvest of mature cutthroat trout by anglers, as well as overharvesting of eggs by hatcheries in the early 20th century. Managers implemented catch and release, which required anglers to return their catches to the lake, and they terminated hatchery operations in the park which allowed the cutthroat trout to recover. Then, in 1994, park officials discovered lake trout  in Yellowstone Lake. Although lake trout were established in Shoshone, Lewis and Heart lakes in the Snake River drainage from U.S. government stocking operations in 1890, they were never officially introduced into the Yellowstone River drainage and their presence there is probably the result of accidental or illegal introductions.  By 2000, the cutthroat trout population had declined to less than 10 percent of its early 20th century abundance. However, aggressive lake trout eradication programs have killed over one million lake trout since 1996, and the hope is that this will lead to a restoration of cutthroat numbers.  Cutthroat trout co-exist with lake trout in Heart Lake, an isolated back-country lake at the head of the Heart River that gets little angling pressure. 

Angling

Cutthroat trout are prized as a gamefish, particularly by fly anglers. They are regulated as a gamefish in every state and province they occur in. From the Yellowstone cutthroat trout fishery in Yellowstone National Park,    the unique Lahontan cutthroat trout fishery in Pyramid Lake in Nevada, and the small stream fisheries of the westslope cutthroat trout to saltwater angling for sea-run cutthroat trout on the Pacific coast,  cutthroat trout are a popular quarry for trout anglers throughout their ranges. The all-tackle world record is 41 lbs caught in Pyramid Lake in December 1925. Their propensity to feed on aquatic and terrestrial insects make them an ideal quarry for the fly angler. 

Sea-run fishing along the Pacific coast

From Alaska to Northern California, coastal cutthroat trout in sea-run, resident stream and lacustrine forms are sought by anglers. Puget Sound in Washington is a stronghold of sea-run cutthroat trout fishing with its many tributaries and protected saltwater inlets and beaches. Fly anglers search for sea-run cutthroat trout along beaches, river mouths and estuaries year round. In the lower reaches of larger rivers, anglers in drift boats float the rivers searching for trout along the wooded shorelines.  In addition to a cutthroat trout sport fishery managed by the Alaska Department of Fish and Game, the Federal Subsistence Management Program  manages coastal cutthroat trout subsistence fisheries in Southeast Alaska. 

Yellowstone fishery

Yellowstone National Park was established in 1872. By the 1890s, the Yellowstone cutthroat trout fishery in the Yellowstone River and Yellowstone Lake were well known and being promoted in national guidebooks.

In 1902, anticipating the completion of the east entrance road from Cody, Wyoming, Captain Hiram M. Chittenden supervised the construction of the first "Fishing Bridge" across the outlet of Yellowstone Lake.  Fishing Bridge was rebuilt in 1919, and reconstructed in
1937, primarily to accommodate vehicle traffic. Between 1916 and 1931, a large development of campgrounds, cabins, stores and service facilities were built just east of the bridge to support anglers. This area is now known as the Fishing Bridge Historic District.  The 1937 bridge boasted pedestrian walkways on either side of the roadway to give more room to anglers. Angling in the river, at Fishing Bridge and in the lake, boomed during the 1950s and 1960s and over harvest caused a significant decline in the fishery. Consequently, in 1973, fishing was no longer permitted from Fishing Bridge.

Pyramid Lake Lahontan subspecies fishery

The Pyramid Lake strain of Lahontan cutthroat trout, source of the 41 lbs world record and native to Pyramid Lake, Lake Tahoe and the Truckee River, was brought to near-extinction in the two decades between the 1920s-40s from overharvest, introduced species and loss of spawning habitat. In the 1970s, Pyramid Lake was stocked with Lahontan cutthroat trout strains still surviving in some nearby lakes, but they were not the large Pyramid Lake strain. Although the Paiute tribe had been successful in reestablishing a cutthroat trout fishery on the reservation in Pyramid Lake, the cutthroat trout were not the large fish of the late 19th and early 20th century. In the late 1970, biologists discovered a surviving population of a pure Pyramid Lake strain in a small headwaters stream on the Nevada-Utah border that had at some point around 1900 been introduced into the stream. In 1995, the U.S. Fish and Wildlife Service began rearing these fish in the Lahontan National Fish Hatchery  in Gardnerville, Nevada. In 2006, the Pyramid Lake strain of Lahontan cutthroat trout were reintroduced into the lake.  As Pyramid Lake has a very shallow shoreline, anglers use ladders to stand comfortably in 3 to of water and cast to trout cruising along shoreline breaks. Considered a "world-class" fishery, anglers routinely catch cutthroat trout exceeding 10 lbs. 

As a symbol

The cutthroat trout is the state fish of Idaho, Montana and Wyoming, while particular subspecies of cutthroat are the state fish of Colorado, Nevada, New Mexico and Utah.
* Idaho  Montana  and Wyoming –Cutthroat trout 
* Colorado–Greenback cutthroat trout 
* Nevada –Lahontan cutthroat trout 
* New Mexico –Rio Grande cutthroat trout 
* Utah –Bonneville cutthroat trout 

Notes A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.   Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish). With 33,100 described species, fish exhibit greater species diversity than any other group of vertebrates. 

Fish are an important resource for humans worldwide, especially as food. Commercial and subsistence fishers hunt fish in wild fisheries (see fishing) or farm them in ponds or in cages in the ocean (see aquaculture). They are also caught by recreational fishers, kept as pets, raised by fishkeepers, and exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds, and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.

The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.

Evolution

Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event. 

Early fish from the fossil record are represented by a group of small, jawless, armored fish known as ostracoderms. Jawless fish lineages are mostly extinct. An extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.

Fish may have evolved from a creature similar to a coral-like Sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case.

Taxonomy

Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications.

Traditional classification divide fish into three extant classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes:  
* Class Agnatha (jawless fish)
** Subclass Cyclostomata (hagfish and lampreys)
** Subclass Ostracodermi (armoured jawless fish) †
* Class Chondrichthyes (cartilaginous fish)
** Subclass Elasmobranchii (sharks and rays)
** Subclass Holocephali (chimaeras and extinct relatives)
* Class Placodermi (armoured fish) †
* Class Acanthodii ("spiny sharks", sometimes classified under bony fishes)†

* Class Osteichthyes (bony fish)
** Subclass Actinopterygii (ray finned fishes)
** Subclass Sarcopterygii (fleshy finned fishes, ancestors of tetrapods)

The above scheme is the one most commonly encountered in non-specialist and general works. Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of phylogenetic nomenclature, the fishes has been split up into a more detailed scheme, with the following major groups:
* Class Myxini (hagfish)
* Class Pteraspidomorphi † (early jawless fish)
* Class Thelodonti †
* Class Anaspida †
* Class Petromyzontida or Hyperoartia
** Petromyzontidae (lampreys)
* Class Conodonta (conodonts) †
* Class Cephalaspidomorphi † (early jawless fish)
** (unranked) Galeaspida †
** (unranked) Pituriaspida †
** (unranked) Osteostraci †
* Infraphylum Gnathostomata (jawed vertebrates)
** Class Placodermi † (armoured fish)
** Class Chondrichthyes (cartilaginous fish)
** Class Acanthodii † (spiny sharks)
** Superclass Osteichthyes (bony fish)
*** Class Actinopterygii (ray-finned fish)
**** Subclass Chondrostei
***** Order Acipenseriformes (sturgeons and paddlefishes)
***** Order Polypteriformes (reedfishes and bichirs).
**** Subclass Neopterygii
***** Infraclass Holostei (gars and bowfins)
***** Infraclass Teleostei (many orders of common fish)
*** Class Sarcopterygii (lobe-finned fish)
**** Subclass Actinistia (coelacanths)
**** Subclass Dipnoi (lungfish)

† – indicates extinct taxonSome palaeontologists contend that because Conodonta are chordates, they are primitive fish. For a fuller treatment of this taxonomy, see the vertebrate article.

The position of hagfish in the phylum Chordata is not settled. Phylogenetic research in 1998 and 1999 supported the idea that the hagfish and the lampreys form a natural group, the Cyclostomata, that is a sister group of the Gnathostomata.  

The various fish groups account for more than half of vertebrate species. There are almost 28,000 known extant species, of which almost 27,000 are bony fish, with 970 sharks, rays, and chimeras and about 108 hagfish and lampreys.  A third of these species fall within the nine largest families; from largest to smallest, these families are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species. The final total of extant species may grow to exceed 32,500. 

Diversity

The term "fish" most precisely describes any non-tetrapod craniate (i.e. an animal with a skull and in most cases a backbone) that has gills throughout life and whose limbs, if any, are in the shape of fins.  Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including hagfishes, lampreys, sharks and rays, ray-finned fish, coelacanths, and lungfish. Tree of life web project – [http://tolweb.org/Chordata/2499 Chordates]. Indeed, lungfish and coelacanths are closer relatives of tetrapods (such as mammals, birds, amphibians, etc.) than of other fish such as ray-finned fish or sharks, so the last common ancestor of all fish is also an ancestor to tetrapods. As paraphyletic groups are no longer recognised in modern systematic biology, the use of the term "fish" as a biological group must be avoided.

Many types of aquatic animals commonly referred to as "fish" are not fish in the sense given above; examples include shellfish, cuttlefish, starfish, crayfish and jellyfish. In earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also seals, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish.  However, according to the definition above, all mammals, including cetaceans like whales and dolphins, are not fish. In some contexts, especially in aquaculture, the true fish are referred to as finfish (or fin fish) to distinguish them from these other animals.

A typical fish is ectothermic, has a streamlined body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with scales, and lays eggs.

Each criterion has exceptions. Tuna, swordfish, and some species of sharks show some warm-blooded adaptations—they can heat their bodies  significantly above ambient water temperature. Streamlining and swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second.  Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapods, gouramis have a structure called the labyrinth organ that performs a similar function, while many catfish, such as corydoras extract oxygen via the intestine or stomach.  Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, pufferfish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked (as in moray eels), or covered with scales of a variety of different types usually defined as placoid (typical of sharks and rays), cosmoid (fossil lungfish and coelacanths), ganoid (various fossil fish but also living gars and bichirs), cycloid, and ctenoid (these last two are found on most bony fish).  There are even fish that live mostly on land. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows.  The catfish Phreatobius cisternarum lives in underground, phreatic habitats, and a relative lives in waterlogged leaf litter.  

Fish range in size from the huge 16 m whale shark to the tiny 8 mm stout infantfish.

Fish species diversity is roughly divided equally between marine (oceanic) and freshwater ecosystems. Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth. Exceptionally rich sites in the Amazon basin, such as Cantão State Park, can contain more freshwater fish species than occur in all of Europe. 

Anatomy

Respiration

Gills

Most fish exchange gases using gills on either side of the pharynx. Gills consist of threadlike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an operculum.

Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.

Air breathing

Fish from multiple groups can live out of the water for extended periods. Amphibious fish such as the mudskipper can live and move about on land for up to several days, or live in stagnant or otherwise oxygen depleted water. Many such fish can breathe air via a variety of mechanisms. The skin of anguillid eels may absorb oxygen directly. The buccal cavity of the electric eel may breathe air. Catfish of the families Loricariidae, Callichthyidae, and Scoloplacidae absorb air through their digestive tracts.  Lungfish, with the exception of the Australian lungfish, and bichirs have paired lungs similar to those of tetrapods and must surface to gulp fresh air through the mouth and pass spent air out through the gills. Gar and bowfin have a vascularized swim bladder that functions in the same way. Loaches, trahiras, and many catfish breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to frogs). A number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. Labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the Clariidae catfish family.

Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation (summertime hibernation) until water returns.

Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish, must breathe air periodically or they suffocate. Facultative air breathers, such as the catfish Hypostomus plecostomus, only breathe air if they need to and will otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.

Circulation

Fish have a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body. In most fish, the heart consists of four parts, including two chambers and an entrance and exit.  The first part is the sinus venosus, a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the second part, the atrium, which is a large muscular chamber. The atrium serves as a one-way antechamber, sends blood to the third part, ventricle. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, bulbus arteriosus, a large tube, and then out of the heart. The bulbus arteriosus connects to the aorta, through which blood flows to the gills for oxygenation.

Digestion

Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the esophagus. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called pyloric caeca, which secrete digestive enzymes and absorb nutrients. Organs such as the liver and pancreas add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.

Excretion

As with many aquatic animals, most fish release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills. Blood wastes are  filtered by the kidneys.

Saltwater fish tend to lose water because of osmosis. Their kidneys return water to the body. The reverse happens in freshwater fish: they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.

Scales

The scales of fish originate from the mesoderm (skin); they may be similar in structure to teeth.

Sensory and nervous system

Central nervous system

Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal.  However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials. 

Fish brains are divided into several regions. At the front are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves. The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the structural equivalent to the cerebrum in higher vertebrates. In fish the telencephalon is concerned mostly with olfaction. Together these structures form the forebrain.

Connecting the forebrain to the midbrain is the diencephalon (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with hormones and homeostasis. The pineal body lies just above the diencephalon. This structure detects light, maintains circadian rhythms, and controls color changes.

The midbrain or mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids.

The hindbrain or metencephalon is particularly involved in swimming and balance. The cerebellum is a single-lobed structure that is typically the biggest part of the brain. Hagfish and lampreys have relatively small cerebellae, while the mormyrid cerebellum  is massive and apparently involved in their electrical sense.

The brain stem or myelencephalon is the brain's posterior. As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration and osmoregulation.

Sense organs

Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's (see vision in fishes). Many fish also have chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears, many fish may not hear very well. Most fish have sensitive receptors that form the lateral line system, which detects gentle currents and vibrations, and senses the motion of nearby fish and prey.  Some fish, such as catfish and sharks, have organs that detect weak electric currents on the order of  millivolt.  Other fish, like the South American electric fishes Gymnotiformes, can produce weak electric currents, which they use in navigation and social communication.

Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination. 

Vision

Vision is an important sensory system for most species of fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rods and cones (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.  Fish vision shows adaptation to their visual environment, for example deep sea fishes have eyes suited to the dark environment.

Hearing

Hearing is an important sensory system for most species of fish. Fish sense sound using their lateral lines and their ears.

Capacity for pain

Experiments done by William Tavolga provide evidence that fish have pain and fear responses. For instance, in Tavolga's experiments, toadfish grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode. 

In 2003, Scottish scientists at the University of Edinburgh and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with pain in other animals. Bee venom and acetic acid injected into the lips resulted in fish rocking their bodies and rubbing their lips along the sides and floors of their tanks, which the researchers concluded were attempts to relieve pain, similar to what mammals would do.   Neurons fired in a pattern resembling human neuronal patterns.

Professor James D. Rose of the University of Wyoming  claimed the study was flawed since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours".  Rose argues that since fish brains are so different from human brains, fish are probably not conscious in the manner humans are, so that reactions similar to human reactions to pain instead have other causes. Rose had published a study a year earlier arguing that fish cannot feel pain because their brains lack a neocortex.  However, animal behaviorist Temple Grandin argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions."

Animal welfare advocates raise concerns about the possible suffering of fish caused by angling. Some countries, such as Germany have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish. 

Muscular system

Most fish move by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the back fin, backward force is applied to the water, and in conjunction with the fins, moves the fish forward. The fish's fins function like an airplane's flaps. Fins also increase the tail's surface area, increasing speed. The streamlined body of the fish decreases the amount of friction from the water. Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that adjusts their buoyancy through manipulation of gases.

Homeothermy

Although most fish are exclusively ectothermic, there are exceptions.

Certain species of fish maintain elevated body temperatures. Endothermic teleosts (bony fish) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). All sharks in the family Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are endothermic, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures.  See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central nervous system processing, and higher rates of digestion.

Reproductive system

Fish reproductive organs include testicles and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused. There may also be a range of secondary organs that increase reproductive fitness.

In terms of spermatogonia distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in atherinomorph fish they are confined to the distal portion of these structures. Fish can present cystic or semi-cystic spermatogenesis in relation to the release phase of germ cells in cysts to the seminiferous tubules lumen. 

Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.  Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct. Secondary gymnovaries are found in salmonids and a few other teleosts.

Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the nucleus, ooplasm, and the surrounding layers characterize the oocyte maturation process.

Postovulatory follicles are structures formed after oocyte release; they do not have endocrine function, present a wide irregular lumen, and are rapidly reabsorbed in a process involving the apoptosis of follicular cells. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.

Some fish, like the California sheephead, are hermaphrodites, having both testes and ovaries either at different phases in their life cycle or, as in hamlets, have them simultaneously.

Over 97% of all known fish are oviparous,  that is, the eggs develop outside the mother's body. Examples of oviparous fish include salmon, goldfish, cichlids, tuna, and eels. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their gametes into the surrounding water. However, a few oviparous fish practice internal fertilization, with the male using some sort of intromittent organ to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the horn shark, and oviparous rays, such as skates. In these cases, the male is equipped with a pair of modified pelvic fins known as claspers.

Marine fish can produce high numbers of eggs which are often released into the open water column. The eggs have an average diameter of 1 mm.

File:Oeufs002b,57.png|Egg of lamprey
File:Oeufs002b,54.png|Egg of catshark (mermaids' purse)
File:Oeufs002b,55.png|Egg of bullhead shark
File:Oeufs002b,56.png|Egg of chimaera

The newly hatched young of oviparous fish are called larvae. They are usually poorly formed, carry a large yolk sac (for nourishment), and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to become juveniles. During this transition larvae  must switch from their yolk sac to feeding on zooplankton prey, a process which depends on typically inadequate zooplankton density, starving many larvae.

In ovoviviparous fish the eggs develop inside the mother's body after internal fertilization but receive little or no nourishment directly from the mother, depending instead on the yolk. Each embryo develops in its own egg. Familiar examples of ovoviviparous fish include guppies, angel sharks, and coelacanths.

Some species of fish are viviparous. In such species the mother retains the eggs and nourishes the embryos. Typically, viviparous fish have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with that of the embryo. Examples of viviparous fish include the surf-perches, splitfins, and lemon shark. Some viviparous fish exhibit oophagy, in which the developing embryos eat other eggs produced by the mother. This has been observed primarily among sharks, such as the shortfin mako and porbeagle, but is known for a few bony fish as well, such as the halfbeak Nomorhamphus ebrardtii.  Intrauterine cannibalism is an even more unusual mode of vivipary, in which the largest embryos eat weaker and smaller siblings. This behavior is also most commonly found among sharks, such as the grey nurse shark, but has also been reported for Nomorhamphus ebrardtii.

Aquarists commonly refer to ovoviviparous and viviparous fish as livebearers.

Diseases

Like other animals, fish suffer from diseases and parasites. To prevent disease they have a variety of defenses. Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the epidermis that traps and inhibits the growth of microorganisms. If pathogens breach these defenses, fish can develop an inflammatory response that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens. Specific defenses respond to particular pathogens recognised by the fish's body, i.e., an immune response.  In recent years, vaccines have become widely used in aquaculture and also with ornamental fish, for example furunculosis vaccines in farmed salmon and koi herpes virus in koi.  

Some species use cleaner fish to remove external parasites. The best known of these are the Bluestreak cleaner wrasses of the genus Labroides found on coral reefs in the Indian and Pacific oceans. These small fish maintain so-called "cleaning stations" where other fish congregate and perform specific movements to attract the attention of the cleaners.  Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner, and the much larger Etroplus suratensis. 

Immune system

Immune organs vary by type of fish. 
In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. These fish rely on regions of lymphoid tissue within other organs to produce immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature.) They resemble primitive bone marrow in hagfish.
Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to Chondrichthyes; the epigonal organs (lymphoid tissue similar to mammalian bone) that surround the gonads, the Leydig's organ within the walls of their esophagus, and a spiral valve in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored.
Chondrostean fish (sturgeons, paddlefish, and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system.) Their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.

Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), which houses many different immune cells.  In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.   In 2006, a lymphatic system similar to that in mammals was described in one species of teleost fish, the zebrafish. Although not confirmed as yet, this system presumably will be where naive (unstimulated) T cells accumulate while waiting to encounter an antigen. 

B and T lymphocytes bearing immunoglobulins and T cell receptors, respectively, are found in all jawed fishes. Indeed, the adaptive immune system as a whole evolved in an ancestor of all jawed vertebrate.Flajnik, M. F., and M. Kasahara. "Origin and evolution of the adaptive immune system: genetic events and selective pressures." Nature Reviews Genetics 11.1, 47-59 (2009).

Conservation

The 2006 IUCN Red List names 1,173 fish species that are threatened with extinction.  Included are species such as Atlantic cod,  Devil's Hole pupfish,  coelacanths,  and great white sharks.  Because fish live underwater they are  more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the Devil's Hole pupfish occupies only a single 3 by pool. 

Overfishing

Overfishing is a major threat to edible fish such as cod and tuna.   Overfishing eventually causes population (known as stock) collapse because the survivors cannot produce enough young to replace those removed. Such commercial extinction does not mean that the species is extinct, merely that it can no longer sustain a fishery.

One well-studied example of fishery collapse is the Pacific sardine Sadinops sagax caerulues fishery off the California coast. From a 1937 peak of 790000 LT the catch steadily declined to only 24000 LT in 1968, after which the fishery was no longer economically viable. 

The main tension between fisheries science and the fishing industry is that the two groups have different views on the resiliency of fisheries to intensive fishing. In places such as Scotland, Newfoundland, and Alaska the fishing industry is a major employer, so governments are predisposed to support it.   On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be wiped out within fifty years.  

Habitat destruction

A key stress on both freshwater and marine ecosystems is habitat degradation including water pollution, the building of dams, removal of water for use by humans, and the introduction of exotic species.  An example of a fish that has become endangered because of habitat change is the pallid sturgeon, a North American freshwater fish that lives in rivers damaged by human activity. 

Exotic species

Introduction of non-native species has occurred in many habitats. One of the best studied examples is the introduction of Nile perch into Lake Victoria in the 1960s. Nile perch gradually exterminated the lake's 500 endemic cichlid species. Some of them survive now in captive breeding programmes, but others are probably extinct.  Carp, snakeheads,  tilapia, European perch, brown trout, rainbow trout, and sea lampreys are other examples of fish that have caused problems by being introduced into alien environments.

Importance to humans

Economic importance

Throughout history, humans have utilized fish as a food source. Historically and today, most fish protein has come by means of catching wild fish. However, aquaculture, or fish farming, which has been practiced since about 3,500 BCE. in China,  is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish.  That proportion is considerably elevated in some developing nations and regions heavily dependent on the sea. In a similar manner, fish have been tied to trade.

Catching fish for the purpose of food or sport is known as fishing, while the organized effort by humans to catch fish is called a fishery. Fisheries are a huge global business and provide income for millions of people. The annual yield from all fisheries worldwide is about 154 million tons,  with popular species including herring, cod, anchovy, tuna, flounder, and salmon. However, the term fishery is broadly applied, and includes more organisms than just fish, such as mollusks and crustaceans, which are often called "fish" when used as food.

Recreation

Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in aquariums in homes, offices, or public settings.

Recreational fishing is fishing for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit. The most common form of recreational fishing is done with a rod, reel, line, hooks and any one of a wide range of baits. Angling is a method of fishing, specifically the practice of catching fish by means of an "angle" (hook). Anglers must select the right hook, cast accurately, and retrieve at the right speed while considering water and weather conditions, species, fish response, time of the day, and other factors.

Culture

Fish feature prominently in art and literature, in movies such as Finding Nemo and books such as The Old Man and the Sea. Large fish, particularly sharks, have frequently been the subject of horror movies and thrillers, most notably the novel Jaws, which spawned a series of films of the same name that in turn inspired similar films or parodies such as Shark Tale and Snakehead Terror. Piranhas are shown in a similar light to sharks in films such as Piranha; however, contrary to popular belief, the red-bellied piranha is actually a generally timid scavenger species that is unlikely to harm humans.  In the Book of Jonah a "great fish" swallowed Jonah the Prophet. Legends of half-human, half-fish mermaids have featured in folklore, like the stories of Hans Christian Andersen.

Fish themes have symbolic significance in many religions. The fish is used often as a symbol by Christians to represent Jesus, or Christianity in general; the gospels also refer to "fishers of men"  and feeding the multitude. In the dhamma of Buddhism the fish symbolize happiness as they have complete freedom of movement in the water. Often drawn in the form of carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span. Among the deities said to take the form of a fish are Ika-Roa of the Polynesians, Dagon of various ancient Semitic peoples, the shark-gods of Hawaii and Matsya of the Hindus.

The astrological symbol Pisces is based on a constellation of the same name, but there is also a second fish constellation in the night sky, Piscis Austrinus. 

Terminology

Shoal or school

A random assemblage of fish merely using some localised resource such as food or nesting sites is known simply as an aggregation. When fish come together in an interactive, social grouping, then they may be forming either a shoal or a school depending on the degree of organisation. A shoal is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. Schools of fish are much more tightly organised, synchronising their swimming so that all fish move at the same speed and in the same direction. Shoaling and schooling behaviour is believed to provide a variety of advantages. 

Examples:
* Cichlids congregating at lekking sites form an aggregation.
* Many minnows and characins form shoals.
* Anchovies, herrings and silversides are classic examples of schooling fish.

While the words "school" and "shoal" have different meanings within biology, they are often treated as synonyms by non-specialists, with speakers of British English using "shoal" to describe any grouping of fish, and speakers of American English often using "school" just as loosely.

Fish or fishes

Though often used interchangeably, in biology these words have different meanings. Fish is used as a singular noun, or as a plural to describe multiple individuals from a single species. Fishes is used to describe different species or species groups.    Thus a pond that contained a single species might be said to contain 120 fish. But if the pond contained a total of 120 fish from three different species, it would be said to contain three fishes. The distinction is similar to that between people and peoples.

Finfish
 
* In biology, the term fish is most strictly used to describe any animal with a backbone that has gills throughout life and has limbs, if any, in the shape of fins.  Many types of aquatic animals with common names ending in "fish" are not fish in this sense; examples include shellfish, cuttlefish, starfish, crayfish and jellyfish. In earlier times, even biologists did not make a distinction—sixteenth century natural historians classified also seals, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish.
* In fisheries, the term fish is used as a collective term, and includes mollusks, crustaceans and any aquatic animal which is harvested. 
* The strict biological definition of a fish, above, is sometimes called a true fish. True fish are also referred to as finfish or fin fish to distinguish them from other aquatic life harvested in fisheries or aquaculture.
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