Salmon

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Salmon is the common name for several species of large food and game fish of the family Salmonidae, a group which also contains trout, whitefish, and their relatives. The species called salmons are placed within two genera, Salmo and Oncorhynchus, with various species of trout. Sometimes, however, the term is used to refer more generally to any member of Salmonidae, the salmon family.

Salmon live in both the Atlantic and Pacific Oceans, as well as in the Great Lakes and other landlocked lakes. They are large, predatory fish, mainly feeding on other smaller fish, and they comprise one of the greatest commercial fish industries in the world. Although some are landlocked in freshwater lakes, salmon typically are anadromous: they are born in fresh water, migrate to the ocean, then return to fresh water to reproduce.

The life cycle of salmon reflects on parental sacrifice and the importance of reproduction and lineage. For all animals, having offspring requires an expenditure of energy. For most salmon, this expenditure is total: In all species of Pacific salmon, the mature individuals die within a few days or weeks of spawning. This reflects a remarkable level of sacrifice in service to one's offspring.

Overview

File:Chinook Salmon.jpeg
The Chinook or King Salmon is the largest salmon in North America and can grow to 1.5 meters (58 inches) in length and to 57 kilograms (125 pounds) in weight. This specimen shows the jaws drawn into a curved "kype," a secondary sex characteristic typical of many male salmon around spawning time.

Salmonidae is a family of ray-finned fish, the only living family of the order Salmoniformes. It includes the well-known salmons and trouts; the Atlantic salmons and trouts of genus Salmo give the family and order their names.

Salmon comprise several species, mainly in the Oncorhynchus genus, but also including the Atlantic salmon (Salmo salar). Species in the Oncorhynchus genus are found in the Pacific Ocean, including the Cherry salmon (Oncorhynchus masu or O. masou), which is found in the western Pacific Ocean in Japan, Korea, and Russia and landlocked in Taiwan (Ching-wen 1990); the Chinook salmon, which is the largest of all Pacific salmon (Dean 1994); and the Pink salmon or Humpback salmon (Oncorhynchus gorbuscha) found in both northern California and Korea, as well as northern Pacific. Of the several varieties of North American salmon, all but one are found off the Pacific coast and most (about 90%) come from Alaskan waters (Herbst 2001).

Salmon generally migrate from saltwater to freshwater to spawn. About ninety percent of the salmon spawning in a stream were born there. Salmon will cross large distances and treacherous river conditions to return to spawn in the stream of their birth. In Alaska, salmon have also been known to colonize new streams, which are often created when a glacier melts. The precise method salmon use to navigate has not been entirely established, though their keen sense of smell is certainly involved.

Salmon typically live for two years, returning to spawn. In all species of Pacific salmon, the mature individuals die within a few days or weeks of spawning, a trait known as semelparity. Atlantic salmon can spawn more than once (iteroparity), though post-spawning mortality is still quite high in those species exhibiting iteroparity. Salmon lay between 2,500 and 7,000 eggs depending on the particular species and size of the fish (Wisnia 1997).

Salmon has long been at the heart of the culture and livelihood of coastal dwellers. In the past, many Native American tribes of the Northern Pacific shore had ceremonies to honor the first return of the year. A famous spearfishing site on the Columbia River at Celilo Falls, located between the states of Washington and Oregon, was known for its large numbers of salmon, but was inundated and its community diminished after the Dalles Dam was built in 1957. The Ainu people of northern Japan taught dogs how to catch salmon and performed rituals to ensure a successful catch.

For many centuries, people caught salmon as they swam upriver to spawn. Now, salmon are caught in bays and near shores. Drift net fisheries have been banned on the high seas except off Northumberland on the east coast of England.

Salmon population levels are of concern in the Atlantic and in some parts of the Pacific, though in northern British Columbia and Alaska, stocks are still abundant. The Skeena River alone has millions of wild salmon returning, which support commercial fisheries, aboriginal food fisheries, sports fisheries, and the area's diverse wildlife on the coast, as well as communities hundreds of miles inland in the watershed. Columbia River salmon levels are estimated to now be less than three percent of what they were when Lewis and Clark arrived at the river (McDermott 2007).

In the southern hemisphere, there is a fish commonly called the Australian salmon, which is a salt water species not related in any way to the salmonidae (it is actually a member of the Arripidae family). Found along the southern coastline of Australia and Tasmania, it is commonly caught there with large beach nets, although its use as a commercial fish has been declining over the last twenty years.

Life History

Eggs in different stages of development. In some, only a few cells grow on top of the yolk. In the lower right the blood vessels surround the yolk, and in the upper left the black eyes are visible, even the little lens
Salmon fry hatching: The larva has grown around the remains of the yolk—visible are the arteries spinning around the yolk and little oildrops, also the gut, the spine, the main caudal blood vessel, the bladder, and the arcs of the gills

In order to lay her roe (egg masses), the female salmon uses her tail fin to excavate a shallow depression, called a redd. The redd may sometimes contain five thousand eggs covering thirty square feet (McGrath 2003). The eggs usually range from orange to red in color. One or more males will approach the female in her redd, depositing his sperm, or milt, over the roe (USFWS 2007). The female then covers the eggs by disturbing the gravel at the upstream edge of the depression before moving on to make another redd. The female will make as many as seven redds before her supply of eggs is exhausted. Many species of salmon then die within a few days after spawning.

The eggs will hatch into alevin or sac fry (still containing yolk sacs). The fry quickly develop into parr with camouflaging vertical stripes. The parr stay for one to three years in their natal stream before becoming smolts, which are distinguished by their bright silvery color with scales that are easily rubbed off. It is estimated that only ten percent of all salmon eggs survive long enough to reach this stage (Rieben et al. 1998). The smolt body chemistry changes, allowing them to live in salt water. Smolts spend a portion of their out-migration time in brackish water, where their body chemistry becomes accustomed to osmoregulation in the ocean.

The salmon spend one to five years (depending on the species) in the open ocean, where they become sexually mature. Generally, the adult salmon returns to its natal stream to spawn when, upon their first return, they are called whitling in the UK and grilse or peel in Éire. Prior to spawning, depending on the species, the salmon undergoes changes. They may grow a hump, develop canine teeth, or develop a kype (a pronounced curvature of the jaws in male salmon). At this time, all salmon change from the silvery blue of a fresh-run fish from the sea to a darker color. Their condition tends to deteriorate the longer the fish remain in freshwater, and then deteriorates further after they spawn, at that point becoming known as kelts.

Salmon can make amazing journeys, sometimes moving hundreds of miles upstream against strong currents and rapids to reproduce. Chinook and sockeye salmon from central Idaho, for example, travel over nine hundred miles and climb to elevations of 6,500 feet in order to return to spawn.

The age of a salmon can be deduced from the growth rings on its scales, examined under the microscope. Each year, the fish experiences a period of rapid growth, often in summer, and one of slower growth, normally in winter. This results in rings (annuli) analogous to the growth rings visible in a tree trunk. Freshwater growth shows as densely crowded rings, sea growth as widely spaced rings; spawning is marked by significant erosion as body mass is converted into eggs or milt.

Freshwater streams and estuaries provide important habitats for many salmon species. They feed on terrestrial and aquatic insects, amphipods, and other crustaceans while young, and primarily on other fish when older. Eggs are laid in deep water with large gravel, and need cool water and good water flow (to supply oxygen) to the developing embryos. Mortality of salmon in the early life stages is usually high due to natural predation and human-induced changes in habitat, such as siltation, high water temperatures, low oxygen conditions, loss of stream cover, and reductions in river flow. Estuaries and their associated wetlands provide vital nursery areas for the salmon prior to their departure to the open ocean. Wetlands not only help buffer the estuary from silt and pollutants, but also provide important feeding and hiding areas.

Species

The various species of salmon have many names, and varying behaviors.

Atlantic Ocean species

Atlantic ocean species belong to the genus Salmo. They include

  • Atlantic salmon or Salmon (Salmo salar), which is the species after which all the others are named.

Pacific Ocean species

Pacific species belong to the genus Oncorhynchus, some examples include:

  • Cherry salmon (Oncorhynchus masu or O. masou) is found only in the western Pacific Ocean in Japan, Korea, and Russia and also landlocked in central Taiwan's Chi Chia Wan Stream (Ching-wen 1990).
  • Chinook salmon (Oncorhynchus tshawytscha) is also known locally as King salmon, Tyee salmon, Spring salmon, Quinnat, Tule, or Blackmouth salmon. Chinook are the largest of all Pacific salmon, frequently exceeding thirty pounds (fourteen kilograms) (Dean 1994).
  • Chum salmon (Oncorhynchus keta) is known locally as Dog or Calico salmon. This species has the widest geographic range of the Pacific species (Buettner 1994a). It is found south to the Sacramento River in California in the eastern Pacific and the island of Kyūshū in the Sea of Japan in the western Pacific; north to the Mackenzie River in Canada in the east; and to the Lena River in Siberia in the west.
  • Coho salmon or Silver salmon (Oncorhynchus kisutch) is found throughout the coastal waters of Alaska and British Columbia and up most clear-running streams and rivers.
  • Pink salmon or Humpback salmon (Oncorhynchus gorbuscha) are found from northern California and [[Korea], throughout the northern Pacific, and from the Mackenzie River in Canada to the Lena River in Siberia, usually in shorter coastal streams. It is the smallest of the Pacific species, with an average weight of 3.5 to 4 pounds (1.6 - 1.8 kg) (Buettner 1994b).
  • Sockeye salmon (Oncorhynchus nerka) is known locally as Red salmon or Blueback salmon (Buettner 1994c). This lake-spawning species is found south as far as the Klamath River in California in the eastern Pacific and northern Hokkaidō Island in Japan in the western Pacific and as far north as Bathurst Inlet in the Canadian Arctic in the east and the Anadyr River in Siberia in the west. Although most adult Pacific salmon feed on small fish and insects, sockeyes feed on plankton that they filter through gill rakers.

Other species

  • Landlocked salmon (Salmo salar sebago) live in a number of lakes in eastern North America. This subspecies of Atlantic Salmon is non-migratory, even when access to the sea is not barred.
  • Kokanee salmon is a landlocked form of sockeye salmon.
  • Huchen or Danube salmon (Hucho hucho), the largest permanent fresh water salmonid.

Aquaculture

File:Salmon newborn.jpg
Artificially-incubated chum salmon

Salmon aquaculture, or salmon farming, is the major economic contributor to the world production of farmed fin-fish, representing over one billion US dollars annually. Other commonly cultured fish species include: tilapia, catfish, sea bass, carp, bream, and trout. Salmon farming is very important in Norway, Sweden, Scotland, Canada, and Chile. Atlantic salmon are also farmed in Russia, Tasmania, Australia, and the UK.

Salmon are carnivorous and in farms are currently fed a meal produced from catching other wild fish and other marine organisms. Consequently, as the number of farmed salmon increase, so does the demand for other fish to feed the salmon. Vegetable proteins are often substituted for animal proteins in the salmon diet. Unfortunately though, this substitution results in lower levels of the highly valued Omega-3 fatty acid content in the farmed product.

Intensive salmon farming now uses open net cages, which have low production costs but have the drawback of allowing disease and sea lice to spread to local wild salmon stocks.

Another form of salmon production, which is safer but less controllable, is the raising of salmon in hatcheries until they are old enough to become independent. They are then released into rivers, often in an attempt to increase the salmon population. This practice was very common in countries like Sweden before the Norwegians developed salmon farming, but is seldom done by private companies, as anyone may catch the salmon when they return to spawn, limiting a company's chances of benefiting financially from their investment. Because of this, the method has mainly been used by various public authorities as a way of artificially increasing salmon populations in situations where they have declined due to overharvest, construction of dams, and habitat destruction or disruption. Unfortunately, there can be negative consequences to this sort of population manipulation, including genetic "dilution" of the wild stocks, and many jurisdictions are now beginning to discourage supplemental fish planting in favor of harvest controls and habitat improvement and protection.

A variant method of fish stocking, called ocean ranching, is under development in Alaska. There, the young salmon are released into the ocean far from any wild salmon streams. When it is time for them to spawn, they return to where they were released and where fishermen can then catch them.

Environmental pressures

Many wild salmon stocks have seen a marked decline in recent decades, especially north Atlantic populations, which spawn in western European waters, and wild salmon of the Snake River system in the Northwest USA. The causes of these declines likely include a number of factors, among them:

  • Disease transfer from open net cage salmon farming, especially sea lice. The European Commission (2002) concluded “The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary.”
  • Overfishing in general, but especially commercial netting in the Faroes and Greenland.
  • Ocean and river warming, which can delay spawning and accelerate transition to smolting (adjustment to salinity). Early smolting may cause salmon to migrate before maturity, decreasing survival rates and reproductive viability.
  • Ulcerative dermal necrosis (UDN) infections of the 1970s and 1980s, which severely affected adult salmon in freshwater rivers.
  • Loss of suitable freshwater habitat, especially suitable material for the excavation of redds (spawning nests).
  • The construction of dams, weirs, barriers, and other "flood prevention" measures, which bring severe adverse impacts to river habitats and the accessibility of those habitats to salmon. This is particularly true in the Northwest USA, where large numbers of dams have been built in many river systems, including over four hundred in the Columbia River Basin.
  • Loss of invertebrate diversity and population density in rivers because of modern farming methods and various sources of pollution, thus reducing food availability.
  • Reduction in freshwater base flow in rivers and disruption of seasonal flows due to diversions and extractions, hydroelectric power generation, [[irrigation] schemes, and slackwater reservoirs, which inhibit normal migratory processes and increase predation for salmon.

There are efforts to relieve this situation. As such, several government and NGOs are sharing and participating in documentation efforts.

  • NOAA's Office of Protected Resources maintains a list of Endangered Species, via the Endangered Species Act
  • Sweden has generated a protection program as part of its Biodiversity Action Plan
  • State of Salmon maintains an IUCN redlist of endangered salmon

The Kamchatka Peninsula, in the Russian Far East, contains the world's greatest salmon sanctuary.

Salmon as food

Edouard Manet: Still Life with Salmon

Salmon is a popular food. Consuming salmon is considered to be reasonably healthy due to the fish's high protein levels and to its high Omega-3 fatty acid content. Salmon is an exceptional rich source of vitamin B12, a rich source of niacin, vitamin B6, copper, and selenium, and a good source of vitamin B1 (Bender and Bender 2005). It is also a source of vitamin B2 and folate, and Pacific salmon is a source of vitamin A and vitamin D, while canned salmon is a source of calcium, given the softened, edible bones (Bender and Bender 2005).

Salmon is also a source of cholesterol, ranging 23-214 mg/g depending on the species (DFF 2006), although high blood cholesterol levels are not correlated with dietary cholesterol but rather dietary fat. Salmon fat is about 20% saturated and 50% mono-unsaturated (Bender and Bender 2005). According to reports in the journal Science, farmed salmon may contain high levels of dioxins. PCB (polychlorinated biphenyl) levels may be up to eight times higher in farmed salmon than in wild salmon, and Omega-3 content may also be lower than in wild caught individuals. According to a 2006 study published in the Journal of the American Medical Association, however, the benefits of eating even farmed salmon still outweigh any risks imposed by contaminants (Mozaffarian and Rimm 2006). It is also noteworthy that salmon generally has among the lowest methylmercury contamination levels of all fish.

A simple rule of thumb is that the vast majority of Atlantic salmon available on the world market are farmed (greater than ninety-nine percent), whereas the majority of Pacific salmon are wild-caught (greater than eighty percent). Farmed salmon outnumber wild salmon eighty-five to one (Montaigne 2003).

File:Rawsalmon.jpg
Raw salmon in Japanese style

Salmon flesh is generally orange to red in color, although there are some examples of white-fleshed wild salmon. The natural color of salmon results from carotenoid pigments, largely astaxanthin (E161j), in the flesh. Wild salmon get these carotenoids from eating krill and other tiny shellfish.

Because consumers have shown a reluctance to purchase white fleshed salmon, astaxanthin, and very minutely canthaxanthin (E161g)), are added as artificial colorants to the feed of farmed salmon because prepared diets do not naturally contain these pigments. Astaxanthin is a potent antioxidant that stimulates the development of healthy fish nervous systems and that enhances the fish's fertility and growth rate. Research has revealed canthaxanthin may have negative effects on the human eye, accumulating in the retina at high levels of consumption.

Today, the concentration of carotenoids (mainly canthaxanthin and astaxanthin) exceeds eight mg/kg of flesh and all fish producers try to reach a level that represents a value of sixteen on the "Roche Color Card," a color card used to show how pink the fish will appear at specific doses. This scale is specific for measuring the pink color due to astaxanthin and is not for the orange hue obtained with canthaxanthin. The development of processing and storage operations, which can be detrimental on canthaxanthin flesh concentration, has led to an increased quantity of pigments added to the diet to compensate for the degrading effects of the processing. In wild fish, carotenoid levels of up to 20-25 mg are present, but levels of canthaxanthin are, in contrast, minor (European Commission 2002).

Canned salmon in the U.S. is usually wild Pacific catch, though some farmed salmon is available in canned form. Smoked salmon is another popular preparation method and can either be hot or cold smoked. Lox can refer either to cold smoked salmon or to salmon cured in a brine solution (also called gravlax).

Raw salmon flesh may contain Anisakis nematodes, marine parasites that cause Anisakiasis. Before the availability of refrigeration, the Japanese did not consume raw salmon. Salmon and salmon roe have only recently come into use in making sashimi (raw fish) and sushi.


References
ISBN links support NWE through referral fees

  • Bender, D. A., and A. E. Bender. 2005. A Dictionary of Food and Nutrition. New York: Oxford University Press. ISBN 0198609612.
  • Buettner, D. 1994a. Chum salmon. Alaska Department of Fish and Game. Retrieved November 11, 2006.
  • Buettner, D. 1994b. Pink salmon. Alaska Department of Fish and Game . Retrieved November 17, 2006. </ref>
  • Buettner, D. 1994. Sockeye salmon. Alaska Department of Fish and Game. Retrieved November 17, 2006. </ref>
  • Ching-wen, L. 1990. Formosan salmon. Taiwan Journal . Retrieved December 13, 2006.
  • Dean, A. 1994. Chinook salmon. Alaska Department of Fish and Game . Retrieved November 17, 2006.
  • Herbst, S. T. 2001. The New Food Lover's Companion: Comprehensive Definitions of Nearly 6,000 Food, Drink, and Culinary Terms. Barron's Cooking Guide. Hauppauge, NY: Barron's Educational Series. ISBN 0764112589.
  • McDermott, J. 2007. Endangered salmon. United States Congressman Jim McDermott website. Retrieved November 17, 2006.
  • McGrath, S. 2003. Spawning hope Audubon Society. Retrieved November 11, 2006.
  • Wisnia, C. 1997. Salmon lesson. Mendocino Community Network. Retrieved June 20, 2007.

External links


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