Salmon

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Sockeye salmon, Oncorhynchus nerka
Many species of salmon are anadromous and migrate long distances up rivers and streams to spawn

Salmon is the common name for several species of large food and game fish clustered into the two genera, Salmo and Oncorhynchus, in the family Salmonidae, whose members also include trout, whitefish, and their relatives. Sometimes the term "salmon" is used to refer more generally to any member of Salmonidae, the salmon family.

Salmon are large, predatory fish, feeding mainly on other smaller fish. Wild salmon are found in both the Atlantic and Pacific Oceans, as well as in the Great Lakes and other landlocked lakes. Although a few salmon are landlocked in freshwater lakes—probably as a legacy of the last period of glaciation—salmon typically are anadromous: they are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. In recent decades, commercial farming of Atlantic salmon in net cages has spread this fish from the Atlantic far beyond the northwestern European countries to such Pacific Rim countries as Australia, Chile, China and even directly into the traditional territory of the Pacific salmon along the northwestern coasts of the North American continent.

The wild salmon life cycle exemplifies a strong parental commitment to reproduce—even after great exertion returning upriver to the place of birth. The Pacific salmon, in particular, is noteworthy because its expenditure of energy to complete reproduction is total as the mature individuals die within a few days or weeks of spawning. The decomposing bodies of the parents nourish a flourishing micro-flora and micro-fauna that helps to sustain the young when they hatch from the eggs some 30-90 or more days later. Wild salmon also play a key role in maintaining a healthy ecosystem as they bring nutrients from the sea back up into the mountains and are an important food source for bears, wolves, eagles, and dozens of other mammals, birds, and even insects.

Contents

Overview

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 the northern Pacific. Of the several varieties of North American salmon, all but one are found off the Pacific coast and most (about 90 percent) come from Alaskan waters (Herbst 2001).

Salmon generally migrate from saltwater to freshwater to spawn. About 90 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 lifespans range from two to seven years. 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 that species. 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.

Wild 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 now to be less than three percent of what they were when Lewis and Clark arrived at the river in 1805 (McDermott 2007).

In the southern hemisphere, the fish commonly called the Australian salmon is a saltwater 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 (in full resolution) 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 the fall, the female salmon is looking for a stream with deep, cool running water and a bed of large gravel. 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 30 square feet (McGrath 2003), but more commonly the female would make more redds with fewer eggs in each. 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.

The eggs will winter over in the stream bed, often under ice and deep snow and hatch in the spring into alevin or sac fry (still containing yolk sacs) and quickly thereafter 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 smolts' 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 salmons 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. Prior to spawning, depending on the species, the salmon undergoes changes. It 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, with the Pacific salmon generally dying within two weeks of spawning, while the Atlantic salmon tend to recover, becoming known as kelts, which journey back to the sea and may even return to spawn again.

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, which feed on terrestrial and aquatic insects, amphipods, and other crustaceans while young, and primarily on other fish when older. Mortality of salmon in the early life stages is usually high due to natural predation. In addition, human-induced changes to habitat, such as siltation, elevated water temperatures, low oxygen conditions, loss of stream cover, and reductions in river flow, also account for considerable mortality. 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 salmon, Salmo salar

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 Ocean 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 to 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 freshwater salmonid.

Aquaculture

An Atlantic salmon farm which holds yearlings for up to two years. Many hold broodstock for even longer in these conditions to help ensure large, sexually mature adults.
Artificially-incubated chum salmon

Salmon aquaculture, or salmon farming, is a major economic contributor to the world production of farmed fin-fish, representing over one billion U.S. dollars annually. Other commonly cultured fish species include: carp, tilapia, catfish, sea bass, bream, tuna and trout. Salmon farming is very important in Norway, Sweden, Scotland, Canada, and Chile. Other countries with significant Atlantic salmon farming industries include Russia, Tasmania, Australia, the United Kingdom, the United States, and British Columbia.

For more than 50 years, the salmon farming industry has been continuously developing its capacity to grow large numbers of salmon in what are essentially fish feedlots where the wild salmon diet featuring fish, shrimp, and squid is replaced by nutrient-dense dry pellets produced from other wild fish and marine organisms. Consequently, as the number of farmed salmon increase, so does the demand for other fish to feed the salmon. The composition of the pellets is varied according to the life stage of the fish and the farm's preferred feeding protocol, which often calls for vegetable proteins to substitute for some of the animal proteins in the salmon diet, a practice that can lead to lower levels of the highly valued Omega-3 fatty acid content in the farmed product.

Intensive salmon farming raises salmon through their full life cycle starting with fresh water hatcheries on land and moving the growing fish at the appropriate time into open net cages in the sea. The approach of using the open net cages immersed directly in the sea, in comparison to the alternative of a closed saltwater system, has low production costs, but has the drawback of allowing disease and sea lice to spread to local wild salmon stocks.

Another approach to increasing the harvest of salmon involves raising them in hatcheries only until they are old enough to swim to the sea, at which time they are released into rivers. Fish hatcheries have been under development at least since the mid-1800s and Scandinavian efforts to enhance salmon runs with fish raised in hatcheries dates back to the early decades of the twentieth century. Those hatcheries laid the foundation for the salmon farming techniques developed in Norway starting in the 1960s, and hatcheries play an important role today in efforts by the Alaska Department of Fish and Game to maintain its highly productive wild salmon fisheries. The practice is not without controversy as some point to the danger of genetic "dilution" of the wild stocks and the competition between wild and hatchery raised salmon for food so that wild salmon returning to spawning grounds may have reduced energy reserves for completing the journey.

Alaska is experimenting with a variant method of fish stocking called “ocean ranching,” in which the young salmon are released into the ocean far from any wild salmon streams. When it is time for the salmon 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 Columbia River system in the Northwestern United States. The causes of these declines likely include a number of factors, among them:

  • Transfer of disease, especially sea lice, from open net cage salmon farming. 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 invertebrate diversity and population density in rivers due to such factors as logging operations and pollution in run-off waters from farms and residential and urban areas.
  • Loss of suitable freshwater habitat, especially suitable material for the excavation of redds (spawning nests).
  • Disruption of river habitats and blockage of the salmons' access to them due to the construction of weirs, flood gates, and small dams as "flood prevention" measures.
  • Reduction in freshwater base flow in rivers and disruption of seasonal flows due to diversion and extraction for irrigation purposes, stream flow regulation to support hydroelectric power generation, and maintenance of slackwater reservoirs for barges—all of which inhibit normal migratory processes and increase predation for salmon.

Departments of several governments as well as several NGOs are sharing and participating in documentation efforts aimed toward developing strategies for relieving the stress on the salmon populations. A few of the key parties are:

  • 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, an NGO, maintains an IUCN redlist of endangered salmon and is compiling a comprehensive database on all things related to salmon.

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

Salmon as food

Still Life with Salmon by Edouard Manet
Pacific salmon for sale at a fish market in Seattle
Raw salmon in Japanese style

As the price of salmon has been radically reduced due to the improvement of farming techniques and the proliferation of salmon farms in countries throughout much of the world, salmon, with its high levels of protein and Omega-3 fatty acids, has become a popular food. Salmon is an exceptionally 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 canned salmon is a source of calcium, given the softened, edible bones (Bender and Bender 2005).

Although salmon is also a source of cholesterol—23-214 milligrams per 100 grams of food depending on the species (DFF 2006), its high levels of Omega-3 fatty acids assure that eating salmon, especially wild salmon, is a widely recommended way to help reduce or control the levels of the harmful LDL cholesterol. Salmon fat is about 20 percent saturated and 50 percent 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 in farmed salmon may also be lower than in wild caught individuals. A study published in 2006 in the Journal of the American Medical Association, asserts nonetheless that 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 99 percent), whereas the majority of Pacific salmon are wild-caught (greater than 80 percent). The generalization applies to the fish species, not to the location from which the fish comes, since salmon raised on farms along the coasts of British Columbia, Washington State, or Chile are most likely Atlantic salmon. Farmed salmon outnumber wild salmon eighty-five to one (Montaigne 2003).

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.

To satisfy the consumers preference for red-toned salmon, salmon farmers add astaxanthin, and very minutely canthaxanthin (E161g) as artificial colorants to the salmon feed because prepared diets do not naturally contain these pigments needed to give the flesh its reddish color. 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 that canthaxanthin may have negative effects on the human eye, accumulating in the retina at high levels of consumption.

Today, the concentration of carotenoids (mainly as canthaxanthin and astaxanthin) in farmed fish generally exceeds eight milligrams per kilogram as the salmon farmers aim for 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 new processing and storage operations that tend to degrade canthaxanthin has led growers to compensate by increasing the quantity of pigments added to the diet. In wild fish, carotenoid levels of up to 20-25 milligrams 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 (raw fish together with rice and other foods).


References

  • 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 August 16, 2007.
  • Buettner, D. 1994b. Pink salmon. Alaska Department of Fish and Game. Retrieved August 16, 2007.
  • Buettner, D. 1994. Sockeye salmon. Alaska Department of Fish and Game. Retrieved August 16, 2007.
  • Dean, A. 1994. Chinook salmon. Alaska Department of Fish and Game. Retrieved August 16, 2007.
  • 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.

External links

All links retrieved August 16, 2007.

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