salmon - Life History, Salmon as food, Environmental pressures, Aquaculture, Species, Further reading
Large, anadromous (ascending rivers to breed) fish (Salmo salar), widespread and locally common in the N Atlantic (Atlantic salmon) and in NW North America (Pacific salmon); length up to 1·5 m/5 ft; adults undertake extensive migrations at sea, feeding on a variety of fishes and crustaceans, returning to the headwaters of freshwater rivers to breed; greatly prized as game fish, and also taken commercially at sea; with the trouts and chars, comprise the very important family, Salmonidae.
Typically, salmon are anadromous: they are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. In Alaska, the crossing over to other streams allows salmon to populate new streams, such as those that emerge as a glacier retreats. The precise method salmon use to navigate has not been entirely established, though their keen sense of smell is certainly involved. In all species of Pacific salmon, the mature individuals die within a few days or weeks of spawning, a trait known as semelparity. However, even in those species of salmon that may survive to spawn more than once (iteroparity), post-spawning mortality is quite high (perhaps as high as 40 to 50%.) Those species average about two or, perhaps, three spawning events per individual.
Salmon has long been at the heart of the culture and livelihood of coastal dwellers. For many centuries, people caught salmon as they swam upriver to spawn. The Ainu, of northern Japan, taught dogs how to catch salmon as they returned to their breeding grounds en masse. Now, salmon are caught in bays and near shore.
Salmon population levels are of concern in the Atlantic and in some parts of the Pacific but 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 and around communities hundreds of miles inland in the watershed. Columbia River salmon levels are now less than 3% than when Lewis and Clark arrived at the river.
Both Atlantic and Pacific Salmon are important to recreational fishing around the world.
In the southern hemisphere, there is the Australian salmon, which is a salt water species not related in any way to the salmonidae.
Life History
In order to lay her roe, the female salmon uses her tail fain to excavate a shallow depression, called a redd. The salmon then die within a few days of spawning. It is estimated that only 10% of all salmon eggs survive long enough to reach this stage. The adult salmon returns to its natal stream to spawn. Prior to spawning, depending on the species, the salmon undergoes changes. They may grow a hump, develop canine teeth, develop a kype (a pronounced curvature of the jaws in male salmon). Chinook and sockeye salmon from central Idaho, for example, travel over 900 miles and climb nearly 7000 feet from the Pacific ocean as they return to spawn, an amazing journey indeed.
The age of a salmon can be deduced from the growth rings on its scales, examined under the microscope.
Freshwater streams and estuaries provide important habitat for many salmon species. 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.
Salmon as food
Salmon is a popular food. Consuming salmon is considered to be reasonably healthy due to the fish's high protein and low fat levels and to its high Omega-3 fatty acids content. According to reports in the journal Science, however, 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 the British FSA (Food Standards Agency), however, the benefits of eating even farmed salmon still outweigh any risks imposed by contaminants. 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%), whereas the majority of Pacific salmon are wild-caught (greater than 80%). Farmed salmon outnumber wild salmon 85 to 1.
Salmon flesh is generally orange to red in colour, although there are some examples of white fleshed wild salmon. The natural colour 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 colourants to the feed of farmed salmon because prepared diets do not naturally contain these pigments.
Canned salmon in the U.S. is usually wild Pacific catch, though some farmed salmon is available in canned form. Alaskan salmon is always wild catch. 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). But Reid Smith holds the record of heaviest salmon caught at 55.6 lbs
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.
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Poached salmon |
White Alaskan Salmon |
Salmon roe at the Shiogama seafood market in Japan |
Ovary of the salmon was opened and loosened |
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Salad with ham and smoked salmon |
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. Ulcerative dermal necrosis (UDN) infections of the 1970s and 1980s which severely affected adult salmon in freshwater rivers. The construction of dams, weirs, barriers and other "flood prevention" measures, which bring severe adverse impacts to river habitat and on the accessibility of those habitats to salmon. Reduction in freshwater base flow in rivers and disruption of seasonal flows, because of diversions and extractions, hydroelectric power generation, irrigation schemes, and slackwater reservoirs, which inhibit normal migratory processes and increae predation for salmon.NOAA's Office of Protected Resources maintains a list of Endangered Species, 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.
Aquaculture
Salmon aquaculture is the major economic contributor to the world production of farmed fin-fish, representing over $1 billion US annually. Salmon farming is very big in Norway, Sweden, Scotland, Canada, and Chile and is the source for most salmon consumed in America and Europe. Atlantic salmon are also farmed in Russia and in Tasmania, Australia.
Salmon are carnivorous and 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. 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 to raise 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. There, the young salmon are released into the ocean far from any wild salmon streams.
Species
The various species of salmon have many names, and varying behaviors. They include,
Atlantic salmon or Salmon (Salmo salar), is the species after which all the others are named.Cherry salmon (Oncorhynchus masu or O. Chinook salmon (Oncorhynchus tshawytscha) is also known locally as King, Tyee, Spring Salmon, Quinnat, Tule, or Blackmouth salmon. Chinook are the largest of all Pacific salmon, frequently exceeding 30 lbs. Chum salmon (Oncorhynchus keta) is known locally as Dog or Calico salmon. 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) is 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. Sockeye salmon (Oncorhynchus nerka) is known locally as "Red Salmon" or "Blueback Salmon". Although most adult Pacific salmon feed on small fish and insects, sockeyes feed on plankton that they filter through gill rakers. This subspecies of Atlantic Salmon is non-migratory, even when access to the sea is not barred. Kokanee salmon is a land-locked form of sockeye salmon. Huchen or Danube salmon (Hucho hucho), the largest permanent fresh water salmonid
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