出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/09/23 14:27:02」(JST)
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| Atlantic salmon | |
|---|---|
| Conservation status | |
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Least Concern (IUCN 2.3) [1] |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Salmoniformes |
| Family: | Salmonidae |
| Genus: | Salmo |
| Species: | S. salar |
| Binomial name | |
| Salmo salar Linnaeus, 1758 |
|
| Distribution of Atlantic salmon | |
The Atlantic salmon, Salmo salar, is a fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and, due to human introduction, the north Pacific.[2][3]
It is also commercially known as bay salmon, black salmon, caplin-scull salmon, fiddler, grilse, grilt, kelt, landlocked salmon, ouananiche, outside salmon, parr, Sebago salmon, silver salmon, slink, smolt, spring salmon, or winnish.[4]
Most Atlantic salmon follow an anadromous fish migration pattern,[3] in that they undergo their greatest feeding and growth in salt water; however, adults return to spawn in native freshwater streams where the eggs hatch and juveniles grow through several distinct stages.
Atlantic salmon do not require salt water. Numerous examples of fully freshwater (i.e., "landlocked") populations of the species exist throughout the Northern Hemisphere.[3] In North America, the landlocked strains are frequently known as ouananiche.
The freshwater phases of Atlantic salmon vary between one and eight years, variably according to river location.[5] While the young in southern rivers, such as those to the English Channel, are only one year old when they leave, those further north, such as in Scottish rivers, can be over four years old, and in Ungava Bay, northern Quebec, smolts as old as eight years have been encountered.[5] The average age correlates to temperature exceeding 7°C (45°F).[2]
The first phase is the alevin stage, when the fish stay in the breeding ground and use the remaining nutrients in their yolk sacs. During this developmental stage, their young gills develop and they become active hunters. Next is the fry stage, where the fish grow and subsequently leave the breeding ground in search of food. During this time, they move to areas with higher prey concentration. The final freshwater stage is when they develop into parr, in which they prepare for the trek to the Atlantic Ocean.
During these times, the Atlantic salmon are very susceptible to predation. Nearly 40% are eaten by trout alone. Other predators include other fish and birds.[citation needed]
When parr develop into smolt, they begin the trip to the ocean, which predominantly happens between March and June. Migration allows acclimation to the changing salinity. Once ready, young smolt leave, preferring an ebb tide.
Having left their natal streams, they experience a period of rapid growth during the one to four years they live in the ocean. Typically, Atlantic salmon migrate from their home streams to an area on the continental plate off West Greenland. During this time, they face predation from humans, seals, Greenland sharks, skate, cod, and halibut. Some dolphins have been noticed playing with dead salmon, but it is still unclear whether they consume them.
Once large enough, Atlantic salmon change into the grilse phase, when they become ready to return to the same freshwater tributary they departed from as smolts. After returning to their natal streams, the salmon will cease eating altogether prior to spawning. Although largely unknown, odor – the exact chemical signature of that stream – may play an important role in how salmon return to the area where they hatched. Once heavier than about 250 g, the fish no longer become prey for birds and many fish, although seals do prey upon them. Grey and common seals commonly eat Atlantic salmon. Survivability to this stage has been estimated at between 14 and 53%.[2]
The Atlantic salmon was given its scientific binomial name by zoologist and taxonomist Carolus Linnaeus in 1758. Later, the differently coloured smolts were found to be the same species.
The name, Salmo salar, is from the Latin salmo, meaning salmon, and salar, meaning leaper, according to M. Barton,[6] but more likely meaning "resident of salt water". Lewis and Short's Latin Dictionary (Clarendon Press, Oxford, 1879) translates salar as a kind of trout from its use in the Idylls of the poet Ausonius (4th century CE).
This is a fairly large salmonid, with average adult specimens measuring 71 to 76 cm (28 to 30 in) long and weighing 3.6 to 5.4 kg (7.9 to 12 lb).[7] Record-sized specimens have been measured to a maximum of 153 cm (60 in) and a weight of 45 kg (99 lb).[8] The colouration of young Atlantic salmon does not resemble their adult stage. While they live in fresh water, they have blue and red spots. While they mature, they take on a silver-blue sheen. When adult, the easiest way of identifying them is by the black spots predominantly above the lateral line, although the caudal fin is usually unspotted. When they reproduce, males take on a slight green or red colouration. The salmon has a fusiform body, and well-developed teeth. All fins, save the adipose, are bordered with black.
At sea, the species is found mainly in the waters off Greenland and in migrations to and from its natal streams, along the northern eastern and western Atlantic coasts. Wild salmon disappeared from many rivers during the twentieth century due to overfishing and habitat change.[2] By 2000 the numbers of Atlantic salmon had dropped to critically low levels.[10]
Young salmon begin a feeding response within a few days. After the yolk sac is absorbed by the body, they begin to hunt. Juveniles start with tiny invertebrates, but as they mature, they may occasionally eat small fish. During this time, they hunt both in the substrate and in the current. Some have been known to eat salmon eggs. The most commonly eaten foods include caddisflies, blackflies, mayflies, and stoneflies.[2]
As adults, the fish feed on much larger food: Arctic squid, sand eels, amphipods, Arctic shrimp, and sometimes herring, and the fishes' size increases dramatically.[2]
Fry and parr have been said to be territorial, but evidence showing them to guard territories is inconclusive. While they may occasionally be aggressive towards each other, the social hierarchy is still unclear. Many have been found to school, especially when leaving the estuary.
Adult Atlantic salmon are considered much more aggressive than other salmon, and are more likely to attack other fish than others. Where they have become an invasive threat, attacking native salmon, such as Chinook salmon and coho salmon, has become a concern.[2]
Atlantic salmon breed in the rivers of Western Europe from northern Portugal north to Norway, Iceland, and Greenland, and the east coast of North America from Connecticut in the United States north to northern Labrador and Arctic Canada. Atlantic salmon which have escaped from the aquaculture industry have also been found breeding in rivers tributary to the Pacific Ocean in British Columbia on Canada's west coast.[citation needed] The species constructs a nest or "redd" in the gravel bed of a stream. The female creates a powerful downdraught of water with her tail near the gravel to excavate a depression. After she and a male fish have eggs and milt (sperm), respectively, upstream of the depression, the female again uses her tail, this time to shift gravel to cover the eggs and milt which have lodged in the depression.
Unlike the various Pacific salmon species which die after spawning (semelparous), the Atlantic salmon is iteroparous, which means the fish may recondition themselves and return to the sea to repeat the migration and spawning pattern several times, although most spawn only once or twice.[3][11] Migration and spawning exact an enormous physiological toll on individuals, such that repeat spawners are the exception rather than the norm.[11] Atlantic salmon show high diversity in age of maturity and may mature as parr, one- to five-sea-winter fish, and in rare instances, at older sea ages. This variety of ages can occur in the same population, constituting a ‘bet hedging’ strategy against variation in stream flows. So in a drought year, some fish of a given age will not return to spawn, allowing that generation other, wetter years in which to spawn.[5]
Very young fertilised salmon eggs, notice the developing eyes and neural tube
Atlantic salmon redd (nest): Sac fry remain in the gravel habitat of their redd until their yolk sac, or "lunch box" is depleted. Once depleted, the young salmon then emerge from the gravel to look for food.
A newly hatched Atlantic salmon alevin which, within around 24 hours, will absorb the yolk sac and become a fry
When in shared breeding habitats, Atlantic salmon will hybridize with brown trout (Salmo trutta).[12][13][14] Hybrids between Atlantic salmon and brown trout were detected in two of four watersheds studied in northern [Spain]. The proportions of hybrids in samples of 'salmon' ranged from 0 to 7-7% but they were not significantly heterogeneous among locations, resulting in a mean hybridization rate of 2-3%. This is the highest rate of natural hybridization so far reported and is significantly greater than rates observed elsewhere in Europe.[15]
In its natal streams, Atlantic salmon are considered prized recreational fish, pursued by fly anglers during its annual runs. At one time, the species supported an important commercial fishery and a supplemental food fishery. However, the wild Atlantic salmon fishery is commercially dead; after extensive habitat damage and overfishing, wild fish make up only 0.5% of the Atlantic salmon available in world fish markets. The rest are farmed, predominantly from aquaculture in Norway, Chile, Canada, the UK, Ireland, Faroe Islands, Russia and Tasmania in Australia. Sport fishing communities, mainly from Iceland and Scandinavia, have joined in the North Atlantic Salmon Fund to buy away commercial quotas in an effort to save the wild species of Salmo salar.[11]
Adult male and female fish are anaesthetised; their eggs and sperm are "stripped" after the fish are cleaned and cloth dried. Sperm and eggs are mixed, washed, and placed into fresh water. Adults recover in flowing, clean, well-aerated water.[16] Some researchers have even studied cryopreservation of their eggs.[17]
Fry are generally reared in large freshwater tanks for 12 to 20 months. Once the fish have reached the smolt phase, they are taken out to sea, where they are held for up to two years. During this time, the fish grow and mature in large cages off the coasts of Canada, the USA, or parts of Europe.[11]
Generally, cages are made of two nets. Inner nets, which wrap around the cages, hold the salmon. Outer nets, which are held by floats, keep predators out.[16]
In the past, some Atlantic salmon have escaped from cages at sea. In the Atlantic Ocean, this has resulted in some breeding with native populations, but generally most surviving offspring were from the domesticated Atlantics, not hybrids.
On the West Coast of Northern America, aquaculturists have taken great care to ensure the non-native salmon cannot escape from their open-net pens, and escape is no longer considered a major concern. Evidence of Atlantic salmon surviving and establishing wild populations in the Pacific is lacking. From 1905 until 1935, in excess of 8.6 million Atlantic salmon of various life stages (predominantly advanced fry) were intentionally introduced to more than 60 individual BC lakes and streams. Historical records indicate, in a few instances, mature sea-run Atlantic salmon were captured in the Cowichan River; however, a self-sustaining population never materialized. Environmental assessments by the US National Marine Fisheries Service (NMFS), the Washington Department of Fish and Wildlife and the BC Environmental Assessment Office have concluded the potential risk of Atlantic salmon colonization in the Pacific Northwest is low.[18]
Human activities have heavily damaged salmon populations across their range. The major impacts were from overfishing and habitat change, and the new threat from competitive farmed fish. Salmon decline in Lake Ontario goes back to the 18th–19th centuries, due to logging and soil erosion, as well as dam and mill construction. By 1896, the species was declared extirpated from the lake.[19] When dams were constructed on the Oswego River, their spawning areas were cut off and they went extinct locally.
In the 1950s, salmon from rivers in the US and Canada, as well as from Europe, were discovered to gather in the sea around Greenland and the Faroe Islands. A commercial fishing industry was established, taking salmon using drift nets. After an initial series of record annual catches, the numbers crashed; between 1979 and 1990, catches fell from four million to 700,000.[20]
Overfishing at sea is generally considered the primary negative factor, though marine exploitation rates estimated for various Newfoundland stocks for the period 1984–1991 averaged 45% on small (<63 cm) salmon and 74.2% on large salmon (>63 cm); closure of the Newfoundland commercial salmon fishery beginning in 1992 has not resulted in general increases in salmon populations through the present.[10]
Beginning around 1990, the rates of Atlantic salmon mortality at sea more than doubled. In the western Atlantic, fewer than 100,000 of the important multiple sea-winter salmon were returning. Rivers of the coast of Maine, plus southern New Brunswick and much of mainland Nova Scotia saw runs drop precipitously, and even disappear. In the mid-1990s, the Atlantic Salmon Federation in cooperation with partners developed sonic tracking technology, and by 2008, the salmon had been tracked from rivers such as the Restigouche and the Miramichi as far along their migration routes as the Strait of Belle Isle, between Labrador and Newfoundland, and halfway to feeding grounds off Greenland. The problems at sea remain, leading to a concerted international effort, called SALSEA, to find out more about the mortality at sea. It is organized by the North Atlantic Salmon Conservation Organization.[citation needed]
Possibly because of improvements in ocean feeding grounds, returns in 2008 were very positive. On the Penobscot River in Maine, returns were about 940 in 2007, and by mid-July 2008, the return was 1,938. Similar stories were reported in rivers from Newfoundland to Quebec. In 2011, more than 3,100 salmon returned to the Penobscot, the most since 1986, and nearly 200 ascended the Narraguagus River, up from the low two digits just a decade before.[21]
Around the North Atlantic, efforts to restore salmon to their native habitats are underway, with slow progress. Habitat restoration and protection are key to this process, but issues of excessive harvest and competition with farmed and escaped salmon are also primary considerations. In the Great Lakes, Atlantic salmon have been introduced successfully, but the percentage of salmon reproducing naturally is very low. Most are stocked annually. Atlantic salmon were native to Lake Ontario, but were extirpated by habitat loss and overfishing in the late 19th century. The state of New York has since stocked its adjoining rivers and tributaries, and in many cases does not allow active fishing.[3][22] Wild salmon on entering rivers as adults have characteristically pointed fins which help scientists distinguish them from farmed or escaped salmon.
In New England, many efforts are underway to restore salmon to the region by knocking down obsolete dams and updating others with fish ladders and other techniques that have proven effective in the West with Pacific salmon. There is some success thus far, with populations growing in the Penobscot and Connecticut Rivers. In Ontario, the Atlantic Salmon Restoration Program[23] was started in 2006, and is one of the largest freshwater conservation programs in North America. It has stocked Lake Ontario with over 700,000 young Atlantic salmon. In October 2007, salmon were video-recorded running in Toronto's Humber River by the Old Mill. In November 2007, a migrating salmon was observed in the Credit River.[19] There has also been some success in establishing Atlantic salmon in Fish Creek, a tributary of Oneida Lake in central New York.
Atlantic salmon, though, remains a popular fish for human consumption.[3] It is commonly sold fresh, canned, or frozen.
The decline in anadromous salmonid species over the last two to three centuries is correlated with the decline in the North American beaver and European beaver, although some fish and game departments continue to advocate removal of beaver dams as potential barriers to spawning runs. Migration of adult Atlantic salmon may be limited by beaver dams during periods of low stream flows, but the presence of juvenile Salmo salar upstream from the dams suggests the dams are penetrated by parr.[24] Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows.[24] A 2003 study of Atlantic salmon and sea-run brown trout/sea trout (Salmo trutta morpha trutta) spawning in the Numedalslågen River and 51 of its tributaries in southeastern Norway were unhindered by beaver.[25] In a restored, third-order stream in northern Nova Scotia, beaver dams generally posed no barrier to Atlantic salmon migration except in the smallest upstream reaches in years of low flow where pools were not deep enough to enable the fish to leap the dam or without a column of water over-topping the dam for the fish to swim up.[26] The importance of winter habitat to salmonids afforded by beaver ponds may be especially important (and underappreciated) in streams of northerly latitudes without deep pools where ice cover makes contact with the bottom of shallow streams.[24] In addition, the up to eight-year-long residence time of juveniles in fresh water may make beaver-created permanent summer pools a crucial success factor for Atlantic salmon populations. In fact, two-year-old Atlantic salmon parr in beaver ponds in eastern Canada showed faster summer growth in length and mass and were in better condition than parr upstream or downstream from the pond.[27]
The first laws regarding the Atlantic salmon were started nearly 800 years ago.[citation needed]
Edward I instituted a penalty for collecting salmon during certain times of the year. His son Edward II continued, regulating the construction of weirs. Enforcement was overseen by those appointed by the justices of the peace. Because of confusing laws and the appointed conservators having little power, most laws were barely enforced.
Based on this, a royal commission was appointed in 1860 to thoroughly investigate the Atlantic salmon and the laws governing the species, resulting in the 1861 Salmon Fisheries Act. The act placed enforcement of the laws under the Home Office's control, but it was later transferred to the Board of Trade, and then later to the Board of Agriculture and Fisheries.
Another act passed in 1865 imposed charges to fish and catch limits. It also caused the formation of local boards having jurisdiction over a certain river. The next significant act, passed in 1907, allowed the board to charge 'duties' to catch other freshwater fish, including trout.
Despite legislation, board effects decreased until, in 1948, the River Boards Act gave authority of all freshwater fish and the prevention of pollution to one board per river. In total, it created 32 boards.
In 1974, the 32 boards were reduced to 10 regional water authorities (RWAs). Although only the Northumbrian, Welsh, northwest and southwest RWA's had considerable salmon populations, all ten also cared for trout and freshwater eels.
The Salmon and Freshwater Fisheries Act was passed in 1975. Among other things, it regulated fishing licences, seasons, and size limits, and banned obstructing the salmon's migratory paths.[2]
Legislation in Scotland to help Atlantic salmon began in 1318 by Alexander II. It prohibited certain types of traps in rivers.
During the 15th century, many laws were passed; many regulated fishing times, and worked to ensure smolts could safely pass downstream. James III even closed a meal mill because of its history of killing fish attracted to the wheel. Because the fish were held in such high regard, poachers were severely punished.
More recent legislation has established commissioners who manage districts. Furthermore, the Salmon and Freshwater Fisheries Act in 1951 required the Secretary of State be given data about the catches of salmon and trout to help establish catch limits.[2][16]
Several populations of Atlantic salmon are in serious decline, and are listed as endangered under the Endangered Species Act (ESA). Currently, runs of 11 rivers in Maine are on the list – Kennebec, Androscoggin, Penobscot, Sheepscot, Ducktrap, Cove Brook, Pleasant, Narraguagus, Machias, East Machias and Dennys. The Penobscot is the "anchor river" for Atlantic salmon populations in the US. Returns in 2008 have been around 2,000, more than double the 2007 return of 940.
Section 9 of the ESA makes it illegal to take an endangered species of fish or wildlife. The definition of "take" is to "harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect, or to attempt to engage in any such conduct".[28]
The federal government has prime responsibility for protecting the Atlantic salmon, but over the last generation, effort has continued to shift management as much as possible to provincial authorities through memoranda of understanding, for example. A new Atlantic salmon policy is in the works, and in the past three years,[when?] the government has attempted to pass a new version of the century-old Fisheries Act through Parliament.
Federal legislation regarding at-risk populations is weak.[citation needed] Inner Bay of Fundy Atlantic salmon runs were declared endangered in 2000. As of 2008, no recovery plan is in place.
It takes constant pressure from nongovernmental organizations, such as the Atlantic Salmon Federation, for improvements in management, and for initiatives to be considered. For example, the technology for mitigation of acid rain-affected rivers used in Norway is needed in 54 Nova Scotia rivers. Yet, an initiative of the ASF and the Nova Scotia Salmon Association raised the funds to get a project in place, in West River-Sheet Harbour.
In Quebec, the daily catch limit for Atlantic salmon is one fish over 63 cm (25 in), two fish under 63 cm or one fish over and one under 63 cm, provided the smaller fish was the first one caught (a provision designed to prevent an angler from continuing to fish if a large fish is already in possession). The annual catch limit is seven Atlantic salmon of any size.
The North Atlantic Salmon Conservation Organization is an international council made up of Canada, the European Union, Iceland, Norway, the Russian Federation, and the United States, with its headquarters in Edinburgh.[29] It was established in 1983 to help protect Atlantic salmon stocks, through the cooperation between nations. They work to restore habitat and promote conservation of the salmon.
In 2010, Greenpeace International has added the Atlantic salmon to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries".[30]
| Wikimedia Commons has media related to: Salmo salar |
| External identifiers for Salmo salar | |
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| Encyclopedia of Life | 206776 |
| ITIS | 161996 |
| NCBI | 8030 |
| WoRMS | 127186 |
| Also found in: Wikispecies, FishBase | |
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| リンク元 | 「Atlantic salmon」「タイセイヨウサケ」 |