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INTRODUCTION Salmon (fish), common name applied to fish characterized by an elongate body covered with small, rounded scales and a fleshy fin between the dorsal fin and tail. These fish belong to the salmon family, which also includes char (see Trout), grayling, and whitefish. Most members of the salmon family are valuable food fish and excellent game fish. Salmon eat smaller fishes, crustaceans, and insects. They are found in both fresh and salt water in the colder regions of the northern hemisphere.
SALMON LIFE CYCLE Many species of salmon are anadromous-they spawn, or lay their eggs, in fresh water; the young migrate to salt water and grow up there; and the fish return to fresh water to breed after they reach maturity. Other populations or species of salmon are landlocked, spending their entire life cycle in fresh water. The migratory instinct of members of the salmon family is remarkably specific, each generation returning to spawn in exactly the same breeding places as the generation before it. Some salmon migrate hundreds or even thousands of miles to reach their spawning grounds. Even those species that do not migrate from fresh water to salt water spawn in the same freshwater streams as did their ancestors.
Although usually drab in color before the breeding season, which varies with the species, members of the salmon family develop bright hues at spawning time. During this season the male usually develops a hooked snout and a humped back. Salmon typically spawn in rapidly flowing, clear streams with gravel and rocks in the bottom. Before mating, one parent excavates a nest, or redd, for the eggs. The female deposits eggs in the nest and the male releases sperm, or milt, over the eggs to fertilize them. The female then stirs up the stream bottom so that earth and stones cover the eggs and protect them. During the migrations and nest-building activity that precede mating, neither the females nor the males consume food.
The eggs hatch in two weeks to six months, depending on the species and the water temperature. The newly hatched young, called alevins, remain buried in the nest, living on nutrients absorbed from a yolk sac attached to the abdomen. When all the yolk has been absorbed, the young salmon, then called fry or fingerlings, emerge from the gravel to seek food. Their diet consists of microscopic plants and small animals, such as insects.
As the fry feed and grow larger, dark vertical bars appear along their sides. At this stage they are referred to as parr or brandlings. The amount of time the young salmon spend in fresh water depends on the species. Eventually the young salmon turn bright silver and, in the case of seagoing forms, descend to the sea. At this stage they are called smolts. When they are fully grown and reach sexual maturity, the salmon begin the migration back to fresh water to reproduce. Different species of salmon spend different amounts of time in salt water before migrating back to their birth stream to spawn.
ATLANTIC SALMON The Atlantic salmon has tasty flesh that is often orange-red. The fish average about 3.6 to 5.5 kg (8 to 12 lb) in weight, but specimens weighing up to 38 kg (84 lb) have been caught. The Atlantic salmon migrates from the sea into cold fresh water in late spring or early summer, swimming upriver at an average rate of up to 6.4 km (4 mi) per day. The female lays as many as 20,000 eggs in October or November, after which time the adult salmon float downstream and return to the sea. An Atlantic salmon returning to its spawning ground for the first time is known as a grilse. After spawning, it is known as a kelt. The Atlantic salmon returns year after year to its breeding place and can live for up to 8 years.
Several subspecies of the Atlantic salmon live in the lakes of the northern United States without ever descending to sea. These landlocked salmon are much smaller than migrating salmon, attaining a maximum weight of about 16 kg (about 35 lb). The two most important landlocked populations of the Atlantic salmon are the Sebago salmon, found in lakes from New Hampshire to New Brunswick, and the ouananiche, of Lac Saint-Jean, Canada.
PACIFIC SALMON Salmon found in the North Pacific Ocean spawn only once, dying after depositing and fertilizing their eggs. Six species of salmon live in the Pacific Ocean: chinook, sockeye, coho, pink, chum, and masu. The best-known and most valuable species is the chinook salmon, which is also known as the king salmon, Columbia River salmon, quinnat, chowichee, and takou. The chinook is the largest species of Pacific salmon, averaging about 7 to 11 kg (15 to 25 lb) in weight. Numerous specimens up to 1.5 m (5 ft) in length and 60 kg (130 lb) in weight have been recorded. The chinook salmon migrates farther than any other salmon, often traveling 1,600 to 3,200 km (1,000 to 2,000 mi) inland to its spawning ground.
The sockeye, also known as red or blue-black salmon, is another valuable species. Young sockeye salmon live for a year or two in a lake before migrating to the sea. A small, landlocked subspecies of sockeye is known as the kokanee. Another valuable fishery is based on the coho, or silver salmon, which has tasty, light pink flesh. Other salmon in the eastern Pacific are the pink, or humpback salmon, and the chum, or dog salmon. The masu salmon, the smallest species of Pacific salmon, lives only in the waters of Japan.
ECONOMIC VALUE Anglers fish for salmon with rod and reel, usually using artificial flies as bait. Commercial fishing for salmon occurs on a much larger scale, employing various types of nets to catch the fish on the way to their spawning grounds. Commercial salmon fishing occurs chiefly during the spawning migrations, so the fishing season varies with the geographic location and the species of salmon, as well as with local, national, and international fishing regulations. The salmon fishing industry is one of the major industries of the states along the American Pacific coast, providing 60,000 jobs and more than $1 billion in income per year for the region. Other countries with a large salmon industry include Canada, Japan, Russia, Britain, Norway, and Denmark.
Hatcheries, where salmon eggs are collected and young produced under controlled conditions, have been used to supplement wild salmon populations for commercial and recreational fisheries. Every year the U.S. Fish and Wildlife Service and other agencies release billions of salmon eggs and young from hatcheries into rivers. However, some experts have raised concerns about hatchery-raised salmon because they compete with wild salmon for food and are more vulnerable to predators than are wild salmon. In addition, hatchery salmon usually have less genetic diversity (see Genetics: Genes in Populations) than wild salmon, which can lead to lowered resistance to disease and other environmental hazards. The annual harvest of wild and farm-raised salmon in the United States averages about 478,000 metric tons, of which about 60 percent is canned.
CONSERVATION ISSUES In the United States, wild Atlantic and Pacific salmon populations are severely threatened. In particular, the drastic decline of wild Pacific salmon populations has raised alarm and become one of the most important conservation issues in the Pacific Northwest. Less than 2 percent of the wild salmon population of the Columbia River Basin (including parts of Washington, Oregon, Idaho, Montana, Wyoming, and British Columbia) remains and only one individual sockeye salmon returned to the Snake River in Idaho in 1994. Coho salmon in the Snake River have been declared extinct by the U.S. Fish and Wildlife Service, as have 106 other salmon populations on the West Coast.
Many factors are responsible for this dramatic decline. The construction of dams on rivers interferes with both upstream and downstream migration of salmon-15 to 30 percent of young salmon die at each dam as they migrate down river. Urban and suburban development has destroyed much salmon spawning habitat, and withdrawal of water from rivers for irrigation or for industrial or municipal use sometimes leaves too little water for fish. Finally, logging and agriculture near waterways lead to erosion, siltation (the clouding of waterways with fine soil), toxic runoff, and high water temperatures, all of which interfere with salmon spawning and migration.
The decline of salmon populations is uniting environmentalists and fishers with industries that extract natural resources, such as hydroelectricity, timber, and water, to find a compromise that saves both the wild salmon populations and the fishing industries that depend on the species' continued health. Efforts to protect salmon populations in the Pacific Northwest include constructing fish ladders to enable mature salmon traveling upstream to get around dams; barging or trucking young salmon around dams; reducing industrial and agricultural water withdrawals from river systems; prohibiting logging near streams or rivers; dramatically limiting salmon fishing seasons; restoration projects to improve degraded habitat; and increasing water flow, or spillage, through hydroelectric dams.
Since the late 1980s, the federal government has declared a number of salmon populations along the West Coast as threatened or endangered under the Endangered Species Act (ESA). By the late 1990s, 14 salmon populations were protected by the ESA. Some of these protected populations live in the metropolitan areas of Seattle, Washington and Portland, Oregon, marking the first time the ESA has been broadly applied to large urban areas. Salmon conservation efforts are expected to intensify as local and state governments work with federal agencies to develop plans to recover wild salmon populations.
Scientific classification: Salmon belong to the family Salmonidae. The Atlantic salmon is classified as Salmo salar. Pacific salmon belong to the genus Oncorhynchus. The chinook salmon is classified as Oncorhynchus tshawytscha, the sockeye salmon as Oncorhynchus nerka, and the coho salmon as Oncorhynchus kisutch.
"Salmon (fish)." Microsoft® Encarta® Encyclopedia 2001. © 1993-2000 Microsoft Corporation. All rights reserved.＜本文发表于: 相约加拿大:枫下论坛 www.rolia.net/f ＞