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Salmon population levels are of concern in the Atlantic and in some parts of the Pacific. Alaska fishery stocks are still abundant, and catches have been on the rise in recent decades, after the state initiated limitations in 1972.〔(【引用サイトリンク】url=http://www.adfg.alaska.gov/index.cfm?adfg=commercialbyfisherysalmon.salmoncatch )〕 Some of the most important Alaskan salmon sustainable wild fisheries are located near the Kenai River, Copper River, and in Bristol Bay. Fish farming of Pacific salmon is outlawed in the United States Exclusive Economic Zone, however, there is a substantial network of publicly funded hatcheries,〔http://media.aprn.org/2008/ann-20080922.mp3|low fish returns in Southeast this summer have been tough on the region's hatcheries〕 and the State of Alaska's fisheries management system is viewed as a leader in the management of wild fish stocks. In Canada, returning Skeena River wild salmon support commercial, subsistence and recreational fisheries, as well as the area's diverse wildlife on the coast and around communities hundreds of miles inland in the watershed. The status of wild salmon in Washington is mixed. Out of 435 wild stocks of salmon and steelhead, only 187 of them were classified as healthy; 113 had an unknown status, 1 was extinct, 12 were in critical condition and 122 were experiencing depressed populations.〔(Johnson et al. 1997)〕 ==Environmental pressures== The population of wild salmon declined markedly in recent decades, especially North Atlantic populations which spawn in the waters of western Europe and eastern Canada, and wild salmon in the Snake and Columbia River system in northwestern United States. The decline is attributed to the following factors: *Sea lice - transfer of parasites from open-net cage salmon farming, especially sea lice, has allegedly reduced numbers. The European Commission (2002) apparently 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.",〔(Scientific Evidence of Sea Lice from Fishfarms Seriously Harming Wild Stocks ), .〕 however it is impossible to very this statement from the reference provided It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms. Unfortunately for Dr Krkosek and his research group their electronic modelling predictions of a collapsed salmon fishery have not eventuated, with two of the best runs ever recorded for wild salmon. This raises real questions over the validity of the models and their dire predictions. For Atlantic salmon smolts, it takes as few as eight sea lice to kill the fish. On the Pacific Coast where the smolt are much smaller, only one or two can be critical, often resulting in death. In the Atlantic, sea lice have been a proven factor in both Norwegian and Scottish salmon declines. In the Western Atlantic, there has been little research at sea, but sea lice numbers in the period after 2000 do not appear to be a significant factor in the critical decline of endangered inner Bay of Fundy salmon. The situation may have been different in the 1980s and 1990s, but we are unlikely ever to know the factual history in that regard. *Overfishing - in general, has reduced populations, especially commercial netting in the Faroes and Greenland. Several seafood sustainability guides have recommendations on which salmon fisheries are sustainable and which have negative impacts on salmon populations. *Warming in ocean and river water can delay spawning and accelerate the transition to smolting. *Ulcerative dermal necrosis - (UDN) infections of the 1970s and 1980s severely affected adult salmon in freshwater rivers. *Habitat - the loss of suitable freshwater habitat, especially degradation of stream pools and reduction of suitable material for the excavation of redds has caused a reduction in spawning. Historically, stream pools were, to a large extent, created by beavers (see section below). With their extirpation, the nurturing function of these ponds was lost. Reduced retention of the nutrients brought by the returning adult salmon in stream pools has lowered populations. Without stream pools, dead adult salmon tend to be washed straight back down the streams and rivers, so the nutrients are not available for the hatchlings. The construction of dams, weirs, barriers and other "flood prevention" measures bring severe adverse impacts to river habitat and on the accessibility of those habitats to salmon, particularly in the northwest USA, where large numbers of dams have been built in many river systems, including over 400 in the Columbia River Basin.〔Moscrip, A., Montgomery, D. ("Urbanization, Flood Frequency, and Salmon Abundance in Puget Lowland Streams". ) JAWRA Journal of the American Water Resources Association.〕 Other environmental factors, such as light intensity, water flow, or change in temperature, dramatically affects salmon during their migration season. Modern farming methods and various sources of pollution have resulted in loss of invertebrate diversity and population density in rivers, thus reducing food availability. Reduction in freshwater base flow in rivers and disruption of seasonal flows, because of diversions and extractions, hydroelectric power generation, irrigation schemes, barge transportation, and slackwater reservoirs, inhibit normal migratory processes and increase predation for salmon.〔Pacific States Marine Fisheries Commission ("When Salmon Are Dammed". ) Pacific States Marine Fisheries Commission, 1997.〕 Agricultural practices, such as the removal of riparian plants, destabilization of stream banks by livestock and irrigation processes, result in a loss of suitable low-gradient stream habitats.〔Bradford, MJ., Irvine, JR. ("Land use, fishing, climate change, and the decline of Thompson River, British Columbia, coho salmon". ) Canadian Journal of Fisheries and Aquatic Sciences, 2000.〕〔Orr, Raymond I. http://www.indiancountrytoday.com/archive/28215419.html "Northwest Salmon Make Legal Headway." Indian Country Today〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Environmental issues with salmon」の詳細全文を読む スポンサード リンク
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