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1 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.
2 The disease transfer from open net cage salmon farming, especially sea lice. The European Commission (2002) concluded “The reduction of wild salmon in 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.” It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.
3 For Atlantic Salmon it takes as few as eight sea lice to kill the fish. On the Pacific Coast where the counts 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 post 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 1980’s and 1990’s but we are unlikely ever to know the factual history in that regard.
4 The 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.
5 The ulcerative dermal necrosis (UDN) infections of the 1970s and 1980s which severely affected adult salmon in freshwater rivers. Loss of suitable freshwater habitat, especially degradation of stream pools and reduction of suitable material for the excavation of redds. Historically stream pools were, to a large extent, created by beavers. With the extirpation of the beaver, the nurturing function of these ponds was lost.
6 The reduction of the retention of the nutrients brought by the returning adult salmon in stream pools. Without stream pools dead adult salmon tend to be washed straight back down the streams and 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. This is particularly true in the northwest USA, where large numbers of dams have been built in many river systems including over 400 in the Oregon Columbia River Basin.
7 Other environmental factors such as light intensity and water flow, or change in temperature dramatically affects salmon during their migration season. The loss of invertebrate diversity and population density in rivers is because of modern farming methods and various sources of pollution thus reducing the food availability.
8 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 slack water reservoirs, which inhibit normal migratory processes and increase predation for salmon.
9 The loss of suitable low gradient stream habitats due to agricultural practices such as the removal of riparian plants and destabilization of stream banks by livestock and irrigation processes. There are efforts to relieve this situation. As such several governments and NGO’s are sharing in research and habitat restoration efforts.
10 Results overall are showing that estuary problems exist for some rivers, but issues involving feeding grounds at sea are impacting populations as well. In 2008 returns were markedly improved for Atlantic salmon on both sides of the Atlantic Ocean, but no one knows if this is a temporary improvement or sign of a trend.
