SeaWeb - Ocean Briefing Book

Atlantic Salmon

The Problem

Catches of wild Atlantic salmon have been in decline throughout the North Atlantic region since 1973; the species is on the brink of extinction in the United States (where it is now confined to Maine) and adjoining parts of Canada, as well as in Portugal, Poland, and Estonia.
Thirty years ago, over 40,000 salmon swam the 32 rivers of the inner Bay of Fundy, between Nova Scotia and New Brunswick; today, fewer than 200 do so, and 17 of the 32 rivers have no salmon at all. During the 1980s, over 1000 salmon returned to the Bay of Fundy’s Magaguadavic River; in 2003, the total number of returning salmon was just six.
Atlantic salmon populations are now extinct in 42 U.S. rivers and are endangered in the remaining eight. These eight rivers – located in Maine – saw a combined total of just 72 salmon returning from the ocean in 2003.

The Causes

Wild Atlantic salmon populations are under pressure from a wide-ranging suite of human impacts. Freshwater acidification as a result of acid rain can have a severe effect on salmon smolt, which are extremely sensitive to changes in pH. Upon exposure to acidified freshwater, Atlantic salmon have been shown to experience reduced feeding and growth, altered behavior, gill damage, endocrine disruption, and lower survivability upon encountering saltwater.
There is some evidence that various chemical pollutants are impacting juvenile salmon in particular, by affecting their ability to make the transition from freshwater to saltwater. Such chemicals are also believed to impact the reproductive abilities of females. There are no definitive linkages between pesticides and salmon numbers in the United States; in Europe, however, atrazine, a herbicide that can persist in the water column for more than a year, has been strongly linked with a number of physiological disruptions in salmon.
Dam construction can impede or obstruct passage to vital spawning grounds, and has been described as the single greatest cause of salmon extirpation in rivers. Construction of reservoirs, and deepening and straightening of water channels, may inundate spawning gravel and reduce oxygen levels, resulting in increased salmon mortality. Other habitat changes as a result of, for example, agriculture, mining, forestry, or industry, can lead to erosion of riverbanks and the subsequent siltation can lead to fatal clogging of gills among other consequences.
Salmon aquaculture has increased significantly over the past quarter century and in the United States the US Fish and Wildlife Service considers that salmon farming is the single largest threat to the remaining wild salmon in Maine. Escaped salmon are regarded as the most important problem; it is assumed that escaped salmon now outnumber wild salmon returning to Maine, the coast of Nova Scotia, and Bay of Fundy rivers. It is likely that competition between wild and escaped farmed salmon results in lower reproductive success for the former; in addition, hybridization between the two results in erosion of genetic adaptations to local environmental conditions. (A similar concern relates to the deliberate “stocking” of salmon runs by hatchery-reared fish). Finally, farmed salmon frequently carry parasites and pathogens possibly contributing to high levels of mortality among wild salmon.
Although commercial fishing is believed to be an historical contributor to declines of wild salmon populations, the extent of the population collapse means that it is no longer a significant issue in United States waters. However, throughout the species’ range, ongoing overfishing of principal prey such as capelin may remain a major factor.
It is likely also that climate change will have several impacts on numerous aspects of wild salmon biology and ecology—and indeed, may have already played a significant role in the decline and extirpation of some populations. There appears to be a negative correlation between increasing sea surface temperature in the North Atlantic, and survival rates of post-smolt juvenile salmon; however, the causality of this correlation has yet to be established. It seems likely, however, that more extreme weather conditions are only likely to add to the stresses faced by salmon as a result of human activity.

The Context

Wild Atlantic salmon stocks have already completely disappeared from approximately 300 river systems in Europe and North America. Nearly 90 per cent of the known healthy populations of wild salmon are found in only four countries – Norway, Iceland, Scotland and Ireland. In the remainder of the range, 85 per cent of wild Atlantic salmon populations are categorized as either Vulnerable, Endangered, or Critical.
In some regions, efforts are underway to maintain or restore wild Atlantic salmon numbers through “stocking”—the ranching and release of wild salmon either into rivers or directly into the sea. However, this, too, entails risks, particularly with regard to competition and genetic impacts.
The collapses in wild Atlantic salmon populations are illustrative of the declines of many previously common and widespread fish species throughout the ocean. The existence of such a wide variety of different stressors, and the difficulty in ascribing specific effects to any of them, demonstrates both the cumulative nature of anthropogenic impacts, and the complexities involved in addressing many environmental issues.