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Salmon Farming

Salmon are farmed in coastal waters of Norway, the United Kingdom (especially Scotland), Chile, Canada, Australia, New Zealand, Ireland, the Faroe Islands, and the U.S. The species of choice is typically the Atlantic salmon, and usually the European strain. At the end of the twentieth century annual production globally was reported at about 700,000 metric tons, an amount which now surpasses what is landed each year by wild salmon fisheries.

There are two forms of salmon aquaculture. Fa r mi n g , currently done commercially in the U.S. in Washington and Maine, involves raising salmon until they are marketable. Following a period of early growth from the hatchling stage in freshwater, they are placed in densely stocked pens in cold marine waters where they continue to grow for an additional one to two years before being harvested. Salmon hatcheries, on the other hand, produce young salmon to enhance fisheries. Operators of these hatcheries col-lect eggs from fecund salmon and rear the young that hatch out until they are fingerlings ready to be stocked in streams, rivers, and lakes.

The Problems

  • Interactions with wild fish: When salmon escape from farms and survive in large numbers or establish their own breeding populations, they compete with wild salmon. Furthermore, if they are the same species as the wild salmon (e.g., Atlantic salmon grown in the Atlantic), there is the possibility of interbreeding between the two; if this occurs there can be a significant change in the genetics of the wild salmon pop-ulation. Escapees can also transfer disease to wild fish.
  • Wild fish for feed: Each pound of farmed salmon produced requires at least three pounds of wild-caught fish as feed, challenging the presumption that fish farming necessarily reduces commercial fishing pres-sure (nor does it result in a reduction of commercial salmon fishing; rather, fishing pressure can increase as markets for salmon expand). Overall, there is a net loss of protein from marine systems when wild catch is converted into meal for aquaculture consumption.
  • Pollution: Pens full of salmon produce large amounts of waste both as excrement and unconsumed feed resulting in the introduction of nutrients, antibiotics, and pesticides to the environment. High nutrient levels, for example, can result in water quality conditions (such as low oxygen) that are unfavorable for both the farmed fish and the surrounding environment. It is suspected that nutrients from salmon farms can stimulate microalgal blooms, but proof of this is lacking because little research has been done.
  • Disease: Densely packed conditions in fish pens promote disease, a common problem in salmon farms. Furthermore, there have been documented disease transfers from farmed salmon to wild populations, and the potential effects are considered serious. While antibiotics are used to treat some diseases, there are concerns about the effects of antibiotic-resistant bacteria on human health. There has been an emphasis on developing vaccines to prevent specific diseases, which reduces the need for antibiotics.
  • Aesthetics: In some areas, salmon aquaculture facilities sited near residential shorelines have been opposed because they were considered unsightly, odoriferous, and/or viewed as interfering with the natural setting of the seascape. In the U.S., aquaculture development has been significantly slowed down by these concerns.
  • Genetically-modified salmon: The farming of genetically-modified salmon in ocean pens a distinct possibili-ty in the near future adds another layer of concern with respect to interactions with wild populations. If interbreeding were to occur as a result of escapes, such genes could be incorporated into the wild gene pool and possibly diminish the vigor of the wild population. Somewhat of a similar process has taken place as a result of salmon hatcheries; the selective survival of large numbers of young from the small numbers of adults that donate eggs and sperm have reduced the genetic variability of various wild popu-lations after interbreeding has occurred.

The Causes

  • Escapes are virtually inevitable in open-water facilities, such as when pens are damaged during storms. Some incidents have involved the escape of tens of thousands of fish. Efforts to secure facilities against these accidents may reduce the size and number of releases, but are unlikely to stop them altogether.
  • The numbers of salmon in individual pens and farms is variable, but the farmer is motivated to pack them at high densities to increase profits. High densities not only cause stress on the fish which leads to disease problems or inferior product quality but also exacerbates the problems of pollution.
  • The siting of salmon farms is often problematic, particularly if it does not adequately take into account the proximity of wild salmon migration routes, water flow and circulation patterns, the fate of waste materials, the number of facilities already in an area, and aesthetic concerns.
  • According to some, the development of genetically-modified salmon for farming is proceeding without concern about the ecological consequences of their escape into the natural environment.

The Context

  • Wild Atlantic salmon in the U.S. now number in the low hundreds and are nearing extinction, a result of past overfishing, dams, and habitat loss. As well, many populations of salmon throughout the North Atlantic have been in recent decline for largely unknown reasons. Pacific salmon have not fared well either: more than 140 distinct populations have disappeared and the species is extinct in 40% of the rivers in which they once spawned. Interactions with farmed fish and transmission of disease from farmed to wild salmon is especially threatening in the context of these declines.
  • Aquaculture is encouraged by governments because it is viewed as economic development. This often leads to the intensive, large-scale farming methods most often associated with environmental damage.
  • The market price of farmed salmon is artificially low because the costs of environmental damage and the value of ecosystem services are not included in production costs. Furthermore, as these external costs are not borne by the salmon farming industry there is little incentive for them to operate in environmen-tally sound ways.

Further Reading

Naylor, R.L. et al. 1998. Nature's subsidies to shrimp and salmon farming. Science 282: 883-884.

Naylor, R.L. et al.. 2000. Aquaculture is both a boon and bane to world fish supplies. Nature 405 (June 29): 1017-1024.

See also SeaWeb's Aquaculture Resources.