There are a number of environmental issues associated with salmon aquaculture:
Interactions with wild fish: When fish escape from farms and survive in large numbers or establish their own breeding populations, they will compete with wild salmon. If they are the same species as the wild salmon (i.e. Atlantic salmon grown in the Atlantic), there is the possibility of interbreeding between farmed and wild fish. When such interbreeding occurs, there is a significant change in the genetics of the salmon population. More.
Genetically engineered salmon: The introduction of genetically altered salmon in ocean pen farms -- a distinct possibility 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. Hatcheries are problematic because the selective survival of large numbers of young from the small numbers of adults that donate eggs and sperm change the genetic pool of the wild population. More.
Wild fish for feed: Farmed salmon are fed meal and oils from wild-caught fish. Each pound of salmon produced requires at least 3 pounds of wild-caught fish, challenging the presumption that fish farming necessarily reduces commercial fishing pressure. In fact, there is a net loss of protein in the marine ecosystem as a whole when wild catch is converted into meal for aquaculture consumption.
Pollution: Pens full of salmon produce large amounts of waste -- both excrement and unconsumed feed. This may result in water quality conditions (such as high nutrients and low oxygen) that are unfavorable for both the farmed fish and the natural ecosystem. It is suspected that nutrients released from salmon farms stimulate micro- algal blooms, but proof of this is lacking because little research has been done.
Disease: The densely packed condition in pens promotes disease, a common problem in most salmon farms. Furthermore, there have been documented disease transfers from farmed salmon to wild populations, and the potential effects are 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. More.
Aesthetics: In some areas, landowners have opposed salmon aquaculture facilities sited near residential shorelines because they are unsightly, odoriferous and/or interfere with the natural setting of the seascape. In fact, in the U.S., aquaculture development has been significantly slowed down by these concerns.
Escapes are inevitable in open-water pens, especially during storm conditions. Some incidents have resulted in the escape of tens of thousands of farmed fish into the environment. Efforts to secure facilities against these accidents may reduce the size and number of releases, but is unlikely to stop them altogether.
The density of salmon in farms is variable, but the farmer is motivated to pack them at high densities to increase profits. This exacerbates the problems of pollution and disease and causes stress on the fish that leads to inferior product quality.
The siting of salmon farms is often problematic, particularly if it does not adequately take into account the proximity to 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 engineered salmon for farming is proceeding without concern for the consequences of inevitable escapes into coastal waters inhabited by wild salmon.
Populations of salmon in the western North Atlantic have recently plummeted for unknown reasons. Meanwhile, in the eastern Pacific, more than 100 populations have disappeared and salmon are extinct in 40 percent of the rivers where they once spawned along the North American Pacific coast. The potential for interactions with farmed fish and transmission of disease from farmed to wild salmon is especially threatening in the context of these declines.
Governments typically encourage aquaculture 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 are not borne by the industry, nor are the value of ecosystem services factored into the cost of production, there is no pressure on the industry to operate in environmentally sound ways.
Source: Environmental Media Services - May 2004.