The negative ecological impacts associated with Salmo salar seem to be limited only to domestic fish farm stocks. Sea cages used are prone to tearing from storms, human error, predators or other causes, resulting in the mass escape of fish annually. For example a single storm in Norway resulted in the release of 490,000 farmed Atlantic salmonwhose total weight exceded the wild salmon harvest there for a whole year. Domesticate farm stocks of S. salar that escape can wreak havoc on wild populations by spreading disease and parasites to, competing with, and hybridizing with native salmon and other fish. Fish farming also fouls sea waters with varies toxicants associated with and produced by fish cultivation (Alaskan Department of Fish and Game, 2002; Thorstad et al, 2006; Volpe, undated; Hindar et al, 2006) .
Crowding fish in net pens increases stress, which makes them more susceptible to disease. Therefore, when outbreaks do occur they tend to spread rapidly through the captive population. Diseases that occur in captive populations, such as furunculosis and sea lice Lepeophtheirus
salmonis can spread to wild fish (Alaskan Department of Fish and Game, 2002; Amundrud & Murray, 2009; Naylor et al, 2005).
Escaped S. salar compete with wild populations and other native fishes for resources. Farmed salmon and hybrids (farm x wild) can be expected to interact and compete directly with wild fish for food, habitat, and territory. Farm juveniles are generally more aggressive and consume similar resources as wild fish. They grow faster than wild fish, which may give them a competitive advantage at some life stages (Thorstad et al, 2006).
Farm-raised S. salar also hybridize with wild stocks and other fishes, thereby reducing the wild stock's ability to survive in the wild by changing the level of genetic variability and frequency and types of alleles in the gene pools. The extremely high abundance of espaced salmon can completely dominate wild populations, comprising up to 80% of all breeders in smaller wild populations. Such an influx in these genetically divergent farmed Atlantic salmon can have dire consequences to wild populations and their genetics (Thorstad et al, 2006; Hindar et al, 2006).
The net 'walls' of sea cages or net pens allow virtually complete interaction between the farm and the surrounding environment. Therefore, clean, oxygenated water is free to pass into the net pen while uneaten food pellets, feces, antibiotics and toxic anti-foulants flow out. The exchange of clean water into the farm and dispersal of industrial wastes away from the farm means that the industry benefits from a subsidy from nature (Volpe, undated).
The likelihood of S. salar establishing reproducing populations in introduced habitats is extremely low. Over 130 attempts to introduce Atlantic salmon across 32 states in the United States, over 60 attempts in British Columbia, Canada, several attempts in Tasmania, and numerous attempts in Chile have all failed (Thorstad et al, 2006).
Location Specific Impacts:
British Columbia (Canada)
Competition: Salmo salar displays significant niche overlap with juvenile steelhead rainbow trout Oncorhynchus mykiss and, under limiting circumstances, are likely to come into vigorous competition for resources (Volpe et al, 2001).
New Brunswick (Canada)
Threat to endangered species: Cultured Salmo salar populations have dominated locations in New Brunswick, specifically the Bay of Fundy. Corresponding with increased fish farming between 1983 to 1997, escaped cultivated S. salar have gone from comprising only 5.5% of the Atlantic salmon population to the majority in the Magaguadavic River by means of hybridization and competition. This significant decline in wild populations of S. salar have caused the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) to classify it as an endangered species (Carr et al, 1997; Dextrase & Mandrak, 2006).
Competition: Escaped Salmo salar from Norwegian fish farms has varied between approximately 250,000 and approximately 600,000 individuals yearly since 1994. These escapees threaten wild populations through competition (Fiske, 2006).
Hybridisation: Escaped Salmo salar from Norwegian fish farms threaten wild populations through hybridization (Fiske, 2006).
Northern Ireland (United Kingdom (UK))
Competition: Adult farmed Salmo salar escapees are less successful at reproducing in the wild but their offspring are highly sexually precocious, outcompeting native salmon in river environments (Stokes et al, 2006).
Scotland (United Kingdom (UK))
Disease transmission: Farmed populations of Salmo salar have been found to transmit sea lice (Lepeophtheirus salmonis) to wild salmon populations in Scotland ( Amundrud & Murray, 2009).
S. salar smolts transported from Scotland to Norwegian fish farms in the 1980's were infected with furunculosis a disease caused by the bacterium Aeromonas salmonicida. The disease spread rapidly and over 250,000 escaped farmed Atlantic salmon were infected with it spreading furunculosis to wild populaitons (Naylor et al, 2005).
Washington (United States (USA))
Disease transmission: Infected Salmo salar cultivated in the Puget Sound have introduced viral hemorraghic septicema (VHS) to the west coast (Fuller, 2009).