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\nsalmonis 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).

Copp et al, (2005) Risk identification and assessment of non-native freshwater fishes presents a conceptual risk assessment approach for freshwater fish species that addresses the first two elements (hazard identification, hazard assessment) of the UK environmental risk strategy. The paper presents a few worked examples of assessments on species to facilitate discussion. The electronic Decision-support tools- Invasive-species identification tool kits that includes a freshwater and marine fish invasives scoring kit are made available on the Cefas (Centre for Environment, Fisheries & Aquaculture Science) page for free download (subject to Crown Copyright (2007-2008)).

The impacts of farmed Salmo salar may be prevented by a number of strategies and technologies. Care should be taken in choosing farm sites as to prevent or reduce the spread of infections diseases and parasites suchs as sea lice (Lepeophtheirus salmonis) to wild fish populations (Thorstad et al, 2006; Amundrud & Murray, 2009).

Sterilization of farm stocks by high pressure induction of triploidy in newly fertilized eggs would reduce the effects of hybridization of escaped S. salar with wild populations and other fishes. It may also reduce the competive effects of these escapees as well. This practice may reduce their survival and growth rate, increase the likelihood of deformities and susceptibility to disease, and induce negative market reactions. Culture of triploid S. salar was attmpted and abandoned in Fundy, Canada due to high susceptibility to the infectious salmon anemia virus. However, promising studies on this method continue (Thorstad et al, 2006).

Domestication of S. salar to the point that individuals can no longer breed or even survive in nature is another possible means of negating possible impacts of farmed stocks on wild populations. It would be a complicated and long term process to do so without comprimising characteristics necessary for a quality yeild (Thorstad et al, 2006).

Physical: The establishment of protected zones that prohibit the cultivation of S. salar is a means of retaining unaffected wild populations of Atalntic salmon. Norway, the top cultivator of S. salar, maintains 29 national salmon fjords and 52 national salmon rivers where new Atlantic salmon farming is prohibited. This protects 75% of wild salmon within the country. Large zones without existing salmon farms exhibited the inteded effect, but existing farms were allowed to remain. Establishing these zones before salmon farms are begun is essential to the effectiveness of this method. Recapture of escaped farmed salmon has been deemed ineffective (Thorstad et al, 2006).