Interactions between farm-raised and wild fish most often occur when the farmed individuals escape from where they are being cultured. Escapes are typically the result of human error, mechanical failure, or damage caused by storms or aquatic animals, such as seals, which may tear at the nets in an effort to eat the farmed fish.
While other aquaculture systems can be problematic, feedlot style net pens, in which there is no effective barrier between thefarmed fish and the environment, result in the highest level of interaction and the largest number of escapes.
The interactions between farmed and wild fish are problematic for many reasons.
Genetics: Selectively bred, farm-raised fish that escape from aquaculture facilities and reproduce with wild fish can cause a decrease in the genetic diversity of wild populations. This problem in trn could effect the wide market as well as the business for fish farming, as there has been little research on this effect and could cause problems for fish farmers.
Competition: Farm-raised fish that escape into the wild can negatively affect wild populations through competition for food, habitat, and mates. This increased competition is not good both for the wild stock as well as the farmed stock, as there will be not enough of resources for both kinds of fish, hence posing as a potential problem in the decline in the population of fish in general.
Disease: The high densities of fish held at aquaculture facilities can lead to increased levels of disease and parasites; these can be transferred to wild fish residing in the vicinity of the farms, as well as spread to wild fish by escapees.
Certain diseases can threaten the existence of wild fish stocks. One such disease is infections with the salmon parasite Gyrodactylus salaris. The parasite infects salmon in rivers.
Another parasite is Lepeophtheirus salaries or salmon lice, which represent a threat to migrating salmon and trout smolt in salt water estuaries and fjord systems.
The fungal infection crayfish plague represents a serious threat to Astacus, a genus of crayfish in river systems. Furthermore, the parasitic infection bonamiosis has virtually eradicated stocks of flat oysters in large parts of Europe.
More information can be obtained from http://www.seaweb.org/resources/aquaculturecenter/documents/Aquaculture.Interactions.pdf