Aquaculture problems: Exotic escapees

Aquaculture is one of the main ways that foreign marine species - both exotic species and domestic forms of native species - are introduced to new areas.

A huge number of farmed fish escape from their pens. In 2002 for example, more than 630,000 farmed salmon escaped in Norway - more than the total number of spawning wild Atlantic salmon in Norwegian rivers.  Farmed rainbow trout, an exotic species not native to the country, have also escaped into Norwegian waters.

Like other introduced species, these escaped farmed species are a serious environmental problem. They cause significant disturbances of natural ecosystems, and threaten local wild species in several ways.

For example, they can compete for food and habitat, displace indigenous species, and interfere with the lifecycles of wild species. Escaped farmed salmon and rainbow trout in Norwegian rivers, for instance, spawn later than wild salmon and can displace eggs laid by wild salmon. If the escaped farmed species have a survival advantage over wild species, they can drive local populations to extinction.

Exotic species can also carry diseases or parasites that can be lethal to native species. In addition, escaped species that are able to breed with the wild stock can lead to a dilution of the natural gene pool. This can potentially affect the long-term survival and evolution of wild species.

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