For example, the migration of wild salmon between fresh and salt water normally keeps sea lice, a salt-water parasite, at bay. Farmed salmon spend all year in coastal areas, which increases their chances of getting parasites.
More fish farms mean more hosts for parasites - indeed, in some parts of Northern Europe, sea lice numbers are now significantly higher as a result of the millions of farmed fish in the sea. Just 10-15 sea lice can kill a fish. In 2001, more than 90 per cent of migrating wild salmon died in one Norwegian fjord because of sea lice infections.
Wild fish can also be exposed to viruses and other diseases through the use of imported, unprocessed fish to feed farmed fish. This problem is particularly associated with farmed tuna, which are usually fed whole fish rather than processed fish pellets.
For example, the use of imported pilchards in Australian tuna farms is believed to have caused the massive viral epidemics in 1995 and 1998 that affected 5,000km of coastline and killed 75 per cent of the wild adult pilchard population in South Australian waters. Mediterranean wild fish are similarly exposed to exotic viruses because of huge quantities of imported fish used to feed farmed tuna.
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