Salmon farms in the sea (as in British Columbia, Scotland and Norway) can, unless very carefully regulated, cause most wild salmon to die. Nobody intends this to happen. It's a 'hidden cost'; an unintended side-effect of another form of intensive industrial farming. The main problem intensive farming causes is sea-lice parasites (creatures that live off other creatures) which infect the wild fish. The lice love fish farms which have lots of food for them - salmon! But they escape through the farm cages into the sea and infect the wild salmon too.

This near extinction of wild salmon means local people who used to make a living catching a few of them lose their jobs. There are other serious worries about contaminated salmon too [1].

It's strange to think that farming fish could actually make wild fish in the oceans even more scarce than they are already. But it's happening... Production of a single pound of fish-eating species such as shrimp, salmon, tuna or cod needs 2 to 5 pounds of wild-caught fish for them to eat [2].

1 . "A fishy tale of salmon, dioxins and food safety", New Scientist vol 181 issue 2430 - 17 January 2004, page 8.
2 . Salmon Farming: background, SeaWeb, .