The health of British Columbia's pink salmon stocks have been greatly improved by a simple shift in the timing of anti-parasite treatments in their waters, according to new research. The study, led by a team at the University of Alberta, looked at salmon farming operations in the Broughton Archipelago, which lies between mainland B.C. and the northern tip of Vancouver Island. They describe the area as "ground zero" for studying how salmon farming affects wild populations - chiefly, in this case, the spread of sea lice infestation from farm pens to surrounding waters. Sea lice killed an estimated 90% of migrating wild juvenile salmon during the early 2000s, the researchers said. But by 2009 the mortality rate fell to less than 4%. The key, the study found, was that farmers gradually shifted the timing of sea-lice treatments to the fall and winter months, which has resulted in far fewer of the parasites in coastal waters when the wild salmon migrate in spring. But while those efforts have been successful, still of concern is the ecological effects of anti-parasite chemicals, which are largely unknown, said lead U of A researcher Stephanie Peacock. Scientists have also seen sea lice develop resistance to treatment in other salmon-farming areas.