This quantitative analysis of parasite transfer is a scientific milestone in a contentious debate. It is the first to isolate and measure the impact of a fish farm on sea lice outbreaks in wild salmon. The study combined new field techniques that allowed comprehensive, individual observations of over 5,500 young wild pink and chum salmon over 60 km of migration route; an enormous data set from months of laborious fieldwork; and state of the art models of disease transfer.
"Our research shows that the impact of a single farm is far reaching," says lead author Marty Krkosek. "Sea lice production from the farm we studied was four orders of magnitude - 30,000 times - higher than natural. These lice then spread out around the farm. Infection of wild juvenile salmon was 73 times higher than ambient levels near the farm and exceeded ambient levels for 30 kilometers of the wild migration route."
This increase in sea lice abundance is likely to be damaging for already dwindling wild salmon populations in British Columbia, where the study took place.
In Europe, transfer of parasites is generally accepted as a significant threat to adjacent wild populations ?although European studies have not measured the direct transfer of sea lice from farms to wild salmon. However, a bitter debate continues in British Columbia and the Pacific Northwest as to whether salmon farms are contributing to sea lice infections of wild salmon at all.
"Parasites are a key negative side effect of fish farms on the local fish stocks," says Andrew Dobson, an epidemiologist from Princeton University who researches infectious diseases in wildlife