The wild pink salmon of western Canada are in trouble: In the early 2000s, their numbers in some locations swiftly dropped by 90 percent or more. One explanation put forth for this steep population decline is that sea lice, parasites ubiquitous on farmed salmon, jumped to the wild variety of the fish. But this week in the Proceedings of the National Academy of Sciences, a new study casts doubt on that idea and says the sea lice are not to blame.
When Gary Marty of the University of California, Davis, and his colleagues looked at that aspect for the Broughton Archipelago of western Canada, they found that salmon survival was not lower in years when the juveniles passed by louse-infested farms. This, they say, suggests that something other than sea lice must be reducing survival rates. [New Scientist]
Marty’s team checked up on a decade worth of data dating back to before the 2002 crash, and found a few interesting things. First, they say, the predominance of the lice in wild populations appears to predict the number found in farms a little later, suggesting the parasites travel from wild salmon to farmed ones and not the other way around. Second, they argue, it does appear that a high number of lice in the farmed fish predicts higher than normal exposure for the juveniles of the wild variety, but that increased exposure can’t account for the huge population drop in the wild salmon.