Lethal Lice

Studies of more than 280,000 fish have revealed that parasites pose a severe threat to wild salmon in the Atlantic Ocean, according to a paper in Proceedings of the Royal Society B.

Parasites can threaten domesticated animals and undermine attempts to save species. The salmon louse, for example, kills salmon by eating their skin, blood, and mucous. These lice sometimes infest salmon farms, which may then pass the parasite to wild fish.

To find out how much the parasites affected wild salmon, the authors analyzed studies of Atlantic salmon conducted between 1996 and 2008. In these experiments, researchers released 283,347 tagged salmon into rivers in Ireland and Norway. About half the fish had been treated with a parasiticide that would help them fight off salmon lice, while the other half had not.

Later, the team retrieved some fish by tracking down their tags or examining catches of adult fish. Parasiticide-treated fish were more likely to survive, the study authors found. They estimate that parasites caused about 39 percent of salmon deaths.

Since the most frequently-used treatment lasted for only a month or two, the treated fish must have gained their survival advantage while still close to the shore. Salmon farms are often located along the coast and “represent a large — but not exclusive — source of sea lice,” the authors note. Roberta Kwok | 7 November 2012

Source: Krkosek, M. et al. 2012. Impact of parasites on salmon recruitment in the Northeast Atlantic Ocean. Proceedings of the Royal Society B doi: 10.1098/rspb.2012.2359.

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