Why are these salmon jumping?

Young sockeye salmon (Oncorhynchus nerka) have a curious behavior: They jump up to 30 centimeters in the air, sometimes skimming along the surface for close to a meter using their tail fins, about nine times a day on average. They even do this when no obstacles are in their way. The reason, according to a new study, is that they’re infested with sea lice—and are trying to splash them off.

Researchers already suspected that salmon leap to dislodge sea lice, a pea-size parasite that feeds on mucus, blood, and skin. Fish plagued by the lice jump out of the water 14 times more often than those that are lice-free do. But scientists didn’t know whether this helped rid them of the bloodsuckers.

So in the new study, researchers caught juvenile sockeye salmon infested with sea lice and separated them into two floating ocean pens in a sheltered ocean bay in British Columbia in Canada—one pen where they were allowed to leap, and one covered with netting just beneath the surface. After letting the experiment run for 3 days, the scientists found that salmon that couldn’t jump from the water had 28% more sea lice than those that could, as they report in an article cheekily titled "Oust the Louse" in the Journal of Fish Biology .

Jumping out of the water isn’t a risk-free proposition for the salmon. It takes an average of 56 leaps to dislodge a single sea louse, during which time the fish could be easy prey for predators like seabirds. The effort also drains energy the salmon need for other things. But for the fish, the danger of these leaps may be nothing compared with the relief of delousing.