http://www.sciencemag.org/news/2018/07/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.