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.
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