THOSE friendly clicks dolphins make could actually be weapons in an
impressive hunting arsenal, say American researchers.
The theory that dolphins stun their prey with sound has been around for
almost twenty years, but no one had ever seen them do it, or even shown that the
type of sounds dolphins make can stun or kill fish.
Now Ken Marten of Earthtrust in Hawaii and Denise Herzing from Florida
Atlantic University say they have hard evidence on videotape. “It’s the first
time anyone’s got shots like this,” Marten says.
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Most data in the field comes from researchers on boats recording dolphin
sounds beneath and guessing what was going on. But Marten says he has taped a
dolphin emitting a sequence of low-frequency “bangs” while chasing a fish.
He had noticed before that dolphins close to herring would emit low bangs at the
frequency the fish hear best at, and had suggested the bangs were designed to
damage the fish’s hearing apparatus. He thinks this is happening on his tape.
“She was aiming right at him. Pow, pow, pow,” he says.
In a further experiment, Marten showed that low sounds with similar acoustic
properties to dolphins’ clicks disorientated anchovies to the point where they
swam in circles, remained still or died. “It could also mess up their
schooling,” he says.
Meanwhile, Herzing has found evidence of a different strategy. She recorded
wild Atlantic spotted dolphins emitting a medium-frequency buzz while searching
for prey in sand on the seabed. She says buried eels jumped out of the sand, and
either stopped completely or moved sluggishly as if they were stunned, giving
the dolphin time to catch them. “Maybe it was just too loud in there,” she
says.
Other researchers remain unconvinced. “If they can prove it, I’d hate to be
negative,” says Pete Tyack from the Woods Hole Oceanographic Institution near
Boston. But Tyack thinks the dolphins may be using sound merely to locate their
prey, not to stun it. “The underwater world is unfamiliar to us and you have to
be careful with interpretations,” he says. Marten and Herzing have submitted
their work to The Journal of Aquatic Mammals.