National Geographic News
Sperm whales that steal fish from fishers' lines have been caught red-handed in new underwater videos released this week.
At depths of 328 feet (100 meters), the massive mammals were shown plucking the fishing line at one end to free the tasty black cod at the other end—like shaking apples from a tree, scientists say.
This behavior, captured by cameras attached to fishing gear, had never before been recorded.
That's because sperm whales—which have the biggest brains of any animal known to have lived on Earth—usually hunt at pitch-black depths of up to 6,500 feet (2,000 meters), where audio and video technology are of little use.
Scientist Aaron Thode spent many hours screening the video footage from the underwater cameras.
When the hulking black shape of the whale finally came into the frame (see video above), "our hearts just stopped for a moment," said Thode, of the Scripps Institution of Oceanography. "We [will] spend years trying to recapture that feeling."
The whales were able to to work the fish free from the hooks without getting injured—a sign of a learned behavior, Thode said.
The sperm whale, with its giant head, is not "built for fine maneuvering"—so its strategy of shaking the fish from the line is an "elegant solution," he added.
Thode and his team received funding from the National Geographic Society, which owns National Geographic News.
Sound Indicates Size
In 2004 and 2006, respectively, scientists sent acoustic recorders and video cameras down with the fishing gear of frustrated cod fishers in Sitka, Alaska.
The fishers had noticed that their fishing boats were targeted by groups of the usually solitary whales.
Scientists wanted to test the theory that a whale's sounds may give clues to its size—assuming that the pulses of sound ricochet inside the animals' heads. By measuring the timing of the pulses, scientists believe they can determine the size of the head.
Based on the Alaska video and sound recordings, Thode and colleague Delphine Mathias found support for this theory: The clicking noises heard in the audio matched the physical sizes of the whales seen on video.
Thode and Mathias were also surprised to hear the sperm whales making clicking noises—the loudest and most intense sounds created by any animal—at such shallow depths, and especially near fishing vessels.
Prompted by Thode's findings, the U.S. National Marine Fisheries Service plans to include acoustic recorders during black cod surveys to determine how many of the fish are being stolen by the whales.
Gathering more data about the whales' behavior may help fishers avoid losing their harvest.
Regardless, Thode said, an "arms race" between whales and fishers will continue as overfishing makes fish increasingly scarce.
"As whale populations recover, [we're] going to see more direct competition between whales and humans," he said.
The study appeared in the May issue of the Journal of the Acoustical Society of America
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