Oceanographers With Flippers

Tagged seals plumb Southern Ocean’s depths.

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By Alexandra Witze, Science News

Seals diving for their dinner near Antarctica have surfaced with an extra morsel: information, gathered by electronic tags on the animals’ heads, about the shape of the seafloor there.

The work has revealed previously unknown undersea channels, through which warm water might flow toward fragile ice shelves. And the seals do it all for a fraction of the cost of traditional seafloor mapping done from ships.

“It gives you a much denser picture of what the water depth is than anything you can conceivably do with ship tracks,” says Laurence Padman, an oceanographer at Earth & Space Research in Corvallis, Ore., and a coauthor of an upcoming paper in Geophysical Research Letters describing the technique.

Seals, walruses, whales and other large marine creatures have moonlighted as oceanographers before. Scientists typically glue sensors to the animals’ bodies that measures factors like temperature and salinity. Researchers have used this information to study water temperatures around Greenland, among other topics.

But the new work is the first to extract information on the shape of the seafloor—known as bathymetry—from the sensors, which also measure pressure and hence depth. “You can actually map the ocean floor,” says team member Daniel Costa, a marine biologist at the University of California, Santa Cruz.

The data came from 57 elephant seals, tagged by Costa’s group over five summers at the U.S. Antarctic Marine Living Resources camp in the South Shetland Islands. As the animals swim, the tags record information every few seconds, then relay it via satellite once the seals surface. About 30 percent of the time seals dive all the way to the bottom to forage for food, says Padman, so by studying enough dives for each animal—some 200,000 dives in total—the researchers can deduce where the seafloor lies.

“It’s a novel and useful technique for gathering bathymetry data,” comments Paul Holland, an ocean modeler at the British Antarctic Survey in Cambridge, England.

Within the seal data Padman’s team discovered several significant troughs cutting across the continental slope off the west side of the Antarctic Peninsula. These features hadn’t been mapped before, says Padman, yet they play an important role in ocean circulation. Warm water can flow through such channels and up under the floating ice shelves that extend off Antarctica, such as the Wilkins ice shelf that partially disintegrated in 2008.

Better information on underwater topography could lead to improved models of how the ocean will respond to climate change, says Padman.

Other researchers might now be inspired to dig through seal data to see what features could be mapped, he adds. Ships can cost tens of thousands of dollars a day to operate in Antarctic waters, whereas there is a wealth of readily available information available on seal tags.

“We want to encourage other people who work with seal data to look into it,” says Padman. “We just thought it was really cool.”

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