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Camera-Fitted Seals Spy Antarctic Sea Life

Bijal P. Trivedi
National Geographic Today
January 23, 2002
 
By mounting video cameras on the heads of Weddell seals in Antarctic
seas, scientists have gained much insight into the life of the seals.
But the technique also provides another unexpected bounty: a rare
spy's-eye look at species the seals prey on.

Antarctic fishes,
particularly those living in the freezing, dark waters beneath the ice
pack, are difficult to capture or even observe. But during studies of
the foraging behavior of Weddell seals, the researchers acquired
considerable data on two important fish species: the Antarctic
silverfish and the Antarctic toothfish.



The silverfish (Pleurogramma antarcticum) and toothfish (Dissostichus mawsoni) are major players in the ecology of the southern oceans, said fish biologist Lee Fuiman of the University of Texas at Austin. He led the study, which is published online in Marine Biology, at a research station in Antarctica's McMurdo Sound.

This far south in the High-Antarctic Zone there are no krill, Fuiman explained. So silverfish replace krill at the bottom of the food chain, providing a major source of food for fish, seals, whales, seabirds, and other creatures.

"The seals feed on these fish like popcorn, getting a hundred or so in a single dive," noted collaborator Terrie Williams of the University of California at Santa Cruz.

New Insight

Silverfish are about 15 to 20 centimeters (six to eight inches) long. Trawling data has revealed that they are one of the most abundant fish in southern oceans, but little is known about where exactly they live.

Toothfish, which range from three to six feet (one to two meters) long, are both economically and ecologically important. Their similarity to the Chilean sea bass—a popular menu item and a heavily overfished species—has made them a recent target of the commercial fishing industry.

Fishing trawlers are heading farther south into the domain of the Antarctic toothfish, which, unlike the silverfish, is higher up on the food chain.

Video film from the seal-mounted cameras enabled Fuiman and his colleagues to get a much more detailed profile of where in the water column these fish spend most of their time, how fast they travel, and their vertical migrations during the day.

But the film, recorded over three years and captured by 15 seals, also revealed tantalizing details of the fishing techniques of the Weddell seals.

Weddell seals sunbathe during the Antarctic summer, hauling their huge blubbery bodies out of the frigid waters. Once the seals were on the ice, Fuiman and his colleagues lightly sedated them and took them to a lab, where the animals were rigged with a fist-size infrared camera; a computer that records depth, speed, and bearing; and satellite and radio tags.

The researchers then released the animals into waters beneath the ice, and the camera-equipped seals recorded undersea activity as they swam to and from the site.

"The really wonderful thing about these animals," said Williams, "is that they are just so mild-mannered." When the seals resurfaced, the researchers removed the camera gear from them with minimal disturbance.

Underwater Cunning

Now, Fuiman's team has shifted to studying free-ranging seals, which the researchers locate through satellite and radio tags. Working with free-ranging animals is more of a gamble when it comes to getting the camera equipment back, said Fuiman, but it enables the researchers to observe the seals as they feed and interact with their own species wherever they roam.

"We need to understand the balance between predators and prey—we need to know who's feeding on what, and how much they are eating," said Williams. "These videos allow us to do this."

One of the most surprising findings for the scientists was the individual variation in hunting behavior. "Every animal seemed to have its own strategy for making a living," said Williams.

Yet how the seals are so skilled in finding food still remains a mystery. "It's cold and dark. The animals dive four to five hundred meters down while holding their breath, and are still efficient at finding food," said Williams.

Co-author Randall Davis of Texas A&M University in Galveston suspects that during summer, when the sun shines, the seals may find food by "silhouetting" prey. They dive down deep and look for prey that appear as dark silhouettes against the brighter underside of the ice.

But the seals also attack fish from above, Fuiman noted. He thinks the seals may detect prey by means of their whiskers, detecting the "wake" of fish as they pass by. "But this is a pure guess," said Fuiman. The video captured whisker movements as a seal pursued its prey.

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