duminică, 31 august 2008

In Shark Vs. Polar Bear Smackdown, Shark Wins

Shark vs. Polar Bear
Natural Born Enemies?
-- Adult polar bears, among the largest and most powerful carnivores, are thought to have no natural enemies, but that assessment might change in light of a polar bear jawbone recently found in the stomach of a Greenland shark.

While Greenland sharks, which can grow up to 24 feet long, are known to have eaten large seals, porpoises, an entire reindeer and parts of a horse, the discovery is rare evidence that they may also feast upon polar bears.

"These sharks eat anything they find dead, and do some active hunting as well," Kit Kovacs, who made the find, told Discovery News.

"We cannot determine whether the young bear was carrion or not -- nobody can," added Kovacs, who is the leader of the Biodiversity Research Program at the Norwegian Polar Institute in Tromso.

Kovacs made the unusual discovery while investigating the deaths of multiple harbor seals at Svalbard, an Arctic archipelago. She suspected that Greenland sharks, one of two species of sleeper sharks there, might be the culprits. Prevalent in the region, they are one of few species that might be capable of taking down polar bears, which grow to around 10 feet and weigh up to 1,500 pounds.

Kovacs and her team performed autopsies on collected Greenland shark specimens. They were shocked to find the polar bear bone, but in hindsight, she said, "this finding is not likely to be anything new."

"Greenland sharks have been in the Arctic for millions of years," she explained, suggesting that fatal encounters between marine and terrestrial predators would be inevitable over such an extended period.

"Habitat loss due to global warming is so much more important in terms of a threat to polar bears," added Kovacs.

It's even possible that climate change could have set the stage for the fatal encounter.

In recent years, scientists have reported that sea ice is melting in the Arctic at unprecedented rates. The International Union for Conservation of Nature and Natural Resources classifies polar bears as a vulnerable species, with global warming mentioned as the bear's most significant threat.

Seals make up the bulk of the polar bear diet, and the predators do most of their seal hunting on the ice.

At least one study, published in journal Polar Biology, found that the bears have become so desperate for food that they are now resorting to cannibalism.

Another climate change factor that could have made the shark-meets-bear encounter more likely is a documented growth of the Arctic sleeper shark population.

Aerial surveys have spotted hundreds of fins circling regions such as Prince William Sound off the south coast of Alaska.

Vince Gallucci, a University of Washington professor of fisheries and aquatic sciences, has studied shark populations for more than a decade. He believes the aerial count "would be a high number of sharks in one spot for any place in the world."

Climate change, regular regional climate shifts, and commercial fishing appear to have altered local ecosystems. The change appears to be hurting some animals, such as sea lions, but benefiting others.

"Sharks, being the more efficient eaters, just may be able to take greater advantage of changes in the food that's available," Gallucci explained.

Greenland sharks can descend to almost 1.5 miles underwater, so it is unclear if the Svalbard polar bear died, sunk and was snatched, or if the shark scavenged or killed it close to the water's surface.

Kovacs joked, "I won't be going swimming there again."

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