Sharks Blamed in Island Seal-Decline Mystery

Brian Handwerk
for National Geographic News
March 10, 2004

Often described as the "graveyard of the Atlantic," Canada's Sable Island guards the secrets of over 400 ships that have come to grief near its shores.

In recent years, the sandy island located 185 miles (300 kilometers) off the southeast coast of Halifax, Nova Scotia, also posed an ecological mystery: Why has the island's harbor seal population nearly vanished while resident gray seals flourish?

Don Bowen, Daryl Boness, and Sara Iverson are researchers who sought the answer. The trio has tracked Sable Island's seals for over two decades. Though the island offers little to most people, it's a treasure island of sorts for pinnipeds—flipper-limbed marine mammals—and those who study them.

"It's an unusual island because of its distance from shore," said Bowen, a research scientist in the Department of Fisheries and Oceans at Canada's Bedford Institute of Oceanography. "It's roughly 300 kilometers [185 miles] offshore—basically just an emerged dune sticking out of the ocean, right at the edge of the continental shelf."

The island's harbor and gray seals are unusually tame. Their relative approachability offers pinniped researchers extraordinarily easy access to the wild animals. That's made it especially heartbreaking for researchers to watch the dramatic decline of Sable Island's harbor seal population.

"We used to see 500 to 600 harbor seal pups born each year," said Sara Iverson, associate professor of biology at Dalhousie University in Halifax, Nova Scotia. "Those numbers peaked sometime in the late 1980s and began to drop. In the early 1990s the number dropped [to] about a hundred pups per year. And by 2002 only eight to ten pups were born. Essentially there is no longer a breeding population up there."

Even as the numbers plummeted, the remaining animals seemed puzzlingly healthy.

"The animals that were there, the pups that were weaned, they were in great shape. They were nice and fat; they had plenty of milk. There was no indication of nutritional stress," Iverson said. Those observations led her to believe that lack of food was not the key source behind the seals' decline.

Harbor seals are broadly distributed throughout eastern Canada. "There has been nothing to suggest that other parts of the range have seen the kind of declines we see on Sable," Bowen said.

Meanwhile, Sable Island's gray seals are doing just fine. The population is thriving, with perhaps 50,000 pups born in 2004.

"It's extraordinary. They have been increasing by about 13 percent annually for four straight decades," Iverson said. "In the 1960s there were only a few thousand, but now there are a few hundred thousand."

Awash With Clues

Continued on Next Page >>


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