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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

Long hours of fieldwork may have finally led researchers to suggest an explanation as to why the island's two seal populations suffered such different fates.

"It's possible that the harbor seal decline was largely caused by shark predation and secondarily affected by competition for food from the much larger numbers of gray seals," Bowen said.

This theory was born right on the beaches—a gift of Sable's unusual location.

"Because of eddy currents, almost everything at Sable ends up on the shores," Iverson said. "So when a shark attacks a harbor seal pup, the carcass washes up on shore. We saw a large increase in shark-kill seals. Those kills increased for gray seals also. But [because their] numbers are so huge, it did not affect their population in the same way."

The scientists presume that most of the attacks are by Greenland sharks, as evidenced by the large spiral lacerations on the head and neck of recovered seal carcasses.

Yet other shark species are also involved. "I also saw, in the June months, adult male harbor seals bitten absolutely in half, and that had to be a great white," Iverson said.

Shark predation is nothing new at Sable Island. So what lay behind sudden devastation to the island's harbor seal population? It's probably not related to an increase in sharks, the researchers say. Rather, the freefall in seal numbers may have been spurred by shifting ocean currents that altered the water temperatures near the island.

"I think that the story was really a year with very cold water levels," Iverson said. "A cold water current sort of brought these sharks in, and they basically stayed," Iverson said.

Bowen added, "It's at least conceivable that fairly large-scale changes in oceanographic regime—the cooling of water on the continental shelf—may have brought cold-water shark species to Sable, where they discovered these gray and harbor seals."

The much larger gray seal population was perhaps more able to withstand the increased number of shark assaults, researchers say.

Seal Competition

Meanwhile, the harbor seals also faced some stiff competition from the surging population of their relatives. "Harbor seals had a double whammy," Iverson said. "They had to compete with a much larger population of gray seals for food while bearing the brunt of the shark attacks."

Researchers say the seal story provides an interesting test case for the study of the ecology of marine carnivores like seals. One lesson that might be learned is how populations competing for the same food resources can affect one another.

It also enables researchers to examine ecological ramifications up and down the food chain from a different perspective.

"We want to try to understand the ecological roles of some of these top marine carnivores," Bowen said. "They are long-lived, wide ranging, and [consume] other animals in the food chain."

Studies of behavior, diet, reproduction, and other characteristics of these large animals can help scientists understand the current state of changing ecosystems.

Iverson cites an example from the United States Pacific Northwest, where she says declining marine mammals, like the Steller sea lion, have generated broad media coverage and become political hot potatoes.

"People have long thought in terms of bottom-up problems, always in terms of a food-limitation resource," she said. "But more than ever, people are beginning to think of top-down effects."

"These two aren't mutually exclusive, but top predators can impact prey populations quite significantly. In this case, it was the sharks having an effect. Perhaps similar things could be happening with sharks or killer whales in the waters of the [Pacific] Northwest."

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Related Web Sites
National Geographic Magazine: Harp Seals
Lab of Sara Iverson
Lab of Don Bowen
Marine Mammal Stranding Center—Harbor Seal
Marine Mammal Stranding Center—Grey Seal
 

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