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 beachesa 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 regimethe cooling of water on the continental shelfmay 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.
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 CenterHarbor Seal
Marine Mammal Stranding CenterGrey Seal