For sharks, it all adds up in Cape waters

By Patrick Cassidy, Cape Cod Times,
12. November 2011

Experts have made the first scientific attempt to quantify the shark phenomena in local waters, and what they have concluded is (spoiler alert): More seals means more sharks.

“That’s it in a nutshell,” said state shark researcher Greg Skomal, who gave the Cape Cod Times a preview of a draft paper on the relationship between a burgeoning gray seal population around Monomoy Island and increasing visits from the great white sharks that feed on them.

Although the basic findings may seem obvious given reports in recent years of shark sightings around Chatham, the 12-page paper lays the groundwork for important future research, Skomal said.

The paper — co-authored by Skomal, John Chisholm and Steven J. Correia — will be a chapter in a peer-reviewed book set to be released early next year called “Global Perspectives on the Biology and Life History of the Great White Shark.”

While the number of great white sharks visiting the waters off Chatham remains relatively low, the number of sightings and attacks on seals has increased significantly in the past decade, according to the authors’ findings.

It’s possible that great white sharks shifted their diet from seals to other prey, including dead whales, after the gray seal population declined in the 17th century, according to the paper.

“With the protection of marine mammals over the last 40 years, the western North Atlantic gray seal population has rebounded,” the authors wrote. “Hence, it is entirely plausible that white sharks are expanding their foraging strategies to once again include active predation on pinnepeds, which may be becoming a viable food source.”

Pinnepeds are a group of marine mammals that include seals, sea lions and walruses. While population estimates are difficult to pinpoint, the number of gray seals in the western North Atlantic is thought to be greater than 200,000, according to the paper.

Pup production by gray seals in a large breeding colony on Muskeget Island west of Nantucket has increased from six in 1991 to more than 2,000 in 2008.

From 1990 through 2009, Skomal and his co-authors documented 50 credible white shark sightings around the Cape, with more than half near Monomoy.

“The first question was, for us, is this perceived increase real?” Skomal said. “Are we actually seeing more white sharks off Chatham, Cape Cod?”

The conclusion is that sharks are increasingly visiting the area, Skomal said.

A second question is whether the increase means there are more sharks in the Atlantic or just a “shift to take advantage of this abundant predictable food resource,” Skomal said.

Skomal and his fellow researchers are scouring data on shark sightings from up and down the East Coast to try to answer that question, he said.

In addition to the increased sightings of sharks, “the number of shark-bitten seals has been increasing off the coast of Massachusetts and, in particular, off Monomoy Island over the last decade,” according to the paper.

It remains difficult, however, to gauge the amount of predation on seals by white sharks, Skomal said.

“We don’t have access to dead white sharks that would allow us to look inside their stomachs,” he said.

Members of the International Fund for Animal Welfare Marine Mammal Stranding and Research Team have consistently seen signs of what appear to be shark attacks on seals during the decade or so they’ve been tracking data, said Katie Moore, the team’s manager.

Much like with the sharks, it is difficult to distinguish population trends among seals from shifts in the habitat they are exploiting, Moore said.

“It may be that animals are moving back into habitat that they once used,” she said.

Strandings of all animals that the rescue team works with are up this year, Moore said.

Even with the high season for strandings still to come, there have been 280 strandings of whales, seals and dolphins so far this year, versus the average of 228, she said.

The research is being followed closely by other shark experts.

Michael L. Domeier, president of the Marine Conservation Science Institute, has studied sharks in the northeastern Pacific, among other locations, but he and others know little about sharks in New England waters.

One of Domeier’s findings was that sharks in the northeast Pacific spend the majority of their time hundreds of miles from shore. He said it will be interesting to see if Skomal’s work finds the same scenario here.

“(Skomal’s) research is also creating an important baseline of information regarding the relationship between the recovery of the gray seal population and the incidence of white sharks near these seal rookeries,” Domeier wrote in an email.

“It seems logical that an increase in white shark prey could result in an increase in white sharks.”

SOURCE: Cape Cod Times




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