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The near extinction of several species of sharks is causing a dangerous ripple effect through the marine food chain, according to a new study that links their virtual disappearance to depletions of other sea life.

The report by a team of researchers at Dalhousie University in Halifax has found that species that were once the primary food source for certain types of large sharks are undergoing a population boom because there aren't as many sharks to prey on them.

The scientists contend that the explosive increase in about a dozen types of smaller sharks, rays and skates has caused a cascading effect throughout the ecosystem as they begin to deplete limited nutrient sources and alter nature's complex food web.

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"It's incredibly serious," said Julia Baum, who co-wrote the report to be released today in the journal Science. "Everyone knows that the oceans are being overfished and it's the top predators that are being disproportionately hit by overfishing.

"Because they structure everything underneath them in the food web, we may be drastically changing and restructuring how the oceanic food web functions and operates."

The report states that shark populations off the eastern United States are in an even steeper decline than originally thought. Using data from fisheries logs and research surveys from 1970 to 2005, the team discovered that the abundance of several types of so-called great sharks has dropped by more than 99 per cent.

The bull and dusty sharks are verging on extinction, while hammerheads and great white sharks are in dangerously low numbers, Ms. Baum said, due largely to overfishing.

The controversial practice of finning -- slicing the shark's fin off and then tossing the carcass overboard -- has led to precipitous drops in most strains of the large predators globally, the report said.

"What we're seeing is a higher risk of extinction of these species in these areas, and the term we use as ecologists is functional elimination," Ms. Baum said, adding that finning kills as many as 73 million sharks a year worldwide for an industry that supplies fins for soups and other uses.

"It means that these great predators can no longer play their roles in the ecosystem as top predators. So they're no longer controlling the species in the food web below them."

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The researchers, including Ransom Myers, who died Tuesday, cited a specific example of how the removal of sharks is affecting other species.

Ms. Baum, a marine biologist, said they have for the first time linked the decimation of bay scallops in waters off North Carolina to an increase in cownose rays, which eat the delicacy. Sharks feed on the rays, but because there are now so few sharks, the ray population has grown to more than 10 times what it was a decade ago.

Cownose rays have wiped out scallop beds to the point that the fishery has been closed every year off North Carolina since 2004.

"This ecological event is having a large impact on local communities that depend so much on healthy fisheries," said Charles Peterson, a professor of marine sciences and biology at the University of North Carolina and co-author of the report.

Ms. Baum said it's not clear what the increase in the other species will mean for the food chain and the wider ecosystem, but it's likely skates, rays and smaller sharks are disrupting the wider natural order in oceans around the world.

The loss of the bay scallop has already caused disruptions to seagrass, an important habitat for other marine life, because rays plow through the growth in search of scallops. Rays may also be inhibiting the recovery of oysters, hard clams and soft-shell clams.

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Ken Frank, a fisheries scientist with the federal Fisheries Department, said the findings add to what he had discovered in an earlier research paper that looked at how the disappearance of cod affected the food chain.

Mr. Frank, whose study was published in Science in 2005, found that the collapse of cod and other large species led to a cascade effect. As the number of large predators declined dramatically, the fish they preyed on -- herring, capelin, shrimp and snow crab -- experienced a population explosion.

"There are interdependencies among the species, and when you cause these imbalances, you're going to get some effect elsewhere," he said from his office in Halifax.

"For many decades, it was thought this type of cascade effect was possible only in simplified systems like ponds, so seeing this occur in the marine system is alarming. It means we're modifying the way energy is flowing through these systems."

This latest scientific paper follows groundbreaking research Mr. Myers and Ms. Baum did in 2003 that found shark populations in the Atlantic had plunged dramatically since the mid-1980s.

"We know better now why sharks matter," Ms. Baum said. "Keeping top predators is critical for sustaining the health of the ocean."

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