The near extinction of great sharks is disrupting the marine ecosystem, according to a new study that links their decline to depletions of marine life lower down the food chain.

Overfishing of sharks has led to a boom in about a dozen types of smaller sharks and fishes such as rays and skates, creating a ripple effect through the ecosystem, according to a team of researchers at Halifax's Dalhousie University

"At this point their numbers have been so drastically reduced that they are no longer performing their role in the oceanic ecosystem, so they are no longer acting as the top predator and controlling the different fish species that needs them in the food web," marine biologist Julia Baum, who co-wrote the report, told CTV's Canada AM.

"The ripple effect that we see in our study, is that as a result, the species that they eat -- in particular, smaller shark species and species of skates and rays -- have increased tremendously since 1970."

In the absence of the ancient predators, the smaller sharks, skates and rays that were once the main food source for the large sharks are seeing a population boom, say the researchers in Friday's edition of the journal Science.

One of the middle links in the food chain -- the cownose ray -- is also thriving and in turn, wiping out scallop beds.

The study finds that shark populations off the eastern United States are even more depleted than originally believed.

Basing their report on data from fisheries logs and research surveys from 1970 to 2005, the scientists found the levels of several kinds of so-called great sharks have dropped by more than 99 per cent. The bull and dusty sharks are near extinction, while hammerheads and great white sharks are in hazardously low numbers, Baum said, due largely to overfishing.

A controversial practice known as finning -- which involves slicing the shark's fin off and then throwing its body overboard -- has led to significant declines in most types of the large predators worldwide, 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," Baum said.

Finning kills as many as 73 million sharks a year worldwide for an industry that supplies fins for soups and other uses, she said.

"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," Baum said.

The researchers, including Ransom Myers, who died Tuesday, said the demise of sharks is clearly showing a cascading effect in the ecosystem.

The scientists have also linked the decimation of bay scallops in waters off North Carolina to an increase in cownose rays, which eat the delicacy. Because there are now so few sharks, which feed on the cownose rays, the ray population has increased to more than 10 times what it was a decade ago.

"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.

The explosion of the number of cownose rays coincides with the near-demise of scallops, which are being consumed even before they have a chance to reproduce.

In fact, the scallop fishery off North Carolina has been closed every year since 2004. The loss of the bay scallop has already caused disruptions to seagrass because rays plow through the growth in search for scallops.

Ken Frank, a fisheries scientist with the federal Fisheries Department, said the findings bolster his findings in an earlier research paper that looked at how the disappearance of cod affected the food chain.

Frank, whose study was published in Science in 2005, found that the collapse of cod and other large species had a ripple effect. As the number of large predators saw significant declines, the fish they preyed on -- herring, capelin, shrimp and snow crab -- saw a population explosion.

"There are interdependencies among the species, and when you cause these imbalances, you're going to get some effect elsewhere," he told The Canadian Press 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."

But Steve Murawski, director of scientific programs at the National Marine Fisheries Service, suggests the links between the large sharks, medium-sized rays, and bay scallops are "tenuous."

Habitat degradation and environmental issues could be factors in the decline of bay scallops, he told The Associated Press.

As for the boom in rays, they used to be widely caught and discarded, he said. Now, however, fishing has declined in their prime habitat, Murawski said.