Many large species of North Atlantic sharks have declined by 97 per cent or more since 1970.

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CHICAGO: The virtual elimination of large sharks from the east coast of the U.S. has disturbed the marine ecosystem and decimated scallops, says a new study.

Massive over-fishing of large predatory sharks in the coastal waters of the Atlantic over the past 30 years has led to an explosion in the ray, skate and small shark species that they prey on, with devastating effects for one of the organisms at the bottom of the food chain.

"Large sharks have been functionally eliminated from the east coast of the US, meaning that they can no longer perform their ecosystem role as top predators," said Julia Baum a fisheries biologist at Dalhousie University in Halifax, Canada and co-author of the study.

"With fewer sharks around, the species they prey upon - like cownose rays - have increased in numbers, and in turn, hordes of cownose rays dining on bay scallops have wiped the scallops out."

Several of the larger shark species in the northwest Atlantic are verging on extinction, according to Baum and colleagues who analysed a dozen surveys dating from 1970 up to 2005.

The numbers of scalloped hammerhead and tiger sharks appear to have declined by more than 97 per cent over that period, while populations of bull, dusky and smooth hammerhead sharks could be down as much as 99 per cent, according to this analysis.

The reasons are not hard to find, say the marine biologists. The growing demand for shark fins and shark meat, particularly in Asia, has led to a rapid escalation in shark fishing.

With the drop in shark population, a dozen species of rays, skates and small sharks have increased in numbers over the past 16 to 35 years, some of them tenfold, according to other data reviewed by the marine biologists.

Perhaps the most conspicuous beneficiary of the decline in shark numbers is the east coast cownose ray, whose numbers have exploded, rising an average of eight percent a year to bring the total population to an estimated 40 million, according to the study published in the U.S. journal Science today.

The rays feed on molluscs including bay scallops, oysters, and clams. The boom in cownose ray numbers has been nothing short of a disaster for North Carolina's bay scallop fishermen.

In the early 1980s, researchers who sampled the bay scallops in the North Carolina sounds before and after the rays' annual summer feeding session, found their numbers were sufficient to sustain a commercial fishery and still replenish themselves every year.

By 1996, the migrating rays were consuming nearly all of the scallops in the area by early fall, except those protected by fences the researchers had put up to keep the rays out. By 2004, the fishermen who harvested the scallops had gone out of business, ending a century-old North Carolina tradition.

"This is just a small window into this domino effect," said Charles Peterson, a marine biologist at the University of North Carolina at Chapel Hill and co-leader of the study.

The effects on the ecosystem of increases in the other rays, skate and smaller shark species is still unclear, but there may be a cascade effect there too, the authors of the paper said. "Our study provides evidence that the loss of great sharks triggers changes that cascade throughout coastal food webs," said Baum.

"Solutions include enhancing protection of great sharks by substantially reducing fish pressure on all of these species and enforcing bans on shark finning both in national waters and on the high seas."