Overfishing of powerful sharks ¡ª a top predator in the ocean ¡ª may endanger bay scallops, a gourmet delicacy. With fewer sharks to devour them, skates and rays have increased sharply along the East Coast and they are gobbling up shellfish, particularly bay scallops, researchers report in Friday's issue of the journal Science. (AP File Photo)

    BEIJING, March 30 (Xinhuanet) -- A tremendous loss in the number of large predator sharks in the Atlantic Ocean during the past 10 years has left smaller species free to pig out on lower organisms in the food chain, according to a study published in the journal Science.

    With the large predators gone, smaller sharks and rays feast on lower organisms like scallops and clams, depleting valuable commercial stocks.

    Ecologists worry this "trickle down" effect will seriously damage, if not destroy, the ocean ecosystem.

    "Large sharks have been functionally eliminated from the East Coast of the U.S., meaning that they can no longer perform their ecosystem role as top predators," said study team member Julia Baum of Dalhousie University in Halifax, Nova Scotia.

    Shark populations worldwide have plunged because of intentional fishing for their fins, which are eaten and used for medicines in Asia, and "bycatch," in which sharks are accidentally caught when fisherman go after other species.

    The researchers looked at surveys of populations of 11 great shark species, conducted between 1970 and 2005. Every species had substantially declined in just those few decades. The smallest observed decline was in sandbar shark populations, which had decreased by 87 percent. Other species, including the bull, dusky and smooth hammerhead sharks, may have declined by more than 99 percent.

    "They're all down dramatically," said study co-leader Charles Peterson of the University of North Carolina.

    Two of the shark species studied have been Endangered Species Act candidates since 1997, but have yet to be added to the list, Baum said.

    When one predator disappears from an ecosystem, others that eat the same prey usually take over and keep the balance of the ecosystem in check. But in this case, where not one, but all, of the top predators are rapidly disappearing, "you lose the resiliency and buffering capacity of one species to step in for another," Peterson told LiveScience.

    The loss of top predators has a domino effect on the rest of the ecosystem; populations of lower-level predators, such as rays, skates and smaller sharks, aren't kept in check, allowing them to overeat and wipe out their own prey. The study looked in particular at cownose rays, which feed on bay scallops along the east coast as they migrate in the autumn.

    In a 1983¨C84 study, Peterson found that as the cownose rays came through, they "didn't make a dent on the scallops."

    But when the researchers repeated the study from 2003 to 20004, "the scallops were essentially eliminated," he said. The only scallops spared were those protected by poles erected by the researchers to keep out the rays, which are broader than the space between the poles.

    Peterson said the U.S. National Marine Fisheries Service is aware of the problem of declining shark populations and has taken some steps to mitigate the problem, but he emphasized the need to manage whole ecosystems rather than specific species.

    In the meantime, Peterson said the problem may be far greater than this study shows: other intermediate predators could be destroying other lower organisms, such as clams and oysters, which are also valuable commercial stocks.

    "We haven't even scratched the surface," he said.

    (Agencies)