A hammerhead shark has wide-set eyes that give it a 360 degree view and stereo vision, which improves depth perception. Image
The precipitous decline in large predator sharks in the Atlantic Ocean in the past decade has made ecologists worry about a trickle-down effect on the ocean ecosystem.
A new study supports the case. With the large predators gone, their prey—smaller sharks and rays—are free to feast on lower organisms like scallops and clams, depleting valuable commercial stocks.
“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 all over the world have plummeted because of intentional fishing for their fins, which are eaten and used for medicines in Asia, and "bycatch," in whicn sharks are accidentally caught when fisherman target other species.
For this study, published in the March 30 issue of the journal Science, 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 nonetheless 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 1983–84 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—04, “the scallops were essentially eliminated,” he said. The only scallops that were spared were those that were protected by poles erected by the researchers to keep out the rays, which are broader than the space between the poles.
Peterson said that 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.
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