Too many sharks have been killed, so they're no longer devouring a voracious predator that feasts on bay scallops, marine researcher Charles "Pete" Peterson concludes. As a result, North Carolina's bay scallops fishery, once worth $1 million a year, has been wiped out.

The finding, reported today in the journal Science, is evidence that harm to one creature in an ecosystem can unexpectedly injure another, Peterson said.

"The marine environment is so vast and three dimensional, there are many linkages," he said. "There are cascading and domino effects."

Sharks don't eat scallops. But the top predators do feast on cownose rays -- kite-shaped creatures that migrate through North Carolina waters. And the rays eat scallops, hordes of them, as they make their late-summer and early-fall travels south.

The timing of the cownose trip past North Carolina is particularly harmful to scallops, Peterson said. The rays arrive from from mid-August to mid-September. Scallops, which live about 18 months, don't start spawning until September. So the rays eat them before they can reproduce.

Fished for centuries

Scallops have been fished from sea grass meadows in North Carolina estuaries for centuries. Native Americans retrieved them from Bogue, Back and Core sounds long before European settlers or modern fishermen.

Into the mid-1980s, annual takes were as high as half a million pounds a year, said Louis Daniel III, director of the N.C. Division of Marine Fisheries. A red tide, a harmful algal bloom, reduced takes in 1987, but they were climbing back -- temporarily.

In 1998, 289,000 pounds of scallops were harvested from North Carolina waters. The last season that the state permitted a harvest, in 2004, less than 100 pounds were recorded.

Fishmonger's, a Durham restaurant, bought North Carolina bay scallops for as little as $3 a pound 10 years ago, manager Mitch Payton said. About two years ago the price spiked to more than $10. The restaurant hasn't bought any since.

"It's hard to make a profit with that," Payton said.

Sharks in demand

Sharks were long fished aggressively for their fins, which are prized in Asia, and for their body meat, which people all over the world eat. The federal government has managed Atlantic shark fisheries in U.S. waters since 1993 and strictly limits the number of sharks that can be taken.

The precise level of decline among big sharks off North Carolina's coast is in hot dispute, Daniel said. But if Peterson's findings are correct, good news might be coming for the scallop fishery, Daniel said.

If populations of blacktips, tigers, bulls, sandbar and hammerhead sharks grow, maybe the scallop population could also revive, Peterson said.

"His hypothesis is interesting," Daniel said.

Peterson, who has studied bay scallops for decades, bases his conclusions on shark census data collected by fellow UNC-CH scientist Frank Schwartz. Schwartz's survey data shows great shark numbers falling in North Carolina waters by as much as 99 percent from 1972 to 2003.

Peterson, who also used population data provided by Canadian researchers, tested his cownose hypothesis himself. During multiple migrations, he shielded clumps of North Carolina bay scallops from hungry rays with wooden poles placed too close to one another for the rays to pass through.

Each time, scallops placed behind the homemade stockades survived in much larger numbers than those left unprotected.

Never lucrative enough

Even when harvests were bountiful, commercial scallop fishing was never lucrative enough to support fishermen year round.

The shellfish traditionally were harvested during the winter months in North Carolina. Fishermen hunted other prey the rest of the year.

David Gaskill, a Cedar Island commercial fishermen who helps Peterson conduct research, said the decline in scallops is profound.

He has sometimes had to work hard to collect juvenile scallops for Peterson's studies at spots once known to yield plenty of bay scallops.

"I've walked around an hour and never found one," he said.

Like Peterson, Gaskill wonders if scallops are the only shellfish that the rays put at risk. If scallops disappear, the rays will find fuel elsewhere, possibly on other types of sea life people like to eat.

"I wouldn't be surprised if they start eating clams," Gaskill said, "and then start on the oysters."