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Overfishing great sharks wiped out North Carolina’s bay scallop fishery

Posted on April 5, 2007April 5, 2007 by Staff

View a chart showing the decline in great sharks and increase in their preyBy UNC News Services

Fewer big sharks in the oceans led to the destruction of North Carolina’s bay scallop fishery and inhibits the recovery of depressed scallop, oyster and clam populations along the U.S. Atlantic coast, according to an article in the March 30 issue of the journal Science.

A team of Canadian and American ecologists, led by the late world-renowned fisheries biologist Ransom Myers of Dalhousie University in Halifax, Nova Scotia, has found that overfishing in the Atlantic of the largest predatory sharks—such as the bull, great white, dusky and hammerhead sharks—has led to an explosion of their ray, skate and small shark prey species.

“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,” said co-author Julia Baum of Dalhousie.

“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 biology and ecology at the Institute of Marine Sciences at UNC and co-leader of the study.

In 2003, Myers and Baum published a study in Science that showed rapid declines in the great sharks of the northwest Atlantic since the mid-1980s. In the new study, funded by the Pew Institute for Ocean Science, the research team examined a dozen different research surveys from 1970-2005 along the eastern U.S. coast and found that their original study underestimated the declines: scalloped hammerhead and tiger sharks may have declined by more than 97 percent; bull, dusky and smooth hammerhead sharks by more than 99 percent.

Sharks are targeted in numerous fisheries and they also are snagged as bycatch in fisheries targeting tunas and swordfish in both U.S. and high-seas fisheries. As many as 73 million sharks are killed worldwide each year for the finning trade, and the number is escalating rapidly.

With an average population increase of about 8 percent per year, the East Coast cownose ray population may now number as many as 40 million. The rays, which can grow to be more than four feet across, eat large quantities of bivalves, including bay scallops, oysters and soft-shell and hard clams in the bays and estuaries.

In the early 1980s, Peterson sampled bay scallops in North Carolina sounds in late summer before and after the cownose rays passed through and found that most scallops survived the ray predation, allowing the scallop population to support a fishery and still replenish itself each year. In contrast, sampling in recent years by Peterson and co-author Sean Powers of the University of South Alabama and the Dauphin Island Sea Lab—after the cownose ray population explosion—showed that the migrating rays consumed nearly all adult bay scallops in the area, except those protected inside fences that the researchers had put up to keep the rays out. By 2004, cownose rays had completely devastated the scallop population, terminating North Carolina’s century-old bay scallop fishery.

“Increased predation by cownose rays also may inhibit recovery of oysters and clams from the effects of overexploitation, disease, habitat destruction and pollution, which already have depressed these species,” said Peterson.

Ecologists have long predicted that the demise of top predators could trigger destructive consequences. Researching such effects, however, has been a challenge.

“This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops,” said Ellen Pikitch, executive director of the Pew Institute for Ocean Science and a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science. “These unforeseen and devastating impacts underscore the need to take a more holistic ecosystem-based approach to fisheries management.”

This research is a contribution of the Pew Global Shark Assessment. The study also was supported by the Killam Trusts, the Natural Sciences and Engineering Research Council of Canada, the North Carolina Fisheries Resource Grants Program, the UNC Institute of Marine Sciences, the Sloan Census of Marine Life and the U.S. National Science Foundation

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