For years, conservationists have warned about overfishing of large sharks in the northwestern Atlantic, as the demand for meat and fins, coupled with slow growth and reproduction rates of many species, has caused sharp declines in populations of hammerheads, duskies and other sharks. Researchers are now reporting repercussions beyond the declining shark populations. Depletion of large sharks, they write today in the journal Science, has led to the destruction of the bay scallop fishery along parts of the Eastern seaboard.

The study, by Ransom A. Myers of Dalhousie University in Halifax, Nova Scotia, and colleagues, is among the few to document the cascading effects that the loss of a top predator can have on a marine ecosystem. In the absence of large sharks, the researchers say, the smaller sharks, skates and rays that they feed upon have thrived. In turn, the study shows that as one of these middle links in the food chain, the cownose ray, has become more abundant, it has wiped out scallop beds in North Carolina.

“People are always asking me why we should care about sharks,” said Ellen Pikitch, a marine biologist at the University of Miami and executive director of the Pew Institute for Ocean Science, a major funder of the study. “This is a great example of why sharks matter to the broader ecosystem.” Dr. Pikitch described the Science paper as a “classic,” and said it would inspire others to examine data in search of similar effects, called trophic cascades, which she said were “going to be seen more and more in the oceans.”

But Steve Murawski, director of scientific programs and the chief science adviser at the National Marine Fisheries Service, said the study had not conclusively shown a connection between declines in certain species and increases in others. “It certainly shows correlation,” Dr. Murawski said. “What we don’t have is a smoking gun in terms of a predation link.” There are alternative hypotheses for some of the population effects described, he said, including habitat loss and pollution.

The study combined long-term data on shark, skate and ray populations, along with experiments that showed the impact cownose rays can have on scallops. Dr. Myers, a leader in the study of the impact of overfishing in the oceans, died Tuesday of a brain tumor. Pete Peterson, a marine biologist at the University of North Carolina and an author of the paper, said the work had its origins in a discussion he and Dr. Myers had at a conference several years ago. “He was working on shark depletions and I was working on the cownose ray and its effect on bay scallops,” Dr. Peterson said. “The light went off — we were working on the same problem.”

Dr. Peterson said the cownose ray was just one of 12 midlevel species that the study showed were becoming more abundant because of the disappearance of sharks. “There’s another 11 out there doing who knows what,” he said. Dr. Peterson said the rays practically denude scallop beds during their end-of-summer southward migration along the Eastern seaboard. The timing is such that the scallops are devoured before they have a chance to reproduce. “Cownose rays aren’t so wise in their sustainable use of the resource,” he said.

“They eat them before they’ve spawned.” Sonja V. Fordham, manager of the international fish conservation program at the Ocean Conservancy, said the study was being published at an opportune time, as the fisheries service is amending its 1993 management plan for shark fisheries. “The sad reality is that the Atlantic shark management plan is one of the most comprehensive and restrictive in the world, and it’s failing,” said Ms. Fordham, who added that the study affirmed her group’s position that the solution was to close certain shark fisheries. “After 13 years of trying, it’s not working,” she said. “It’s time to shut the thing down.”

(By Henry Fountain, NYT, 31/03/2007)