Global shark population threatened

The Mediterranean Sea, Francesco Ferretti says, is "a very dangerous place for a shark."

So dangerous that in the past two centuries, the shark population there has plummeted by more than 97 percent, both in relative numbers and collective weight, according to a study by the graduate student, two colleagues at Dalhousie University in Nova Scotia and an Italian researcher.

The paper, co-authored with the late Dalhousie marine biologist Ransom Myers and others, is only the latest evidence that some of the oceans' most feared predators are themselves in dire danger.

Another team of scientists has shown in recent months that the peril is global; it concluded all but two of 21 species of open-ocean sharks and their cousins, the rays, are facing the risk of extinction. Another found that the decline of sharks at the top of the food chain is disrupting marine ecosystems around the globe.

"Sharks are just one part of the ocean's web of life," said Margaret Bowman, who directs the nonprofit Lenfest Ocean Program, which helped fund all three studies. "But these studies show if you pull out that one thread, the whole web suffers."

The shark researchers — who hail from Australia, Canada, New Zealand, the United States and several European countries — are engaged in a huge detective project, much of it inspired by Myers, who pioneered the first global shark assessment before his death in late 2006. Culling both unconventional and traditional sources such as commercial and recreational fishing data, museum records and scientific studies, they are tracking not only how drastically sharks' numbers have dropped in recent decades but also how their disappearance is transforming the marine world.

Several factors help explain why the shark population has declined in the Mediterranean, Ferretti said. Fishing vessels are targeting them to meet the Asian demand for shark-fin soup, he said, while simultaneously trying to compensate for the fact that they have depleted other fisheries.

"Some fishers have decided to switch to sharks because they cannot make up their product with bony fish," he said, noting that the presence of so many countries bordering the Mediterranean has contributed to the fishing pressure there.

"At these levels, these sharks can be considered functionally extinct, meaning that they cannot perform their role of top predators in the Mediterranean marine ecosystems anymore," he said. Ferretti and his colleagues published their findings in this month's issue of the journal Conservation Biology.

Other papers published this spring suggest that once these predators disappear, the species they prey on not only increase in numbers but also behave differently once they are in less danger of being eaten.

For instance, in Prince William Sound, Alaska, Pacific sleeper sharks keep harbor seals from eating too many walleye pollock, Dalhousie marine biology professor Boris Worm wrote in an e-mail. Depleting the sleeper sharks in turn hurts the pollock population.

"We now understand that both on land and in the sea, large predators play important roles in regulating both the total number and the behavior of their prey," Worm wrote. "Unchecked by their predators, some of these prey species can wreak havoc on ecosystems — this is one important reason to keep predators around in sufficient numbers."

Bowman said she and other advocates hope fishery managers will "figure out how to control fishing to prevent further declines" of sharks, and policymakers are responding. On June 19, the National Oceanographic and Atmospheric Administration announced it would ban the removal of shark fins at sea in the Atlantic and Gulf of Mexico by late July.