In Mediterranean, the Predator Is the Hunted
Monday, June 30, 2008
The Mediterranean Sea, says Francesco Ferretti, 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.
They based their conclusion on evidence scoured from an unusually wide variety of records, including documents drawn from universities and archives, from fish markets and recreational fishing clubs, and from local accounts of shark sightings.
The paper, co-authored with the late Dalhousie marine biologist Ransom A. 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, concluding that 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 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 in a telephone interview last week from his native Italy. 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.
Two 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.
In Prince William Sound, Alaska, Pacific sleeper sharks keep harbor seals from eating too many walleye pollock, wrote Dalhousie marine biology professor Boris Worm, the lead author of a recent paper in Trends in Ecology and Evolution, in an e-mail. Depleting the sleeper sharks in turn hurts the pollock population.