Photograph by Brian J. Skerry, NGS
National Geographic News
December 3, 2009
Many of the hammerhead sharks that are butchered to feed Asian demand for shark-fin soup start their lives in American waters, a new forensic study shows.
For the first time, scientists have used DNA from shark fins to determine where they came from. The researchers traced finds from the scalloped hammerhead shark species—collected at the world's biggest fin market in Hong Kong—back to rare populations in the Atlantic and Indo-Pacific oceans.
The trade in shark fins supplies Asian markets with the key ingredient in the luxury dish shark-fin soup. The practice claims up to 73 million sharks annually, including up to 3 million hammerheads. The finless fish are usually tossed back into the ocean to die.
Because the vast flow of shark fins to global markets usually operates in secret, conservationists have been left in the dark about where the sharks are killed. And governments can't control the trade if they don't know how many sharks are being taken from their waters.
The shark-fin market is "like this big Wild West show [that] no one is monitoring," said study leader Demian Chapman, now at the Institute for Ocean Conservation Science at New York State's Stony Brook University.
But the new DNA technique, which Chapman worked on at Nova Southeastern University, may be a tool for controlling the shark trade, he said.
For example, governments could use DNA-derived finning data to develop quotas to prevent overfishing.
Hammerhead Shark DNA Mapping
Chapman and colleagues took small tissue samples from 62 hammerhead fins at the Hong Kong market.
Hammerhead fins—which can sell for U.S. $120 each in Hong Kong—are highly coveted, since their fins boil down into noodle-like pieces that give the soup texture.
The team then ran a DNA sequence of a particular part of the shark's genome and compared it to a "map" of hammerhead DNA sequences culled from global research efforts.
The results showed that 57 of the 62 Hong Kong fins had come from sharks in either the Atlantic or the Indo-Pacific.
Twenty-one percent of the 57 fins had originated on sharks from the western Atlantic—including the Gulf of Mexico and the North American Atlantic coast as far south as Brazil. These populations are considered endangered by the International Union for Conservation of Nature.
Finding so many sharks from the depleted waters of the western Atlantic surprised the team, Chapman said, and it suggested that overfishing is still a problem in the region.
Sharks, Top of the Food Chain
Sharks are declining rapidly worldwide, and losing the top of the marine food chain could have vast repercussions for the rest of the ocean—and even us, Chapman said.
For example, sharkless seas may allow swarms of prey species, such as stingrays, to crowd beaches and wipe out commercial fish species.
Also, the sharks' decline is disturbing because shark species have existed on Earth for nearly five hundred million years, Chapman added.
"I don't think anybody wants to be part of a generation that's snuffed out an animal that's been in the oceans that long."
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