DNA "Zip Code" Could Track a Shark Fin to Its Source

Overfishing has caused shark populations to plummet—and often they are hunted simply for their fins. Marine forensics might help scientists ease the pressure: Patterns buried within shark fins' DNA could reveal what region the fish came from—ultimately targeting those populations for conservation.

Forensic analysis can help finger a criminal or track down a long-lost relative, but in recent years it has increasingly been used to trace non-human animals, too. Now, DNA detective work could help protect endangered sharks from overfishing, according to scientists mapping the genetic signatures of shark populations around the world.

Humans kill up to 73 million sharks a year, according to a 2006 estimate, mostly to feed appetites for shark fin soup. Demand for this delicacy has grown with the Chinese economy, making sharks a valuable catch for fishermen­—who often slice off the fins and throw back the rest of the animal. This practice has taken an extreme toll on shark populations, says Demian Chapman, a biologist at Stony Brook University's Institute for Ocean Conservation Science. "When I first started studying sharks, the animals were disappearing faster than I could study them," he says.

Most of the global shark trade is unmonitored and unregulated, so it's not easy for scientists like Chapman to find out which populations are being hit the hardest. So he and an international team of scientists began working backward from fish on the market. In a study published in April in Endangered Species Research, they analyzed DNA from the fins of dusky sharks, a species particularly targeted for soup, to create a map of genetic "zip codes." These sequences identify which part of the world the sharks are from and can ultimately be used to trace fins back to their place of origin.

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During the course of their research, Chapman and his collaborators collected tissue samples from more than 250 dusky sharks, some alive and some dead, and extracted and sequenced a section of non-coding (sometimes called "junk") DNA. Unlike DNA that codes for vital proteins—where mutations are usually detrimental and aren't passed on to the next generation—non-coding DNA can mutate all it wants and "you don't end up with a shark with twelve heads or anything like that," Chapman says. As a result, mutations pile up rapidly and spread among populations that interbreed; comparing the sequences reveals relationships between the animals.

While standard DNA bar-coding only distinguishes one species from the next, DNA "zip-coding," as Chapman calls his analysis, can identify distinct groups of the same species living in different parts of the world. Chapman found unique mitochondrial zip codes for populations in regions like the U.S. Atlantic, South Africa and Australia, for example. That means these particular sharks breed in geographically separate groups, even if they migrate widely­—and, unfortunately, isolated breeding groups aren't replenished by sharks from elsewhere. (Chapman estimates that even with protections on dusky sharks in the U.S., it would take 400 years for the Atlantic stock to fully recover). If shark fins on the market were regularly tested and matched to the zip code map, Chapman says, scientists could better monitor the toll the fin trade is taking on sharks in the wild.

His team has already begun to do that. In 2009, Chapman and his colleagues created a similar DNA map for hammerhead sharks, then compared them to samples from fins for sale in Hong Kong. They found that about a quarter of the hammerhead fins on the Asian market came from the western Atlantic, where hammerhead populations have dropped by about 85 percent in recent years. "In some areas, species are in much worse shape," says John Carlson, a research biologist at the NOAA Fisheries Service who was not involved in Chapman's study. "If you can have a 'zip code' for where a species is coming from," he says, "it helps us tailor where we do our research and our conservation."

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Chapman isn't the first researcher to harness forensic analysis for conservation. Similar DNA techniques have helped international authorities trace black-market elephant ivory to the regions where it was poached, and revealed that whale meat sold in some sushi restaurants came from endangered species—in these instances, both targeted animals are regulated by CITES, the Convention on International Trade in Endangered Species. Hammerhead sharks came within a few votes of being added to the CITES list last year.

While many shark species are yet to be listed, there's no question that they are in trouble. Sharks reproduce more slowly and less frequently than other kinds of fish, and because they're top predators in the ocean, their demise throws entire ecosystems out of balance. Dusky sharks, the species in Chapman's recent study, have declined by more than 80 percent over the last two decades in the western Atlantic, according to a National Marine Fisheries Service assessment.

In the next few years, according to Chapman, scientists plan to create DNA maps for all the major species in the fin trade. And researchers are now working on developing rapid DNA tests that could be used to sample shark parts on the spot, NOAA's Carlson says. But even with zip-coding as a conservation tool, forensic research won't be able help sharks very much until there are laws governing the fin trade. "The technology is not the limiting factor here," Chapman says. "What we need is political will and some resources to go in and monitor this trade."

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