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CSI Sharks: Tracking the fish in Florida by what they leave behind

Blacktip sharks are common around Tampa Bay. University scientists want to see if they could one day help protect the fish by just sampling the water.
Researchers with another university work to bring a blacktip shark aboard during a research cruise on in April 2017
Researchers with another university work to bring a blacktip shark aboard during a research cruise on in April 2017 [ Times (2017) ]
Published Aug. 12, 2020
Updated Aug. 12, 2020

Like suspects on a crime show, when sharks swim through a bay, they leave pieces of themselves behind.

The fragments are known as environmental DNA, coming from bits of feces and flakes of skin. Similar to the unfailingly shrewd investigators of prime-time television, researchers are starting to think they can learn something from that genetic evidence.

Scientists from Florida International University, teaming with colleagues from New College of Florida and a conservation group, have pulled samples of water from Terra Ceia Bay, a popular nursery for blacktip sharks south of the Sunshine Skyway bridge. Filtering that water, they have identified shark DNA, the frequency of which they say roughly corresponds to breeding periods.

Unlike a crime drama, though, the point is not to catch a culprit.

“It’s going to be about saving sharks from people, really,” said Demian Chapman, a biological sciences professor at Florida International.

The hope is that environmental DNA sampling can provide a safe, affordable and easy way to track shark populations in the future, crucial as regulators try to maintain the health of a species. Blacktips are important to the Gulf of Mexico fishery for their meat and skin. Catches have gone up in recent years, in part because regulators restricted harvesting of another type of shark, said Jayne Gardiner, a New College of Florida biology professor involved in the research. At the seafood counter in Publix, she said, shark meat is now often blacktip.

As their popularity has increased, populations have remained high, according to the National Oceanic and Atmospheric Administration. That is a success, Gardiner said, but not a reason to get complacent on managing the fishery.

Gardiner’s team employs a traditional method, pulling in blacktips using gill nets — banned in Florida for commercial harvesting — while watching closely to make sure other marine animals avoid the trap and keeping count of the sharks. Meanwhile, the Florida International group dips a bottle into the bay to collect two liters of water at a time. They use filters and lab analysis to focus on blacktip DNA. Each filter test, Chapman said, costs about $12. The concurrent surveying is married to see how DNA results match up to population counts.

“If you can move to something where all you’re doing is sampling water, it really only takes a couple of people to do that,” Gardiner said. By comparison, fishing for sharks demands bigger crews and closer contact with the animals.

Blacktips can grow to about 6.5 feet in length, according to the Florida Fish and Wildlife Conservation Commission. They come near shore in schools, feeding on creatures like smaller sharks, crabs and rays. They favor Terra Ceia as a spring nursery, where they give birth to pups before moving south to deeper water. The Fish and Wildlife Conservation Commission notes they have also “been implicated in attacks on bathers.”

That, however, is not the point of keeping count, Chapman said, and water sampling would probably not reduce already “very, very rare attacks.”

“Off the beach in most places there are usually sharks,” Chapman said. “It’s like literally a lightning strike.”

The research, he said, will continue to confront many outstanding questions. Biologists need to learn more about how long environmental DNA lingers in the water and how they can tie DNA results to precise population figures. The Florida International researchers, he said, are working with a group from Florida Atlantic University that uses drones to survey the density of blacktips near West Palm Beach.

“It’s very hard to find sharks in actuality. They’re elusive animals,” Chapman said. “It’s a lot easier to go out and collect some water.”