Shark of the High Seas Actually a Homebody
February 20, 2013
by Erik Stokstad
Feared for its feeding frenzies in the high seas, the oceanic whitetip (Carcharhinus longimanus) is also highly prized for shark fin soup. That culinary predilection has contributed to a steep decline in the number of whitetips, which are critically endangered in the Atlantic. Now, the first detailed tracking study of these sharks, published today in PLOS ONE, provides some good news: Female whitetips stay close to the protected waters of the Bahamas. "It's a key question for conservation," says Elizabeth Babcock, a fisheries biologist at the University of Miami in Florida who was not involved in the study. "It really makes a big difference whether there are parts of the population that are in protected areas."
Oceanic whitetips are more mysterious than most other shark species. Only four tagged whitetips have been recaptured, and just a few have been tracked by satellite in the central Pacific. Data from the satellite showed that the sharks travel up to 4000 kilometers but neither data set had revealed any pattern to their movements. Demian Chapman, a marine biologist at Stony Brook University in New York, wanted to know more, so he teamed up with Lucy Howey-Jordan, a classmate from graduate school, whose family runs Microwave Telemetry Inc. in Columbia, Maryland. The company donated 11 satellite tags, which cost upwards of $4000 each.
Chapman and Howey-Jordan headed to Cat Island, Bahamas, where scuba dive operators have reported congregations of whitetips. "It was amazing no one had taken the opportunity to study them in the Bahamas," Chapman says. In May 2011, the team caught 11 adults and outfitted them with hand-sized tags that would monitor the shark's movements, detach at a predetermined time ranging from 30 to 245 days, and upload location data to a satellite.
The team discovered that six sharks stayed within or very close to the Bahamas Exclusive Economic Zone, an area larger than France, for the entire study. Five others stayed within 500 kilometers of Cat Island for a month, and then fanned out across more than 16,000 square kilometers of the western North Atlantic. All but one came back to the Bahamas by the time the last tag had dropped off. "The Bahamas is home to these guys, they're not just passing through," says Robert Hueter of Mote Marine Laboratory's Center for Shark Research in Sarasota, Florida.
The pattern might be related to the 2-year reproductive cycle, Chapman says, with females heading out to breeding grounds to find mates or to birthing grounds to give birth. To distinguish between these two possibilities, the team is trying to figure out whether females are pregnant by using ultrasound and developing a hormone-based blood test. The researchers also returned last summer and tagged another 41 sharks to get more data on movements.
The pattern of staying close to one area means that well-enforced protected areas can not only help reef sharks , which tend to stay in one place, but also some big ocean-going sharks. The Bahamas banned long-line fishing, a technique that is prone to accidentally catching sharks, in the 1990s and then banned the trade of sharks in 2011.
These kinds of data are very helpful for developing ecological risk assessments that can help international fisheries managers make decisions, says John Carlson, a shark expert with NOAA Fisheries Service in Panama City, Florida. "We haven't had a lot of good information on oceanic whitetips," he says. Next year, the International Commission for the Conservation of Atlantic Tuna will revise its risk assessment for pelagic sharks, including the oceanic whitetip.
Meanwhile, environmental groups, including the Pew Charitable Trusts, which helped fund Chapman's research, are advocating for more protection of oceanic whitetips and other sharks. In March, the Convention on International Trade in Endangered Species of Wild Fauna and Flora will vote on a proposal to ban the international trade in whitetip sharks.
Article on Science Web site