National Geographic : 2016 Jul
96 national geographic • JULY 2016 they feed. And each year most sharks they see are the ones they saw in previous years. This raised an intriguing question: With enough observations, could you use the sharks you see to estimate how many you can’t see? In 2011 a team in California did just that and came up with just 219 adults in California’s most shark- rich region. Even among top predators, gener- ally less abundant than their prey, that’s a tiny number. The study shocked the public and came under immediate attack from other experts. Of course, counting great whites is a lot harder than counting land animals or even marine mammals. So scientists make mas- sive assumptions about shark movements and then extrapolate. In California the biggest as- sumption was that a few feeding grounds were representative of the entire hub. Other teams crunched the same data using different assump- tions, and one study estimated about 10 times more sharks. (That count was bolstered by add- ing juveniles, which the first excluded because so little is known about them.) Pretty soon sci- entists began quantifying white sharks in the other hubs. A team in South Africa estimated the population there at around 900, while another team put Mexico’s Guadalupe Island population, Biologist Greg Skomal tries to record video of a shark near a popular swimming area off Cape Cod. For the first time in modern history, great whites have begun regularly returning to the waters of this vacation spot. n Grant Brian Skerry’s fieldwork was funded in part by your National Geographic Society membership.