National Geographic : 2016 Jun
96 national geographic • June 2016 since. They’re now common off Hawaii’s shores and are a familiar food for tiger sharks. Tiger sharks and sea turtles have a long, shared history. They both hark back to the dinosaur age, and the fossil record suggests they may have evolved in tandem. With wide jaws and heavy, angled teeth that resemble old-style can openers, tiger sharks are able to crush and slice through an adult turtle’s shell in a way most sharks can’t. This robust morphology might help explain the tiger’s famously unselective eating habits. Tires, license plates, paint cans, farm animals, unexploded mu- nitions, a suit of armor—all these things have been found in tiger sharks’ stomachs, proving they’re willing to bite just about anything (apparently with minimal adverse effects). So if more turtles are sharing the water with more people, more shark bites might be the result. But this is where the story becomes much more than just a “shark bites man” story, be- cause the relationship between tiger sharks and sea turtles could have broad implications for the health of ocean ecosystems around the globe. On a remote part of Australia’s western coast unpredictable. And according to scientists who study them, tigers are especially unpredictable. AFTER TIGER BEACH I flew to Oahu to meet Carl Meyer at the University of Hawaii to discuss his research on the recent spike in tiger shark at- tacks. Meyer and his team have tagged hundreds of tiger sharks with satellite tags and acoustic tracking devices, and he says they’re just begin- ning to understand the animals. The movements of most shark species are fairly predictable, he says. “They’ll go one place during the day, and one place at night. But for the most part we don’t see that with tiger sharks. They can show up any time of day or night, and In the Bahamas, which prohibited longline fishing in 1993 and designated its waters a shark sanctuary in 2011, the marine ecosystems are relatively healthy. they may be there one day and back the next day, or there one day and then gone for three years.” At least some of this unpredictability is likely caused by the sharks’ hunting habits, he says. As ambush predators, tiger sharks rely on surprise to catch their prey, and “if you’re predictable, your prey is going to adapt to that predictability. So it makes sense to suddenly appear in an area and not be there very long.” Meyer says he doesn’t know why attacks in Hawaii have spiked in recent years, jumping from an average of fewer than four a year from 2000 to 2011 to almost 10 a year from 2012 to 2015. But he says he would expect to see a long- term rise in attacks because of the increasing number of people in Hawaii’s waters. As for why attacks occur mostly in the fall, he points out that’s when tiger sharks come to the main islands to give birth. Female tiger sharks make a huge energy investment when they ovulate. Their eggs are “enormous”—the size of baseballs—and they can have as many as 80 pups in a litter. What that might mean— although it’s a “completely untested hypothesis,” he cautions—is that pregnant sharks get to the islands hungry, and this makes them even more indiscriminate eaters than usual. But the uptick in attacks in the fall, a pattern noticed by native Hawaiians for generations (surfers call it Shark- tober), might also be a function of having more sharks around the islands at that time of year. Besides Hawaii’s growing human population, another possible factor is a proliferation of sea turtles. Green sea turtles received federal pro- tection in 1978, after decades of intense exploita- tion. Their numbers have been increasing ever n Grant Brian Skerry’s fieldwork was funded in part by your National Geographic Society membership.