Large-Shark Hunting Habits Exposed by Crittercam
for National Geographic News
|January 23, 2004|
This story is one of a series looking at National Geographic Crittercam research. Crittercam is a research instrument worn by wild animals and equipped with a video camera and other information-gathering equipment. Crittercam is used on animals both in the ocean and on land.
To learn more about the Crittercam's field test, tune in to the Crittercam: Large Sharks episode on the National Geographic Channel in the United States on Saturday at 8 p.m. ET. Got a high-speed connection? Click here to watch previews of the Crittercam television documentaries on the National Geographic Channel Web site.
Mike Heithaus is used to the public image of sharks as mindless killing machines. But one thing he's learned from using crittercam is that "sharks are a lot more boring than you'd expect."
"Tiger sharks may be capable of taking out big prey," said Heithaus, who is known as one of the world's leading shark scientists. "But they'll turn toward a turtle and if the turtle just looks at him, the shark will not even try to attack. He'll wait to make the sneak attack where he's not going to have to put a lot of effort into killing the prey."
Now a marine biology professor at Florida International University in Miami, Heithaus was a research fellow with National Geographic Society's Remote Imaging Program and the host of the Crittercam series. He's been fitting crittercams on sharks since 1997, first on tiger sharks in Australia and more recently on bull sharks and hammerheads in Florida.
His mission: to learn more about the sharks' foraging behavior and where they spend their time. The research has already shown that sharks have the power to change their underwater environment.
"Because sharks are top predators, they can influence the population sizes of their prey," Heithaus said. "They keep their prey in check, and that in turn helps their prey's prey. These effects can cascade through the whole ecosystem."
Heithaus was studying dolphins before turning to shark research in 1997. He says he stumbled into it by accident.
"I was interested in where dolphins spend their time," Heithaus said. "It turned out that during some seasons dolphins were nowhere near where we expected them to be, based on their food. I thought, maybe it has something to do with their predators. So we started to look at tiger sharks."
He was surprised to learn that virtually nothing was known about tiger sharks, even though they're big animals. The average tiger shark is about ten feet (three meters), but some can grow over 15 feet (4.6 meters) long. Heithaus says he was immediately hooked.
"They're perfectly adapted predators, really efficient hunters," he said. "And they've been around for hundreds of millions of years."
In some places, tiger sharks may be the only species to eat large prey like turtles, dugongs, and dolphins. Their teeth, which cut in both directions, are like razor blades, perfectly evolved for cutting through turtle shells and bone.
However, they may be considered somewhat lazy. "They are not a super-fast and maneuverable shark," Heithaus said. "They really do rely on surprise to catch things."
Since 1997, Heithaus has used the Crittercam to study tiger sharks in Shark Bay on the West Coast of Australia. The technology has helped the researchers establish the critical role that tiger sharks play there.
"Tiger sharks may influence where dugongs spend their time, and since dugongs are major grazers of sea grass, that could influence how the sea grass beds are structured, and because sea grass beds provide the foundation for whole ecosystems, that might affect where juvenile fish can be, which may in turn influence where turtles can be," Heithaus said. "You see how the effects go through the whole ecosystem."
The Crittercams show the sharks spending most of their time on shallow sea grass beds, where they find most of their prey. Heithaus and his team have also been able to show that tiger sharks cause dolphins to abandon shallow sea grass beds, their best feeding grounds. The dolphins will rather not eat much, but be safe, than risk becoming shark food.
"The Crittercam also showed us how the tiger sharks kind of bounce through the water," he said. "They swim along the surface, drop to the bottom, and then swim up to the surface again. This is probably a strategy to surprise animals that are close to the surface."
Now, Heithaus and his team, including his wife Linda, a marine biologist, are using Crittercam to study two kinds of sharks in Florida waters: bull sharks and hammerheads.
"We're taking our first tentative steps," Heithaus said. "We're still trying to figure out how to work with these sharks and what information we can get from the Crittercam. It's far too early to draw any conclusions, except to say that the technology works well on both those species."
The bull shark is relatively small. It may grow to be nine feet long. A pregnant female may hit 500 pounds (227 kilograms), small compared to a 2000-pound (910-kilogram) white shark. But they're one of the few species of sharks that will go after potential prey of the same size. Heithaus calls the bull shark "the pit bull of the sea."
"They're one of the top predators in the Florida waters," he said. "I've seen footage of them body-slam boats when they get annoyed."
The bull sharks are also special because they can survive in fresh waters. Some have swum 4,000 kilometers (2,500 miles) up the Amazon River in South America. One stray bull shark was even found in the Mississippi River all the way up to Illinois.
Meanwhile, the hammerhead sharks, with their bizarre head shaped like a hammer, have confounded scientists for years. Why on Earth would the shark have a head like that?
"It's probably evolved for a number of reasons," said Heithaus. "It will increase their maneuverability, and it also helps their sensory capabilities."
The hammerheads can use their heads to pin their prey to the bottom of the sea while they eat it. Their favorite foods are rays and other sharks.
But hammerhead sharks are very susceptible to overfishing. Populations in the Atlantic Ocean may have dropped as much as 90 percent in some places.
"They're one of the hardest species of shark I've worked with, because they're so fragile," said Heithaus. "In general, we should be concerned about the future of our oceans. Some hammerheads are among the shark species we should be most worried about."
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