The Crittercam footage and more about Shark Bay can be seen on the National Geographic Channel TV special Crittercam: Green and Loggerhead Turtles.
Healthy Populations, Imperiled Individuals
Unlike their counterparts in most of the world, the tiger shark and sea turtle populations in Shark Bay are healthy and robust. The turtles have evolved to deal with predation by the sharks, reaching an equilibrium that places neither shark nor turtle at risk of extinction.
"But individual turtles are certainly in peril, and they have to make good decisions about where to spend their time and how to behave if they are to avoid the sharks," Heithaus said.
The green sea turtles average 3 feet (1 meter) long and 350 pounds (160 kilograms). Their long flippers, sleek bodies, and smooth shells allow them to zip through the water at speeds greater than 10 miles (16 kilometers) an hour.
The researchers theorized that perhaps the greens' speed allows them to escape tiger sharks more readily than loggerheads can. Loggerheads are about the same size as greens but are handicapped by stubby flippers and barnacle-encrusted shells.
But the researchers were in for a surprise.
"The Crittercam showed us the loggerheads spending a lot more time at the surface than the greens, and based on the behavior of tiger sharks, that seems to be a really bad thing to do," Heithaus said. "It is probably one of several reasons we see a lot more injured loggerheads than greens."
The researchers are uncertain as to why the loggerheads spend so much time gulping for air at the surface. The team believes the practice may have to do with a difference in the species' diving abilitiesthe loggerheads may not be physically capable of diving after just a few quick breaths.
The footage also revealed a few tidbits about the greens. For one, they keep their shells nice and polished by rubbing up against sea sponges. Also, jellyfish make up a much greater percentage of their diet than the researchers had suspected.
Heithaus and his colleagues hope future deployments of Crittercam on Shark Bay's sea turtles will reveal how they change their behavior based on the presence or absence of tiger sharks.
Dill said such information could be important in helping devise schemes to prevent sea turtle deaths due to human factors such as boat traffic.
"We're thinking of boats as analogous to predators, so how [the sea turtles] react to predators may be similar to how they react to boats," he said. For example, Dill said, the presence of a predator may force a sea turtle to dive before it is ready, thus affecting how much it can forage while underwater.
Such knowledge, Heithaus said, "might help us figure out how to change human behavior to minimize the impacts on the turtles."
In addition to Heithaus and Dill, project scientists include Linda Heithaus of Florida's Mote Marine Lab; Simon Fraser University graduate students Aaron Wirsing and Alejandro Frid; and graduate student Lars Bejder of Dalhousie University in Halifax, Nova Scotia.