"It's not that hard to tag them," said Pyle. "We watch for an attack on a seal, and then go to where the blood and slick are. The sharks come right up to the boat, circling around it."
"The problem," Pyle explained, "is once you get one [satellite tag] on, it's real difficult to then get it off again."
The tags used in the new study were state-of-the-art, Pyle said. The tags are designed to detach from the animal at a designated time. When the tag reaches the surface of the water, the stored data is transmitted via satellite. The tags provided information on where the sharks went, how deep they dove, and water temperatures.
The data collected while the sharks were near shore confirmed expectations. The sharks were found to spend most of their time in waters between the surface of the ocean down to about 100 feet (30 meters) deep. Water temperatures ranged from about 50 to 57 degrees Fahrenheit (10 to 14 degrees Celsius).
Surprisingly, though, the data revealed that white sharks spend up to five months in deep ocean waters. They also travel farther and dive deeper than had been previously assumed.
One shark traveled 2,360 miles (3,800 kilometers) to waters off the coast of Hawaii; three others traveled to waters in the subtropical eastern Pacific.
In deep ocean waters, the sharks spent their time mainly in one of two depthseither from the ocean surface to 16 feet (5 meters) deep, or in depths from 985 to 1,640 feet (300 to 500 meters).
The sharks were found to inhabit a much wider range of water temperatures than those near shore. Sea-surface temperatures ranged from 68 to 79 degrees Fahrenheit (20 to 26 degrees Celsius). At the deepest dives, 2,130 to 2,230 feet (650 to 680 meters), the temperature was about 40 degrees Fahrenheit (4.8 degrees Celsius).
The researchers have not yet determined why the sharks spend long periods off-shore and deep in the ocean; it may be related to feeding or breeding migrations.
Despite the white shark's position as the top predator in the marine food chain, the species is considered vulnerable. Although white sharks exist all around the world, the total population is low.
The sharks' breeding habits contribute to that vulnerability. The rate of reproduction is extremely important for population stability; a low reproductive rate makes a species particularly vulnerable to population crashes.
"Some of the data show that the females return only every other year, meaning they breed once every two years," said Pyle.
Other threats come from human activities.
Although not specifically a target of fishing fleets, white sharks often scavenge from commercial long-line and net fisheries, making them susceptible to accidental entrapment, according to Ian Fergusson, a member of the Shark Trust and IUCN Shark Specialist Group.
In some regions white sharks are hunted for their curio valuethe jaws and teeth are sold to touristsand for their meat.
The lack of baseline data on white sharks has made it difficult to devise protective measures. Guidelines now in effect are patchy.
The white shark is protected off the coast of California, the eastern United States coastal waters, and the Gulf of Mexico, but not off the coasts of Washington, Oregon, and Mexico.
Given the finding that white sharks travel over a wide range, effective conservation measures may be needed at the global level.
"We need to know where they go in order to consider conservation initiatives," said Pyle. "Right now we're investigating the possibility that the females may be going all the way to Japan or Australia to breed. This study has broadened our perspective."
SOURCES AND RELATED WEB SITES