Satellites Clear Up White Shark Mysteries
for National Geographic News
|January 3, 2002|
The first thing a wildlife biologist will tell you about white sharks is
that their near-legendary status as man-eaters is undeserved. The second
thing they'll tell you is that very little is known about white
How long they live, where they go to breed, how many
there are, how often they reproduce, how deep they divethe
questions are endless.
Now, thanks to satellite technology, scientists are beginning to obtain answerssome of them surprising.
The findings show that white sharks travel farther and dive deeper than previously thought. They also spend nearly half of their time in deep ocean waters, which challenges earlier assumptions that white sharks spend most, if not all, of their time close to the coastline along the continental shelf.
The results came from a study by a team of California-based researchers who tagged six white sharks and tracked their movements to the depths of the ocean. The study, published in the January 3 issue of the journal Nature, offers the most extensive record ever compiled on the ecological niche of white sharks.
Information such as this is crucial to develop effective conservation measures. The World Conservation Union (IUCN) considers white sharks (Carcharodon carcharias) threatened globally.
Filling in the Blanks
Until now, scientists believed that white sharks lived in temperate, near-shore waters, where they occasionally attack swimmers.
"They might make a mistake and bite a person thinking it's a seal, but they don't usually come back to eat a person," said Peter Pyle, a marine biologist at the Point Reyes Bird Observatory in California and a co-author of the study in Nature. "The few fatalities [that occur] are usually the result of a particularly hard first hit," he noted.
To determine the sharks' range, the researchers applied data-recording tags to six sharks off the coast of San Francisco, then followed their movements.
"We knew the sharks left the area every year, and that the males returned each fall," said Pyle. "But we didn't have any idea where they went. We thought they might be going down to the waters off the coast of Baja, Mexico. To find they traveled west and so far was a surprise."
Earlier efforts to track white sharks had failed, in part because the available technology was less advanced.
"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."
|© 1996-2008 National Geographic Society. All rights reserved.|