Satellite Tags Keep Track of Great White Sharks
When Sean Van Sommeran was a boy, out for a day's fishing with a family friend near Ano Nuevo Island, a mile off the coast northwest of Santa Cruz, California, they saw a great white shark devour an elephant seal.
"That left an impression," said Van Sommeran, a former commercial fisherman and now founder and CEO of the Pelagic Shark Research Foundation in Santa Cruz. "It was back in the mid-seventies, and I'd just seen the movie Jaws and read the book."
Now, in the same area, Van Sommeran comes face to face with great white sharks almost every day in season. His mission is to help scientists better understand the behavior of Carcharadon carcharias by tagging the animals and documenting their movements.
During late summer and early fall, when a large breeding colony of northern elephant seals gathers at Ano Nuevo, great whites—at about 21 feet long and 4,800 pounds—come to feed.
Van Sommeran and his six-person team head out in a launch and maneuver close enough to a shark to reach out with a pole, pierce the skin behind its dorsal fin and attach a metal acoustic or satellite transmitter.
Enticing the Great White
Waves around Ano Nuevo can reach 12 feet. "So, you're in a small boat, big swells, big sharks, you know, what could go wrong?" Van Sommeran said.
"One of the main goals is to find out what the population structures are and where the home bases are," said Heidi Dewar, a marine biologist at the Southwest Fisheries Science Center in La Jolla, California.
"It helps us to know what the sharks are encountering in the wild and if they are going to waters controlled by countries where there's no protection afforded them," she said.
Conservation rules offer limited protection to sharks off California and in some other countries, including Australia, Mexico, and South Africa. Elsewhere, sharks are fair game for fishermen and poachers, some of whom take the animals only for their fins, valued in Asia for shark-fin soup.
Van Sommeran's team entices the white sharks with plywood lures designed to look like a seal from below. "They come up slowly and just sort of nudge the lure," he said. At just the right moment a tagger makes his move.
"Between mid-October and mid-November, sharks spend a lot of time near the coast looking for seals but they really only kill very few before they move on," said Peter Klimley, a marine biologist at the University of California–Davis.
Seals have so much fat, Klimley explained, that a shark that eats just half of one can keep going for a month and a half.
Satellite and Acoustic Tags
Scientists know little about where great white sharks migrate when they stray from coastal feeding areas.
Satellite tracking has surprised researchers by showing that some sharks even make round trips from California to Hawaii between winter and spring—up to 40 miles a day, said Andre Boustany, a graduate student at Stanford University in Palo Alto, California.
After six months the satellite tags pop off the shark and float to the surface, where they transmit data.
"Knowing that sharks travel so far brings up new conservation issues," Boustany said. "We may need to look at protecting the animal on an international level."
According to estimates, 100 to 300 great white sharks frequent areas near the California coast.
"We don't really think they're terribly endangered, but it's worth looking at," Klimley said. "There's not been a good population estimate because it involves significant tagging of individuals and tracking over long periods of time."
The science of tagging sharks is still relatively new. Researchers use acoustic tags, which transmit radio signals over a short range, in addition to the newer satellite tags.
Acoustic tags send continuous radio signals, even while underwater, providing a real-time window into the shark's world: how far and how fast the shark is moving, its body temperature and water depth and even how many tail flicks it takes to propel the shark along.
For Van Sommeran, the close encounters are brief but intimidating. "Afterwards you do notice that your hands are trembling," he said. "This is 'Jaws' after all."
Van Sommeran doesn't believe that the great whites, for all their ferociousness, want to prey on him. "They're essentially indifferent to humans," he said. "We're just not fat enough."
Sharks have an image as terrifying, mindless feeding machines.
In fact, the great white, Klimley said, is "a beautiful, awe-inspiring,
gigantic animal, but it also has interesting, complex behavior that
we're only starting to understand."
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For More Information About Sharks:
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Smithsonian References on Sharks
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Shark Sites for Kids on Nationalgeographic.com:
Creature Feature: Great White Sharks
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Print 'N' Go Coloring Book: Great White Sharks
Shark Surfari: Online Quiz
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