SAN FRANCISCO — Far fewer great white sharks live off California's coast than scientists had expected, according to what researchers call the first census of its kind of the fearsome predator.
Biologists believe only 219 full-grown or near-grown adults inhabit the coastal waters between Bodega Bay and Monterey each fall. Based on populations of similar top-level predators such as killer whales and polar bears, which number in the thousands, scientists thought they would find more great whites.
Still, the shark-counters say it's too soon to tell what the low number means.
"This estimate only represents a single point in time," said Taylor Chapple, who led the study as a doctoral student at the University of California, Davis. "Further research will tell us if this number represents a healthy, viable population, or one critically in danger of collapse, or something in between."
Great whites typically linger along California's north-central coast from late summer through the end of fall to feed on a rich stock of seals, sea lions and other marine life. Come January, the sharks begin a massive migration that takes them as far as Hawaii, as demonstrated by satellite tracking of the animals in recent years.
To conduct the shark census, researchers ventured by small boats into waters known to be frequented by sharks starting in 2006. Using a seal-shaped decoy, they lured great whites close enough to take photos of the jagged edge of the sharks' dorsal fins, which serve as a kind of fingerprint unique to each individual shark.
From these photos taken over the course of three years, they counted 131 individual sharks and used a statistical model to extrapolate to 219 sharks total.
Chapple said that he had researched other shark species before tackling great whites but had the same fears as anyone else when going out in a small boat to see them for the first time.
His first encounter changed his perceptions completely, he said. Rather than the boat-biting feeding machines portrayed in the "Jaws" movies, these sharks were mostly cautious around researchers' decoys.
"They were more beautiful to see in the water than they were scary," Chapple said.
John McCosker, a marine biologist with the California Academy of Sciences who has researched great whites for more than 30 years, praised the rigor of the study, in which he was not a participant.
McCosker said he thought the number of sharks could be slightly higher than the study determined because of how infrequently great whites come to the surface, but not by much.
As top-level predators, McCosker said the sharks play a crucial role in keeping the rest of the marine ecosystem in balance.
"We need them very badly," he said. "If there are as few as they suggest, that means we should be very careful in maintaining the population level."
The results of the study are being published this week in the journal Biology Letters.
Video of great whites being tagged for research: http://www.youtube.com/user/UCDavis