Great white sharks coming closer to shore than thought, researchers find
Wednesday, November 4, 2009
For years, humans have thought of great white sharks wandering the sea at random, only occasionally venturing close to shore.
We were wrong.
Pacific white sharks spend months near the northern and central California coast between August and February foraging among elephant seals, sea lions and other prey, according to a new study published online Tuesday in the Proceedings of the Royal Society. The team of 10 California-based researchers determined that these sharks probably pass close to populated beaches and have been spotted as far inland as the mouth of the San Francisco Bay, east of the Golden Gate Bridge.
"It shows you how wild it is off our West Coast of North America. This is Yellowstone," said Stanford University marine sciences professor Barbara A. Block, who co-wrote the paper.
By tracking their movements, scientists determined that the fearsome predators make such precise, regular migrations each year between the California coast and the Hawaiian islands that they have become genetically distinct from their counterparts on the other side of the Pacific.
The fact that "a major concentration" of great whites can ignore the humans who might have crossed their path there "shows us the sharks are really minding their own business. The number of interactions with people is very small, considering," said Stanford University post-doctoral scholar Salvador J. Jorgensen, the paper's lead writer.
The findings represent nearly a decade of work, during which scientists tagged 179 great white sharks that roam the Pacific. They lured the creatures to their boat with a carpet decoy designed to look like a seal, and used a lance to attach the tags with the aid of 2.3-inch titanium darts.
They used three technologies to track the sharks' movements: satellite tags, which archive travel data by measuring the light in the sea and using astronomical math to determine where they are swimming; acoustic tags, which register a precise location when a shark comes within about 820 feet of a receiver; and mitrochondrial DNA sampling, which maps the animals' genetic lineage through their maternal line.
While researchers set up acoustic receivers in four central California locations where they knew the sharks would congregate -- Año Nuevo Island, South Farallon Island, Point Reyes and Tomales Point -- they discovered by accident that several white sharks entered the mouth of the San Francisco Bay. That is because the five great whites set off receivers established by another team, which had put them there to track migrating salmon.
Just as important, the scientists were able to determine through satellite tagging that great whites left the California coastline each winter and traveled 1,240 to 3,100 miles to the Hawaiian islands. Scientists have called a certain location along the route the white shark cafe, because they suspect that mating or foraging may take place there.
Tagging records from May and June show that male white sharks "converge in a very specific area of the cafe," Jorgensen said, while female sharks move in and out of the area. "It adds a little more evidence to the argument that this could be an important reproductive area."
Jorgensen said the great whites swimming off California probably descended from migrants that came from areas near Australia and New Zealand during the late Pleistocene Epoch, as many as 150,000 to 200,000 years ago.
The new findings have several significant conservation implications for great whites, which rank as one of the world's most protected shark species. Researchers are conducting a census of the creatures off California's coast because their exact numbers are unknown, and they may be able to identify areas of their migration routes that need additional protection.
Even Block, however, said she looks at places such as Carmel Point with a new perspective now that she knows the extent to which white sharks frequent the area. The scientists hope to put additional receivers near popular beaches to monitor shark movements.
"When I go to the beach there, I look at it differently," she said. "These animals are coming in so close to shore because that's where the pinnipeds [seals and sea lions] are."