Nov. 7, 2009— -- Great whites aren't all alike. Even though the sharks travel all over the Pacific Ocean to hunt, they tend to mate with others from the same area, forming genetically distinct groups.
That's what local great whites revealed to Barbara Block of Stanford University in California and her colleagues.
The team headed out into the Pacific to find the sharks, which they lured to the surface using a silhouette of a seal. They then used a pole to attach two different tags to the sharks and took a sneaky biopsy at the same time.
GPS tags were used to track the long-distance movements of the creatures, allowing the team to follow their migration during the colder months from coastal areas to the deep ocean.
The other tags gave off sonic "pings" that were picked up by sensors moored in coastal areas, providing more precise location fixes than the satellite measurements, so that the team could tell if the sharks returned to the same areas.
Block noted that the sharks tended to follow a predictable, if mysterious route every year.
"They go to an area we call 'the white shark café'," she says. Why this is such a cool place to hang out isn't clear, however.
The tags revealed that the females weave in and out of the males, and because there doesn't seem to be much in the way of food at the café 2000 kilometres off the California coast, Block thinks they may be mating.
The tags also confirmed the team's theory that the sharks return to specific coastal sites in the summer. "These animals aren't just wandering aimlessly through the sea," says Block. "They seem to maintain neighbourhoods."
The tissue samples taken from the sharks showed that their mitochondrial DNA, which is inherited from the mother, was significantly different from that of great whites from the south-west Pacific.
The group reckon that great white sharks originated around Australia and New Zealand and formed a separate group on the other side of the Pacific around 200,000 years ago, during the late Pleistocene epoch.
Knowing Migratory Routes Could Aid Conservation Efforts
"Californian sharks share a female ancestry with the Australian sharks from which they are derived," says geneticist Carol Reeb, co-author of the study. "The fact that males and females show similar, constrained migration patterns suggests that males are not breeding with other populations."
"This study adds important new layers of information to the story of sharks," says Kevin Weng, manager of the Pelagic Fisheries Research Program at the University of Hawaii at Manoa. "The genetic analysis confirms what previous satellite-tracking data suggested – that north-eastern Pacific sharks are separate from the south-western Pacific sharks near Australia and New Zealand."
The team hopes that knowing the migratory route of the great whites will aid conservation efforts, as the sharks are still threatened with extinction.