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| Great white sharks move predictably, mate mysteriously |
Tim Henry
2009-11-19 |
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The most exhaustive study of great white sharks in North America found that the animals swim over 4,000 miles from Point Reyes in the summer to Hawaiian waters in the winter. Their precise migrations can bring them within hundreds of meters of their starting points. This predictable pattern, along with slow reproduction, make them vulnerable to over-fishing.
“For decades we’ve assumed the ocean is a big open space with few barriers to migration and gene flow. Instead, we found they stick to the usual hotspots, swimming along the same regular routes, and never straying too far off course,” said Carol Reeb of Stanford, the geneticist for the project.
The ten-year study, “Philopatry and Migration of Pacific White Sharks,” was a collaboration of the Point Reyes Bird Observatory, UC Davis, Stanford, the Pelagic Shark Research Foundation and Monterey Bay Aquarium. Researchers tagged and tracked 179 northeast Pacific (NEP) white sharks and took DNA biopsies of tissue samples. The data revealed that NEP sharks are a small, isolated group that descended from white sharks in the southern hemisphere.
“We now know they have a relatively limited home range, which has caused them to become isolated and genetically divergent from other populations. Because of this isolation, if white sharks in California go extinct, it could be a long time before they repopulate our shores again,” Reeb said.
Northeast Pacific sharks are found in the nutrient-rich waters around Bodega Bay, Point Reyes, the Farallon Islands and Ano Nuevo Island from August through February. “Rocky headlands or islands have seals and sea lions, and dead whale carcasses. Point Reyes is perfect for them,” said co-author Scot Anderson, who has been studying white sharks in the area for 20 years.
Each winter, white sharks leave coastal areas and travel 1,500 to 4,000 miles offshore to sub-tropical and tropical pelagic habitats. They feed on tuna and billfish in the Hawaiian archipelago, but what they do in between, and where they mate, remains a mystery.
“Where white sharks are mating has never been observed. We’re still trying to figure that out,” said Salvador Jorgensen, principle author of the paper.
In May, male white sharks converge in a shark “café” roughly halfway between California and Hawaii. Female sharks move in and out during the month, possibly taking a break from the taxing activity of mating.
It is believed that male white sharks bite females during copulation. Bite marks have been observed on females in California waters, but these marks have appeared old and healing.
“Why do white sharks go offshore and what do they do there? It could be elaborate courtship, but no one could say they are going out there to breed,” Anderson said.
Anderson tagged 11 sharks off the Farallones this year. A fake seal made out of neoprene wetsuit material lures sharks to his boat. When a shark swims close, a titanium dart is shot into the shark’s skin, which simultaneously attaches a tag and pulls a tissue sample. Photographs of dorsal fins serve as finger prints.
Some tags transmit data directly to satellites, and some tags transmit to a receiver, providing a reliable measure of regional activities. Sharks usually shed tags within 200 to 365 days.
“Acoustic tags can be heard with 400 to 500 meters of the receivers. They give us local movements of high precision,” said co-author Barbara Block, a professor of marine sciences at Stanford.
Anderson said that tags are valuable to shark research, but that the process is slow, and does not provide a long-term record.
Despite their protection under the Convention on International Trade of Endangered Species, white sharks are illegally traded, often for their fins. A lack of knowledge about the size and movements of shark populations has hindered conservation efforts, but scientists hope that the new data will help illustrate the fragility of the animals.
“Our role is to come up with the information and pass that on to the National Fishery Service—it’s up to them and the general public to come up with the policy,” said Taylor Chapple, a doctoral student at UC Davis and co-author of the paper.
The tagging and tracking efforts of the sharks are part of a broader base of data gathering, including the Tagging of Oceanic Pacific Predators, which is part of the larger Census of Marine Life project. Check topp.org and coml.org. The paper can be found at rspb.royalsocietypublishing.org.
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