Saturday, November 14, 2009

Researchers share their findings about the migratory patterns of Great White sharks

Stanford scientists are learning a great deal about great white sharks.

Scientists at the Hopkins Marine Station have concluded that great white sharks, like salmon, have a specific migratory pattern. Their research, published in the scientific journal Proceedings of the Royal Society B on Nov. 4, also found that Pacific white sharks, genetically distinct from other great whites, are swimming in the San Francisco Bay.

Prof. Barbara Block, who facilitated and co-authored the study in collaboration with scientists from UC-Davis, among others, compared the decade-long observation of great whites in the Pacific Ocean to studies of lions and tigers in Africa.

“We’re doing [the study] on the largest ocean in the planet – it was a tall task,” Block said. She and other marine scientists tagged a total of 179 great white sharks using Pop Up Satellite Archival Tags (PATs) and monitored their swimming in the Pacific.

The PATs, according to doctoral student in biology, field project leader and co-author Chris Perle, allowed for the sharks to be tracked via satellite. Perle, who helped attach approximately 20 tags to the sharks, explained in an e-mail to The Daily that each tag is pre-programmed to “pop-off” the shark and send data back for analysis. The sharks’ travel is then measured by the light and sea surface temperature data.

Researchers found that great white sharks travel from the central California coast out into the open ocean to Hawai’i and a nearby popular location dubbed the “White Shark Café.”

“What we learned the most about the white sharks was that they were making round trips – 4,000 nautical mile trips,” Block said. “But they were coming back with precision to the place we let them go.”

In an e-mail to The Daily, Bing Director in Human Biology Carol Boggs said the research offered a clearer conception of the relationship of sharks to their environment.

“It is cool that we’re discovering that large marine organisms behave more like what we’re used to thinking of as standard for large terrestrial animals, with defined geography for the population, and even something resembling individual ‘home ranges,’” Boggs said.

“This blows the idea out of the water that the ocean is a vast, trackless melting pot,” she added.

Great white sharks leave from “hot spots” on the shore around where sea lions and elephant seals gather to feed. Using additional acoustic tags acting as a microchip that can be transmitted from 300 meters away, the sharks’ shore movements were tracked by receivers sunk at the bottom of the ocean.

“We call it ‘homing infidelity,’” Block added. “Just like a salmon going up the stream, we didn’t know white sharks had a home spot.”

According to postdoctoral fellow and co-author Salvador Jorgensen, scientists were already familiar with the fact that white sharks migrate. The main discovery is the consistency in these migratory routes, which Jorgensen called “a virtual highway,” and that the sharks did not venture on to other areas of the Pacific.

“This was further confirmed with genetics data based on maternally inherited mtDNA markers, suggesting that we are looking at a distinct population that is demographically isolated from other known populations of white sharks in the Indo-Pacific (i.e. South Africa and Australia/New Zealand),” Jorgensen wrote in an e-mail to The Daily.

Geneticist Carol Reeb, a research associate in biology and co-author, also determined that Pacific white sharks are genetically distinct within their species. She found the difference by comparing a previous study of great white shark genetic sequences from South Africa and New Zealand with the data Jorgensen collected.

The population of white sharks off of the California coast has a 200,000 year-old ancestry with Australian sharks, migrating during the Pleistocene epoch period at the same time humans migrated out of Africa.

“[The Pacific white sharks] didn’t really go anywhere else which was interesting because these are highly migratory animals and they have the potential to go anywhere in theory,” Reeb said.

Male sharks were previously considered to be free-roaming. Conversely, female sharks, according to Reeb, are known to be philopatric, meaning they return to their original birthplaces. This study refutes the original behavioral predictions and confirms a pattern among both sexes.

Reeb is currently working on a similar study with Mexican sharks to determine if they display distinct behaviors and potentially different genes as well. Boggs, who works in population dynamics, finds the potential for further species conservation in this genetic discovery.

“The result that the ‘local’ population is genetically distinct, with a very old separation date from other populations of the same species, is both interesting and surprising,” Boggs said. “It indicates that these sharks may be adapted to the environment of the northeastern Pacific, which has implications for conservation efforts.”

The tracking devices made an additional observation that many found to be more surprising. Great white sharks were detected swimming as far in the San Francisco Bay as the Golden Gate Bridge. However, many of the scientists working on the study were not at all astonished with this finding.

“We learned this only because a row of sensors set up by our colleagues to detect salmon migrations was compatible with our tags,” Jorgensen wrote. “So it was unanticipated but not entirely surprising.”

Five out of 75 great white sharks were detected up to one mile inside the Bay, chalking up to 0.07 percent of the total tag detections in the study.


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