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Magnificent White Sharks Return to the Farallones
By Dr. Peter Klimley and Dr. Barbara Block
Anyone who visits the islands year-round appreciates this season, when the often-turbulent waters in Gulf of the Farallones are usually calm, and the smooth greenish-blue sea surface slowly undulates with a swell originating from the Gulf of Alaska.
Every day or two, the white-bodied Western gulls that are scattered in individual territories over the islands, ascend in a loud cacophony to fly away toward Mirounga Bay, Maintop Bay, or another stretch of the shoreline. A few gulls see a large splash and a spreading patch of crimson water, alerting them that the top (or apex) predator of the island, the white shark, has seized a young seal that tried to swim unnoticed through the high risk zone close to shore.
Scientists aboard a small boat spot the gulls circling above the seal and quickly motor their boat towards the site to place a miniature electronic tracking device on the shark. This tag will enable these scientists to study the behavior, physiology, and ecology of the shark, not only when it is resident within the Gulf of the Farallones, but also when it migrates out into the wide expanse of the Pacific Ocean.
Scientists from Stanford University, PRBO Conservation Science, and the University of California, Davis, with support from the Monterey Bay Aquarium, will place two types of electronic tags, ultrasonic and satellite transmitters, into the stomach of a shark by hiding it within a small portion of seal meat; the satellite tag will be darted into the thick dorsum of the shark as it is lured to the side of the boat with a seal-shaped decoy.
The satellite tags provide a wealth of exciting information. This data-storage—or “archival”—tag will stay on a shark up to a year, then release to the surface, where it will transmit behavioral and environmental information stored within its memory to a satellite. The tag’s contents, which consists of daily records of the shark’s geographical coordinates, the depths to which it dives, and the temperatures experienced during these dives, is received by a satellite passing over the tag, and is sent back to earth. Data from this tag will reveal the path of the shark moving away from the Gulf of the Farallones into the Pacific Ocean.
The work will continue an on-going study, conducted by the Stanford University and PRBO scientists under the leadership of Barb Block, focusing on understanding the movements and behaviors of white sharks off the Central Coast. Already, twenty-five adult sharks have been tagged and each tracked for nearly a year. These sharks remain at the Farallon Islands during the fall, but during winter make long journeys out into the Pacific Ocean. During their migration, the sharks dive as deep as 600m. A satellite tag carried by a shark named “Tip Fin” revealed that it traveled 4,000 km to the waters off the western coast of Kahoolawe, a Hawaiian island. Why this shark and other white sharks make long-distance migrations, and what it is they do in the waters of the central and eastern Pacific remains a mystery. More recently, a white shark tagged off South Africa made an even longer migration of 11,000 km, traveling from the southern tip of South Africa to the western coast of Australia, and then returning to South Africa.
This team of researchers will continue to examine whether sharks tagged in the Gulf of the Farallones make similar migrations, and whether they return year after year to feed on seals at the same pinniped colonies. This is a real possibility. Two white sharks, one identified by its notched dorsal fin and the other by its bitten caudal fin, were observed to feed on seals at the same side of the Farallon Islands on three out of four consecutive years, on dates separated by three days. Recordings of satellite tags have demonstrated similar fidelity for Tip Fin, who swam from the Farallon Islands to Hawaii and back over consecutive years.
By tagging a large sample of sharks, it is possible to determine whether individuals of different sexes and sizes have different distributions in the ocean and occupy different thermal niches. This information will help scientists learn about the life history of white sharks, and answer basic questions about changes in behavior through a white shark’s life. For example, do smaller white sharks, which prey primarily upon fish, favor warmer waters? And do they lack the physiological adaptations for temperature retention, which adult sharks possess?
This year scientists from the University of California, Davis and Stanford University will place another type of electronic tag, one transmitting an ultrasonic signal, on white sharks to reveal more about what they are doing near the pinniped colonies on the islands in the Gulf of the Farallones. The ultrasonic transmitter, with its unique signature and sensors that record the shark’s stomach temperature and depth, will be recorded by listening devices—each the size of a quart soda bottle—attached to a line that leads to a buoy floating just two meters above the bottom. The scientists will first place monitors offshore of two beaches, one occupied by a seal colony and the other by a colony of sea lions, at the Point Reyes Headlands, a hammer-shaped peninsula north of San Francisco. Additional monitors will be installed later near Año Nuevo Island and Southeast Farallon Island, close to the beaches occupied by their resident elephant seal populations.
The monitors will detect the constantly pulsing signal of the transmitter when the tagged shark arrives at the seal colony, record how long it remains near the colony, how often it feeds there (based on an elevation of the stomach temperature in the shark when it swallows a warm-bodied seal), how often it moves among different pinniped colonies, and whether it returns to particular “home” site during three successive years—the life of the electronic tag.
A research team, led by Dr. Peter Klimley from UC Davis, tracked five white sharks carrying ultrasonic beacons at Año Nuevo Island for nearly a month, during October and November 1999, with three similar sound-receiving buoys anchored in a large triangle just offshore of the seal colony. The sharks spent over ten hours a day within range of the buoys. Only twice did a white shark appear to feed on a seal. This was a surprising result, suggesting that the white shark may not be a frequent feeder, but may gorge itself after long periods of fasting. The same coded ultrasonic tags can be used to estimate the size of the population of sharks at the three locations within the Gulf of the Farallones.
The researchers will also conduct genetic analyses of the tissue samples from the sharks. The results of these studies will help them determine whether the sharks actually mate with individuals when they cross the ocean to inhabit distant locations. Furthermore, a technique involving the analysis of repeated sequences of the genetic code can actually indicate whether individuals are related to each other. Such an experiment might reveal whether two sharks observed to feed at Southeast Farallon Island on successive years are offspring from the same mother shark.
Despite their high profile in popular culture, basic questions remain about the biology of white sharks. The research being done over the next three years will help to answer some of these questions, providing important insights into these animals’ lives. Such information will be critical as scientists are called upon to help resource managers and policy makers ensure that white sharks continue to thrive in the oceans.
Bonfil, R., M. Meÿer, M.C. Scholl, R. Johnson, S. O’Brien, H. Oosthuizen, S. Swanson, D. Kotze, and M. Paterson. 2005. "Transoceanic migration, spatial dynamics, and population linkages of white sharks." Science, 310: 100-103
Boustany, A.M., S.F. Davis, P. Pyle, S.D. Anderson, B.J. Le Boeuf, B.A. Block. 2002. "Expanded niche for white sharks." Nature, 415: 35-36.
Klimley, A.P. and S.D. Anderson. 1996. Residency patterns of white sharks at the South Farallon Islands, California. Pp. 365-373 in Klimley, A.P. and D.G. Ainley (Eds.), Great White Sharks: The Biology of Carcharodon carcharias. Academic Press, San Diego, 528 pp.
Klimley, A.P., B.J. Le Boeuf, K.M. Cantara, J.E. Richert, S.F. Davis, S. Van Sommeran, and J.T. Kelly. 2001. "The hunting strategy of white sharks at a pinniped colony." Marine Biology. 13: 617-636.
Dr. A. Peter “Pete” Klimley (APK), Adjunct Associate Professor, University of California Davis, has published many scientific articles on sharks and two books, The Secret Life of Sharks, an easy to read personal account of what it is like to be a marine biologist and learn about sharks, and Great White Sharks: The Biology of the Carcharodon carcharias, a compendium on the evolution, physiology, behavior, and ecology of the species. He and his graduate students have used coded ultrasonic tags and listening stations to study the behavior and ecology of half a dozen species, including rockfish, tunas, and white sharks.
Dr. Barbara “Barb” A. Block (BAB), Prothro Professor at Stanford University, is a leader of the Tagging of Pacific Pelagics (TOPP) program of research, centered at Stanford University. Her graduate students and she have been using satellite tags to track the ocean-wide movements of the white shark and the related, salmon shark, and the results of their studies have recently appeared in the scientific journals of Nature and Science.
Click here to see footage of Tipfin's tag:
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