Saturday, October 08, 2005
Shark Travels from Africa to Australia and Back
A great white shark, nicknamed Nicole, has made the journey between South Africa and Australia and back in under nine months. The new research published in Science is the first proof of links between populations of different continents.
"This is one of the most significant discoveries about white shark ecology and suggests we might have to rewrite the life history of this powerful fish," said researcher Dr. Ramon Bonfil, shark expert and lead author of the study. "More importantly, Nicole has shown us that separate populations of great white sharks may be more directly connected than previously thought, and that wide-ranging white sharks that are nationally protected in places such as South Africa and Australia are much more vulnerable to human fishing in the open oceans than we previously thought."
The story of Nicole began on November 7, 2003, when Bonfil and his colleagues from the Marine and Coastal Management Department of South Africa and the White Shark Trust attached a satellite tag to Nicole’s dorsal fin as part of a large study on white shark migrations. The tags—specifically known as pop-up archival tags—record data on time, temperature, water depth, and light levels as the shark moves through its habitat. On a pre-recorded date, the tag detaches from the shark and floats to the surface, where it transmits its data sets to a researcher’s computer via satellite.
An additional 24 white sharks were tagged with similar tags and seven more with real-time satellite tags during this study, with Bonfil and his team often having to wrestle the dangerous sharks while on a cradle in order to fix the satellite transmitters to their dorsal fins. While most of the tagged white sharks revealed at least three different movement patterns, including wide-ranging coastal migrations up and down the eastern side of South Africa, Nicole headed out into the vast and deep basin of the Indian Ocean. The track estimated from the data transmitted by the tag revealed that Nicole followed a strikingly direct route towards Australia, on a path void of oceanic islands. Although Nicole took frequent plunges to depths as great as 3,215 feet (980 meters—a record for white sharks) while crossing the Indian Ocean, she spent most of her time (61 percent) swimming along the surface, leading researchers to suspect that perhaps great white sharks use celestial cues for transoceanic navigation.
Ninety-nine days later, Nicole was swimming about a mile from shore just south of the Exmouth Gulf in western Australia, where her tag detached and floated to the surface with all of her secrets.
This leg of the journey alone—some 6,897 miles (11,100 kilometers)—was one for the record books. However, Nicole would resurface again on August 20, 2004, not in Australian waters, but back in Gansbaai, South Africa, where she was tagged just under nine months before. Her distinctively notched dorsal fin was photographed by Michael Scholl, one of Bonfil’s team researchers and compared to previous photographs he had taken over a period of six years. After a detailed comparison of images of dorsal fin notches and markings, there was no longer any doubt: Nicole had returned to her home waters.
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