The Great White Homing Device

Reader Nicole Thayer of Miami asks: How do scientists track great whites?

For a long time, technology limited marine scientists to studying sharks within confined areas, but the recent development of pop-up archival transmission tags has enabled them to monitor great whites over huge distances.

A typical tag, which scientists attach to a shark's dorsal fin, records temperature and pressure, but the real genius behind the technology is its ambient-light sensor. Radio transmitters or GPS devices can't track a shark's position as it cruises hundreds of meters below the surface. So scientist A. Peter Klimley of the University of California at Davis hit on the idea of using a light sensor that records the time at which the sun reaches its zenith every day. Based on the time of local noon and the length of each day, a software program derives the approximate daily longitude of the sensor. Though accurate to only within 100 or so miles (latitude is even less precise), this still provides important details. "Does the shark cross the Atlantic?" Klimley asks. "Does it go over to England to catch a football game?"

This September, Gregory Skomal, a shark specialist at the Massachusetts Division of Marine Fisheries, tagged a 14-foot-long female great white shark trapped in a tidal bay on Naushon Island, off Cape Cod. Unfortunately, Skomal and his group won't get a chance to find out where she went.
The tag they used has a built-in timer that was supposed to run down on April 1 of this year, triggering the device to release from the shark's fin and float to the surface, where it would have transmitted data by satellite. While the shark was in the shallows, the tag misinterpreted the lack of vertical movement to mean that she had died, and it released itself prematurely. Others have fared better: Klimley tracked a shark that dove below 3,000 feet several times during a five-day journey--a finding that shows that despite their technological limitations, the tags are already giving scientists an unprecedented look into the secret lives of sharks.