Oct. 7, 2005 — Whale sharks, the world's largest fish, dive nearly a mile in search of food, according to high-tech electronic tags that have recorded every aspect of the fish's life.

Rachel Graham, of the U.S.-based Wildlife Conservation Society (WCS) and colleagues from the University of York, U.K., attached electronic tags to nine whale sharks swimming at the Gladden Spit reef off Belize, the world's second-largest barrier reef system.

Thousands of snappers gather there under full moons in the spring to spawn, turning the sea a milky color and drawing normally solitary whale sharks, which feast on snapper fish "caviar."

Part of the folklore of the reef's fishermen, the spectacle has proved to be a unique possibility for the researchers to attach satellite-controlled tagging devices to the fish's thick skin.

The tags made regular recordings of temperature, water pressure and light level for 206 days. After this preprogrammed period, they automatically detached from the fish, floating to the surface and sending their data back via satellite.

Downloaded to a computer, the recorded information showed that whale sharks made dives beyond 979.5 meters (3,213 feet) — the tag's maximum depth recording — to temperatures below 7.6° Celsius (46° Fahrenheit).

"A whale shark may be able to withstand low temperatures due to its subcutaneous fat layer," the researchers wrote in the current issue of the Royal Society's journal Interface.

While at night the sharks remained in shallow waters, they made deep dives during the day. These often ended with fast ascents, to re-oxygenate the gills after time spent in less-oxygenated depths.

Growing up to 65 feet (20 meters) long and weighing up to 15 tons, the nonaggressive whale shark (Rhincodon typus) lives in tropical seas and feeds mainly on plankton.

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