Big mouth. A basking shark strains plankton-rich seawater through its gills.

Nick Caloyianis

How Do You Hide a 5-Ton Shark?

Call it the case of the disappearing filter feeders. Every summer, hundreds of basking sharks emerge off the northeast coast of the United States, jaws agape to capture the tiny zooplankton that make up their diet. But by winter, the world's second-largest fish seems to vanish. Now, researchers have used satellites to solve the mystery of the basking sharks' winter home. The findings could help conservationists better protect the 10-meter-long sharks, which may number fewer than 10,000 worldwide.

Tracking the sharks wasn't easy. Gregory Skomal, a marine biologist with the Massachusetts Division of Marine Fisheries in Oak Bluffs, and his colleagues hired airplane pilots to spot groups of basking sharks feeding off the Massachusetts coast. When sharks appeared, the researchers rushed out to sea in boats and maneuvered close enough to the placid creatures to clip satellite tags to the animals' dorsal fins with a modified harpoon. The tags recorded depth, temperature, and light information as the sharks moved around. Then, on a preprogrammed date, the tags popped off and began transmitting the data they'd collected. Of the 25 tags the researchers set, they received data back from 18.

The sharks traveled much farther than anyone expected. Although eight of them hung around the eastern seaboard, 10 ventured far outside known basking shark territory, traveling to offshore Bermuda and Puerto Rico, the Caribbean Sea, and as far south as Brazil--the first time these sharks have been found in the tropics. What's more, the sharks dived as deep as 1000 meters on these journeys, the researchers report online today in Current Biology, which might explain why they've been able to remain undetected until now. "I've been working with sharks for over 26 years, and this is probably one of my most exciting discoveries," says Skomal.

But there's still plenty of mystery surrounding these big fish. No one has ever seen a baby basking shark, and no one knows when or where they mate and give birth, or why some of them head south for the winter. Pinpointing the sharks' winter vacation spot is the first step to answering those questions, says Skomal. He plans to tag more sharks to investigate further.

"This is really good, new information" on a species that scientists know very little about, says Mahmood Shivji, a conservation biologist at the Save Our Seas Shark Research Center in Fort Lauderdale. Basking shark fins can fetch up to $50,000 on the black market as trophies, Shivji says, and the new findings indicate that protecting the animals from illegal fishing will require international cooperation.