© Chris Gotshalk
After half a century of searching, scientists have finally discovered what happens to the world's second largest shark every winter: It has a Caribbean hideout.

Basking sharks, which can grow up to 33 feet long and weigh more than a Hummer H1, spend the late spring, summer and early fall in the temperate regions of the world's oceans. But then they pull their great disappearing act, eluding scientists throughout the winter months.

"It's been a big mystery for the past fifty years," said Greg Skomal, an aquatic biologist at the Massachusetts Division of Marine Fisheries and lead author of the study in Current Biology May 7. "For a while people thought they were hibernating on the sea floor, even though hibernating is not really something sharks do."

Skomal tagged the giant fish off the coast of Cape Cod, Massachusetts and tracked them by satellite, piecing together their mysterious winter wanderings. He discovered the beasts were absconding to the depths of the Caribbean, some voyaging as far as the Brazilian coast, though the attraction of these destinations poses yet another mystery. The findings have implications for conserving the sharks, whose fins are much-desired delicacies in Chinese cuisine.

The basking shark is a benign behemoth. It swims at about three miles per hour with its four-foot-wide mouth gaping open, filtering through almost 500,000 gallons of water every hour for its plankton sustenance.

Like most large fish, they're difficult to keep track of because they rarely come to the surface, where tags need to be to transmit information to satellites. Skomal got around this hurdle by harpooning the fish with special tags that tracked and stored depth, temperature and light level, which then popped off at a pre-programmed date and rose to the surface. Once a tag hits the surface, it transmits the entire archive of the fish's journey via satellite. Skomal used a novel analysis technique that could determine the sharks' locations at every time point, allowing him to retrospectively track them to their secret hiding places.

He found the sharks were traveling well-outside their known range, spending months in the warm waters of the Caribbean and even deep into the southern hemisphere. They also periodically dove to more than 3,000 feet, and often stayed at those depths for months at a time. One shark remained at a depth of nearly 600 feet for upward of five months.

"What they're doing there - therein lies the mystery," said Skomal. "If you're a basking shark you can go to Georgia in the wintertime and be at the right temperature and depth and have plenty of food, so that's optimal. So why travel three to four times that distance?"

He hypothesizes the trip may have to do with reproduction, another area that has long baffled basking shark researchers.

"No one has ever seen a baby basking shark, no one's found a pregnant shark, knows when they reproduce or what their gestation period is," said Skomal.

One possibility, he said, is that the sharks mate in the waters further north, where food and potential mates are plentiful. Then the females may migrate to the deep waters down south, which provide a stable and predator-free environment for the young sharks to grow.

"It's really hypothetical," he acknowledged. "We don't know the genders of the sharks we tagged because we tagged them from the boat. Next time we'll jump into the water so we can pull down their fly."

The extended range of the sharks suggests that the different Atlantic subpopulations - near the east and west coasts of both hemispheres - may actually be the same population.

"They might even be crossing into other oceans," he said, "meaning there might actually be one population in the entire world."

This possibility has implications for conservation biologists. The sharks are currently listed as "vulnerable" on the IUCN Red List of Threatened Species, due to the high value of their fins which are the key ingredient for shark fin soup, a tasteless but symbolic Chinese delicacy. The sharks' livers, which can make up 25 percent of their total body weight, also fetch a high price for their oil.

"This tells us that if we allow sharks off British Columbia to be harvested, we might be impacting the entire population," said Skomal. "We can't just save the fish off of New England, we have to coordinate with all the fisheries. We have to divide up the pie instead of each having our own pie."