How do you lose track of the world's second largest fish?
For decades, that's what scientists have been doing each winter, when basking sharks mysteriously disappear from the cool waters of the Atlantic and Pacific Oceans.
Now the baffled experts have at least part of the answer: Giant basking sharks from New England take tropical vacations.
Previously thought to inhabit only temperate waters, a new study shows that the sharks, which grow up to 32 feet (10 meters) long, make vast migrations to deep, warm-water hideouts.
Before the annual winter disappearance, scientists tagged 25 basking sharks off New England with floating, timed-release satellite transmitters.
Swimming at depths of between 600 and at least 3,000 feet (200 and 1,000 meters), some of the fish moved to Florida. But others kept on going south—thousands of miles, in some cases.
"When a tag popped up in the Caribbean Sea, I was really blown away," said study co-author Gregory Skomal, a marine biologist from the Massachusetts Division of Marine Fisheries.
Even more surprising: One shark crossed the Equator to the mouth of the Amazon River off Brazil, where the fish stayed for a month, according to the study, published online today by the journal Current Biology.
(Related: "World's Largest Shark Species at Risk, Expert Says.")
Why So Far?
Exactly what is driving these giant sharks to migrate remains a mystery. The cold-blooded sharks probably leave the Gulf of Maine seeking warmer waters and more abundant plankton, their main food, Skomal explained.
One theory is that they're heading to undiscovered nursery grounds.
Scientists have never seen a young basking shark. "We still have no idea where they give birth," Skomal said.
Mauvis Gore, a biologist from Marine Conservation International, was involved in a 2007 study that tracked a basking shark crossing the Atlantic, east to west.
"Tracing basking sharks on these journeys begins to tell us much more about the population structure," Gore said.
For example, study co-author Skomal said, based on tracking data, "What were thought to be regional stocks may in fact belong to a single, oceanwide population."
Key to Saving Sharks?
The International Union for Conservation of Nature lists basking sharks as being vulnerable to extinction. And "any kind of impacts on basking sharks in one region may affect the entire population," Skomal said.
The new discovery could help address threats facing the sharks, Skomal said—perhaps sooner rather than later.
In August the Save Our Seas Foundation is holding a workshop to bring together basking shark researchers from around the world. "We hope to develop a program of how best to move forward in working out just what is happening with populations of basking sharks worldwide," Marine Conservation International's Gore said.
Giant Shark Mystery Solved: Unexpected Hideout Found