Emily Sohn, Discovery News
May 7, 2009 -- For centuries, scientists have wondered where basking sharks go in the wintertime. Now, they have an answer -- and it's full of surprises.
In the western Atlantic, the world's second largest fish swims all the way from New England to the Bahamas and across the equator to South America, a new study finds. Scientists have long thought that basking sharks spent all of their time in cooler waters.
"This is equivalent to finding polar bears in Kansas," said lead researcher Greg Skomal, a marine biologist with Massachusetts Marine Fisheries in Martha's Vineyard. "This was a mind-blowing discovery for us."
Basking sharks have long been shrouded in mystery. These fish, which can measure 35 feet or longer, are known to live in temperate waters around the world. Yet, no one has ever examined a newborn basking shark. No one has seen a pregnant female. No one knows where the animals give birth.
And until recently, no one knew where they spent much of their time, particularly in the winter.
To learn more, Skomal and colleagues attached a new type of satellite tracking tag to 25 basking sharks off the coast of Cape Cod, Mass. Once each tag was attached to a shark, it collected data on depth, temperature and light levels for a preprogrammed amount of time -- from 12 days to more than a year. When its time was up, the device popped off the animal and sent its data back to the researchers.
Tags came off in a wide range of locations, the scientists reported today in the journal Current Biology, from New England to the coast of Brazil in the Southern Hemisphere. Other locations included the Sargasso Sea, the Puerto Rico Trench, and the Guyana coast.
Five sharks traveled more than 2,400 kilometers (nearly 1,500 miles). One spent a month hanging out near the mouth of the Amazon River.
A closer look at data from six sharks showed that the animals traveled at depths of between 200 meters (650 feet) and 1,000 meters (3,300 feet). Some stayed at those levels for up to five months. The animals appeared to seek out water in an ideal temperature range.
In the eastern Atlantic, recent tagging studies showed that basking sharks that live near the United Kingdom spend their winters hibernating in deep waters offshore. But basking sharks in the western Atlantic appear to do something completely different, according to the new work. Skomal was so surprised by the results, he reanalyzed the data.
"We've opened up a whole new world of implications into the life history and ecology of these fish and how they live," Skomal said. "To be able to do that in the 21st century for an animal that has been studied for eons is what fascinates me."
It's still not clear why some basking sharks travel such long distances. After all, it takes a lot of energy to make the trip, and there is plenty of food closer to home. Skomal suspects that tropical waters aid with reproduction, by offering a safer nursery habitat, a better food supply, or warmer conditions for pregnant sharks and their newborns.
"This is hypothetical," Skomal said, "To the level where I'm just putting my neck out there."
The new findings may help people better protect basking sharks, said Robert Kenney, a biological oceanographer at the University of Rhode Island in Narragansett. The species is currently listed as threatened by the International Union for Conservation of Nature.
"If all of a sudden, you are finding that a population is spending part of its life somewhere that you didn't consider," Kenney said, "Then you have to expand what you think about when you are trying to manage them."
The study also helps fill in what has long been a black hole in our understanding of what makes basking sharks tick.
"The more we learn about big fish," Kenney said, "The more we find out they do things we didn't expect them to do."
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