Once thought of as a strictly cool-water species, basking sharks migrate to tropical waters each winter, according to research published in the June 2009 issue of journal Current Biology.

“While commonly sighted in surface waters during summer and autumn months, the disappearance of basking sharks during winter has been a great source of debate ever since an article in 1954 suggested that they hibernate on the ocean floor during this time,” said Gregory Skomal of Massachusetts Marine Fisheries. “Some 50 years later, we have helped to solve the mystery while completely re-defining the known distribution of this species.”

Using new satellite-based tagging technology, the researchers found that basking sharks make ocean-scale migrations through tropical waters of the Atlantic Ocean during the winter, travelling at depths of 200 to 1,000 meters. Their data show that the sharks sometimes stay at those depths for weeks or even months at a time.

“In doing so, they have completely avoided detection by humans for millennia,” Skomal said

The discovery was the result of marking 25 basking sharks off the coast of Cape Cod with satellite tags and studying their movements. The migratory paths of the sharks were then estimated by coupling tag data with a novel geo-positioning technology technique.

Several factors had made basking sharks a challenge to study. On top of the fact that they disappear for long periods of time, they also feed exclusively on plankton. That means they can’t readily be captured with traditional rod-and-reel methods. And even when the sharks are found closer to the ocean surface, they spend their time in the cool-temperature, plankton-rich waters that limit underwater visibility and make diving difficult.

The basking shark is the second largest shark after the whale shark. It filters plankton through its gills whilst swimming with its huge mouth open.

Journal Reference:
Transequatorial Migrations by Basking Sharks in the Western Atlantic Ocean. Skomal, Gregory B.; Zeeman, Stephen I.; Chisholm, John H.; Summers, Erin L.; Walsh, Harvey J.; McMahon, Kelton W.; Thorrold, Simon R.