Why Might Sharks Gravitate To The Hurricanes Of The Ocean?

Preliminary research suggests that eddies may hold tasty surprises.

Emily DeMarco, Staff Writer

(Inside Science) -- Around Cape Hatteras, North Carolina, the Gulf Stream veers east, flowing away from the coastline of the United States and out into the Atlantic Ocean. Away from the stabilizing influence of the continent, the powerful current revels in its new freedom, spawning large circulating masses of water called eddies that scientists describe as the hurricanes of the ocean.

Now, in preliminary research, scientists are trying to figure out how great white sharks may use the spinning features.

Eddies are ubiquitous in the surface waters of the world's oceans. Many form when parcels of moving water pinch off from the main body of fast-moving currents like the Gulf Stream, which carries warm water from the subtropics and tropics to regions of the North Atlantic Ocean. On average, the swirls can stretch nearly 125 miles across and typically last weeks or months.

In the new research, Peter Gaube, a biophysical oceanographer at the Applied Physics Laboratory at the University of Washington in Seattle, and his colleagues investigated how great white sharks may interact with the swirling pools of water. They partnered with OCEARCH, a nonprofit that tracks the movements of a number of sharks using satellite tags. Every time a tagged shark surfaces, its tracking device "pings" a satellite, creating a record of the shark's movements over time.

The scientists paired the travel histories of two female great white sharks -- named Mary Lee and Lydia -- with the locations of eddies in the North Atlantic. One of the animals was also tagged with a device that recorded water temperature and depth every 15 seconds. They found for the first time that the sharks appear to prefer the cores of anticyclonic eddies -- those that have centers of water warmer than the surrounding ocean -- to cyclonic, or cold-core, eddies while swimming in this region of the ocean. In particular, the depth data revealed that the tagged female appeared to spend more time and dive deeper in the warm-core eddies found near the Gulf Stream.

Credit: NASA's Goddard Space Flight Center

The researchers suggest that the sharks' need to regulate their body temperatures could explain the longer, deeper dives observed in warm-core eddies. And it’s possible that the animals are targeting those areas to feed, said Gaube, who presented the research at the 2016 Ocean Sciences Meeting in New Orleans last month.

That would be a surprising finding, said Scarla Weeks, a biophysical oceanographer at the University of Queensland in Brisbane, Australia, because "in warm-core eddies, there generally isn't forage food."

Instead, the animals that she's studied -- including manta rays and seabirds -- all target the boundaries of cold-core eddies because that's where the food hangs out, said Weeks, who wasn't involved in the research.

In general, the centers of cold-core eddies have more nutrient-rich water than the warm-core ones. Those nutrients, plus sunlight, fuel the growth of tiny organisms like phytoplankton, which tend to accumulate at the periphery of the cold-core eddies. Animals higher up the food chain follow.

The marine delicacies that may lure sharks to the heart of a North Atlantic Charybdis remain, for the moment, a mystery. But Gaube has a guess.

"Atlantic pomfret. They'd be the perfect shark snack.” Anecdotal evidence from a colleague suggests the silvery fish, which can grow up to a few feet long, appear to congregate in anticyclones at depths similar to those the shark dove to, he said.

Although the researchers can’t draw firm conclusions from data comprising just two sharks, "it’s a good start . . . and quite exciting science," said Weeks. Teasing out how animals use oceanic eddies -- for foraging or for migration, for example -- is important because it can "help us understand and, therefore, potentially conserve some of our species,” she said. "And it can also help us understand our oceans better and what impact climate change is having."

Scientists are still figuring out how eddies evolve in time and space and how they transport heat, salt, dissolved carbon dioxide and other materials throughout the ocean, said Bo Qiu, a physical oceanographer at the University of Hawaii at Manoa in Honolulu, who wasn't involved in the research. That movement of heat and other materials in the ocean plays an important role in regulating Earth's climate, he said. Knowing how eddies work is crucial to understanding how climate change may be affecting them and what the effects of such changes might be.

For now, the next step for the researchers is uncovering what might lurk in the warm-core eddies. Figuring out that missing piece could help explain why the sharks seem to use the features differently than other species.

To do just that, Gaube is planning to return to the region in May.

"There's something good here. I can feel it," he said. "This is going to be really fun."