Magazine article Oceanus

Sharks Take 'Tunnels' into the Depths: SPIRALING EDDIES OFFER CONDUITS TO FIND FOOD DEEP WITHIN THE CHILLY OCEAN TWILIGHT ZONE

Magazine article Oceanus

Sharks Take 'Tunnels' into the Depths: SPIRALING EDDIES OFFER CONDUITS TO FIND FOOD DEEP WITHIN THE CHILLY OCEAN TWILIGHT ZONE

Article excerpt

As the Gulf Stream current curves away from North America and heads east across the Atlantic, it swirls at its edges. If one of these swirls is large enough, it will pinch off, sending a whirling pocket of water--more than 60 miles in diameter--moving through the ocean like a slow-spinning underwater hurricane. These swirling pockets, called eddies, may also be full of sharks.

No, this isn't the plot of the next Sharknado movie. It's a discovery by researchers from Woods Hole Oceanographic Institution and the University of Washington, who tracked the movements of several tagged great white sharks. They found that white sharks in the open ocean seem to seek out eddies for a surprising reason: The eddies offer a beeline to a banquet of food.

The Gulf Stream can spin off both warm- and cold-water eddies, and surprisingly, the sharks seem to have a preference. The warmer eddies are spawned when the Gulf Stream snakes farther northward, drawing warm water up from the Sargasso Sea. When the eddy spins away from the current, it traps that warm water in its center. But because that water is typically low in nutrients, these eddies aren't thought to contain much life.

"The paradigm is that they're like these ocean deserts," said Camrin Braun, who earned his Ph.D. degree from the MIT-WHOI Joint Program in Oceanography in 2018 and was co-author of a study published in May 2018 in the journal Scientific Reports.

Cold-water eddies are just the opposite. "They trap cold, nutrient-rich water from north of the Gulf Stream," Braun said. "They're anomalously cold and anomalously productive." The extra nutrients can fuel enough phytoplankton growth to make these eddies visible from space.

So it might seem logical that when adult white sharks leave the seal-filled waters of coastal New England and head for the open ocean, they might seek out these whirling blobs of cold water, where phytoplankton at the base of the food chain are blooming. But apparently not. To the scientists' surprise, the sharks spent most of their time in the warmer eddies.

PLAYING TAG WITH SHARKS

The research team followed two mature white sharks. Mary Lee, 16 feet long and 3,460 pounds, was tagged off the coast of Cape Cod, Mass., in September 2012. Lydia, slightly smaller at 14.5 feet and just under one ton, was tagged off Jacksonville, Florida, in March 2013. (Their tracking tags have reached the end of their five-year battery life, but both sharks' full tracks can be seen on the OCEARCH Shark Tracker.)

Lydia gained some notoriety as the first white shark to be tracked crossing the Atlantic, while Mary Lee tended to stay a little closer to the coast. Both demonstrated the unexpected preference for warm-water eddies. But it was Lydia--equipped with a second, specialized tag recording her depth--who hinted at a possible explanation.

Both sharks had SPOT tags, an acronym for Smart Position Or Temperature. When the sharks came to the surface, the SPOT tags reported their position to a network of satellites.

Lydia, however, carried a second tag called a PSAT (Pop-up Satellite Archival Transmitting), which could record the depth and temperature of the water around her every five minutes. After six months, this tag was programmed to release itself, and it bobbed to the surface to broadcast the data it recorded. All that accumulated information allowed the scientists to reconstruct her vertical movements as she swam through the eddies and painted a picture of her behavior. …

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