Whether you’re a nervous beachgoer, a fishing boat trying to avoid illegally catching endangered marine life, or a biologist hoping to conserve this vulnerable species, it’s good to know where the great white sharks are swimming.
University of Washington scientists and their colleagues at the Woods Hole Oceanographic Institution have found that adult female sharks in the Gulf Stream and Sargasso Sea in the North Atlantic prefer warm-water eddies—the clockwise-spinning whirlpools in the ocean—and tend to spend extensive time deep inside these slowly spinning features.
While the great white, which can be 20 feet long and weigh between 4,000 and 7,000 pounds, is perhaps the most famous of the sharks, little is known about its behavior. The World Wildlife Federation lists the creature as vulnerable to extinction and notes that its population is in a long decline due to poaching and illegal sports fishing, death by nets set up to protect beaches and bycatch (the term for marine life caught unintentionally.)
“We’ve decimated some open-ocean shark populations to a fraction of what they were 100 years ago. And yet we don’t know the basics of their biology,” says Peter Gaube, a senior oceanographer at the UW’s Applied Physics Laboratory and lead author of a recent study published in Scientific Reports, an online journal of Nature magazine. “If we know where those sharks or turtles or whales might be in the open ocean, then the fisheries can avoid them and limit their bycatch.”
Gaube investigates how ocean eddies, or whirlpools, influence the behavior of marine animals. His previous study, on loggerhead sea turtles, similarly found that they prefer the anticyclonic eddies, which spin clockwise north of the equator. These eddies trap large amounts of water at the ocean’s surface and are most often warm, clear and low in nutrients.
The new study, which among its authors includes UW research scientist Alice Della Penna and Camrin Braun, a postdoctoral researcher, analyzes the movements of two female great white sharks tagged by a nonprofit in September 2012 off Cape Cod and in March 2013 off Jacksonville, Florida. One shark just had a position tag, while the other had a second tag that also recorded water temperature and depth. The sharks, known to the researchers as Mary Lee and Lydia, were tracked for nearly six years as they swam north with the Gulf Stream and then out into the open ocean.
The satellite tags, built by Wildlife Computers of Redmond, transmit the animal’s location as soon as it surfaces. Early shark-tagging projects could offer only rough ideas of where sharks were swimming, Gaube says. But since precise satellite position networks were made available to the public, and with improvements in computing and batteries, the tags can now collect detailed information as sharks travel throughout the marine environment. Researchers, funded by the National Science Foundation, NASA and Woods Hole, took the data from the two sharks and compared their position in the ocean with sea-surface height data from satellites showing where the huge, swirling warm- and cold-water eddies were located at that time.
“These eddies are everywhere; they cover 30 percent of the ocean’s surface,” Gaube says. “It’s like what you see if you’re walking along a river and these eddies form behind rocks, but it happens on a different scale in the ocean: Instead of being a little thing that disappears after a few seconds, they can be the size of the state of Massachusetts, and can persist for months to years. You could be in the middle of an eddy in a ship and you’d probably never know it. The water may be a little warmer, and it could be a little clearer, but otherwise you wouldn’t know.”
The two tracked sharks spent significantly more time in warm-water eddies than the cold-water eddies that spin the other way. They lounged the longest at about 450 meters (about a quarter of a mile) deep inside the warm-water eddies, especially during daytime, likely feeding on the abundant fish and squid found at these depths. The sharks were more likely to come to the surface at night.
This preference goes against common wisdom, because it’s the cold-water eddies that generally bring nutrient-rich water up from the depths of the ocean, and satellite images show that these eddies are rich in marine plant life. The study is the first to show that sharks gravitate toward eddies, and that they prefer the warmer variety.
“White sharks are effectively warm-blooded,” Gaube says. “They have to keep their body temperature elevated. We believe that these warm eddies allow white sharks to forage longer at depth, where most of the biomass in the open ocean is found. One reason that the sharks might prefer them is that by diving in these warm eddies, they can spend more time in the deeper water.”
Second, recent studies suggest that the “twilight zone,” below the depths that satellites can see, contains many more fish than previously believed—and much more than at the surface. Those patterns might be different than the ones we can easily detect from space. “Could these ‘ocean deserts’ actually be super-productive at depth? That’s what we think might be happening,” Gaube says.
Data collected through the sharks could help to protect this twilight zone as it’s just beginning to be targeted by major fisheries, Gaube says. And information about where great white sharks like to hang out could help conserve this vulnerable species. “Maybe if we understand the biology of these animals, how they use these features, we could say, ‘OK, do not fish anticyclonic eddies during this time of year, because you’re more likely to catch white sharks,’” Gaube says. “Instead of cordoning off a particular area, we could say there’s this feature, it moves every day, let’s make a ‘mobile marine protected area’ and not touch it because we know it’s a hot spot for great white sharks.”