A new study published by Scientific Reports found that great white sharks in the Atlantic often dive to great depths to hang out in deep-water whirlpools along the ocean floor.
The study, conducted by the University of Washington and Woods Hole Oceanographic Institution, examined adult female white sharks in the Gulf Stream and North Atlantic Ocean and found that many of these sharks spent significant portions of their time below the surface hanging out in these warm-water eddies.
“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,” senior oceanographer at the UW’s Applied Physics Laboratory and lead author of the study Peter Gaube said. “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.”
The six-year study was done using data collected from sharks tagged by the OCEARCH research program, in which satellite-enabled tracking devices would measure environmental conditions such as depth, location and water temperature.
Researchers 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 said. “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.”
Analysis shows that the two sharks spent significantly more time in warm-water eddies than the cold-water eddies that spin the other way. Sharks lounged the longest at about 450 meters (about a quarter of a mile) deep inside the warm-water eddies, especially during the daytime, likely feeding on the abundant fish and squid at these depths. They were more likely to come to the surface at night.
This 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 said. “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 by diving in these warm eddies, they can spend more time in the deeper water.”
