A new study analyzed movements of two great white sharks and found that in the open ocean, the sharks spend more time deep inside warm-water eddies.
The study, published May 9, 2018, in the peer-reviewed journal Nature Scientific Reports, tracked two adult female white sharks in the Gulf Stream and North Atlantic Ocean for nearly six years. The researchers were surprised to discover that the sharks prefer warm-water eddies — ocean whirlpools that spin clockwise north of the equator — and tend to spend more time deep inside these slowly spinning loops of water.
Watch the video of how OCEARCH tagged and released the first great white in Florida waters, Lydia!
The tricky job of tagging the animals was done by OCEARCH, a nonprofit that focuses on tagging and tracking sharks. The shark named Mary Lee was tagged in September 2012 off Cape Cod and the shark called Lydia in March 2013 off Jacksonville, Florida. Six years later, one of them still reports her position regularly, as they swim north with the Gulf Stream and then out into the open ocean.
See real-time tracks of all sharks tagged by OCEARCH here.
Study lead author Peter Gaube is a senior oceanographer at University of Washington’s Applied Physics Laboratory. Gaube said that early shark-tagging projects could just offer rough ideas of where sharks were swimming. But since precise satellite position networks were made available to the public, and with improvements in computing and batteries, the new high-tech tags can now collect detailed information as sharks travel throughout the marine environment.
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. Gaube said in a statement:
These eddies are everywhere. They cover 30 percent of the ocean’s surface. 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 a quarter of a mile (450 meters) 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 preference goes against common wisdom, Gaube said, 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 cold-water eddies are rich in marine plant life. This study is the first to show that sharks gravitate toward eddies, and that they prefer the warmer variety.
[White sharks] 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.
Recent studies also suggest that the “twilight zone,” below the depths that satellites can see, might contain many more fish than previously believed — and many more than at the surface. Gaube said:
Could these “ocean deserts” actually be super-productive at depth? That’s what we think might be happening.
Some recent deep-sea net surveys have found larger toothy fish like pomfret below the surface in anticyclonic eddies, which could provide a motivation for the sharks to dive there. Gaube said:
These sharks are 2,800 pounds. It’s hard to imagine that they’re just eating krill and small fish all of the time they’re in the open ocean. If they can find pomfret and lots of squid in these eddies, then sharks can really get a meal out of that.
Data collected by tagged sharks could help to protect this “twilight zone” as it’s just beginning to be targeted by major fisheries, Gaube said. And information about where great white sharks like to hang out could help conserve this vulnerable species. Gaube said:
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. 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.
Bottom line: A study tracked two female great white sharks and found that in the open ocean, the sharks spend time deep inside warm-water eddies.