The open ocean looks like a vast and featureless expanse to land-lovers like us. But to its aquatic inhabitants, the ocean is a dynamic environment, guided by the constant motion of the wind and currents. One key feature of the ever-changing ocean surface are mesoscale eddies: huge areas of swirling water that peel off from larger currents and look like whirlpools from space. Eddies are the ocean's ‘internal weather’ system, transporting heat, salt and even plankton around the globe. But what happens to animals when they run into these underwater storms? Led by Camrin Braun, a team of researchers from the Massachusetts Institute of Technology, Woods Hole Oceanographic Institute, the University of Washington, and the Massachusetts Division of Marine Fisheries, all based in the USA, used a unique combination of tools to track how a large voyaging predator, the blue shark (Prionace glauca), navigated these unstable ocean swirls.
Scientists wishing to simultaneously monitor the eddies – which can stretch across hundreds of kilometres – while tracking the movements of individual animals were hampered until recently by the technical challenges. However, the US-based team overcame this hurdle by using an inventive approach – combining satellite tracking of animal movements, ocean remote sensing and modelling – to record specific interactions between the blue sharks and eddies. The team also explored how ocean weather systems influence the behaviour of sharks living along the northeast coast of North America.
The team tagged 15 individual blue sharks with a pair of electronic tags: one that monitored water temperature and depth, and another that used satellites to precisely track the shark's physical location. By compiling over 2000 days of tracking data and nearly half a million measurements, the team constructed precise 3D models that linked the sharks’ movements, the water temperatures they experienced and the positions of eddies in the region. They found that blue sharks frequented warm water eddies during the day and sought out these areas in preference to cold water eddies. Once inside the eddies, the sharks swam in characteristic foraging patterns, diving fast before making leisurely returns to the surface. And these new observations back up earlier reports of cephalopods and other deep-sea fish species found in the stomachs of caught blue sharks, leading the researchers to conclude that the sharks used the warm eddies to tunnel far beneath the surface in search of deep-sea prey.
But why do the sharks bother tunnelling into the ocean at all? Why didn't they just dip and dive as they please, regardless of where the eddies form? The researchers reasoned that it all comes down to the thermal physiology of blue sharks. Unlike some other large open ocean fishes, such as tuna, which selectively heat up parts of their body, blue sharks are true ectotherms; their body temperature is dictated entirely by their environment. By hanging around in eddies, which were frequently 10oC hotter than the surrounding ocean, blue sharks could dive deeper than the theoretical limits of their cold-blooded biology and make a meal of the most abundant fish community on Earth.