A new study by the Woods Hole Oceanographic Institution and the Applied Physics Lab at the University of Washington found that sharks ride fast-moving currents to dive to depths where prey is plentiful.
Using satellite tags attached to more than a dozen blue sharks off the Northeast Coast of the U.S., the researchers found that the sharks were using swirling, warm-water eddies to drop to the oceanic “twilight zones” between 600 and 3,500 feet below the surface. Their finding were published in the most recent issue of the Proceedings of the National Academy of Sciences.
Each shark was “double tagged” on their dorsal fins with one tag monitoring ocean temperatures and depth as the sharks moved through the ocean and the other tag tracked their location. This double-tagging strategy allowed the scientists to reproduce the three-dimensional tracks of the sharks with the resolution and accuracy needed to link their movements to the positions of ocean currents like eddies.
They found that the sharks spent a good portion of their days diving these warm-water tunnels where they would spend an hour foraging before returning to the surface. The data found that they would repeat this behavior throughout the day and would end during the night.
“Sharks are all about opportunity, so with fewer prey items down there at night, they’re just not going to make the trip,” Camrin Braun, an ocean ecologist at UW and lead author of the study, said. “Going down there is costly for them from an energetic and metabolic standpoint.”
The study can show the importance of the ocean twilight zone as a critical biomass resource so that informed decisions can be made when implementing marine protected areas.
“The twilight zone is vulnerable to overfishing,” Simon Thorrold, a senior scientist at WHOI and co-author of the study, said. “If we’re harvesting low-value fish there at the expense of high-value fish like blue sharks and other pelagic predators, that’s probably not a good tradeoff.”