Researchers from Woods Hole Oceanographic Institution (WHOI) and the Applied Physics Lab at the University of Washington (UW) have discovered that blue sharks use eddies, the ocean’s large, swirling currents, to quickly get to the ocean twilight zone, which is located between 200 to 1000m below the ocean surface, where the largest fish biomass on Earth is found.
The scientists, whose findings were published in the journal Proceedings of the National Academy of Sciences, monitored over a dozen blue sharks, which they tagged off the northeastern coast of the United States, for nine month. Data from the tags was relayed back to the scientists by satellite and showed that the sharks spent a large part of their days diving pockets of warmer currents to reach the twilight zone, where they would forage for an hour or more on small fish and squid, returning to the surface to warm up so they could dive again.
“Blue sharks can't regulate their body temperature internally to stay warmer than the ambient seawater like white sharks can,” said Camrin Braun, an ocean ecologist at UW and lead author of the study. “We think this is why they show a clear preference for the warm-water eddies—it removes a thermal constraint to deep diving.”
The study helped to fill in knowledge gaps about blue sharks, which are threatened by over-fishing across the globe, and underscored how important the ocean twilight zone is as a biomass resource. “The twilight zone is vulnerable to overfishing,” said Simon Thorrold, a coauthor of the study and Braun's former advisor. “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.”