Blue sharks use eddies for fast track to food

Researchers tagged more than a dozen blue sharks off the U.S. Northeast Coast and monitored them for nine months. The tags relayed data back to the researchers via satellite, revealing that the sharks had spent a good portion of their days diving these whirling pockets of warm water down to the ocean twilight zone hundreds of meters below the surface. There, they'd spend an hour or so foraging for food like small fish and squid before returning to the surface to warm up before diving again.

Dives were less frequent at night, when many twilight zone animals make their daily migration from the ocean's mid-water to feed at the surface. Camrin Braun, an ocean ecologist at UW and lead author of the study, said that an evening trip likely isn't worth the effort for hungry blue sharks since the zone isn't particularly well stocked after dark.

Braun, who conducted the research as a PhD student in the MIT-WHOI Joint Program in Oceanography, says the behavior of the blue sharks was generally similar to that of the white sharks the team tracked in a previous study last year. However, the two species had different preferences when it came to water temperature. White sharks, which are warm-blooded animals, used a combination of warm and cold-water eddies as a conduit to the twilight zone, while blue sharks -- a cold-blooded species -- relied exclusively on warm-water eddies.

"Blue sharks can't regulate their body temperature internally to stay warmer than the ambient seawater like white sharks can," said Braun. "We think this is why they show a clear preference for the warm-water eddies -- it removes a thermal constraint to deep diving."

Blue sharks are considered a "near threatened" species due to heavy fishing pressure on populations across the globe. This research helps fill important knowledge gaps about where they go and why, which can inform decision making to help protect them, said biologist Simon Thorrold, a coauthor of the study and Braun's former advisor.

The work also underscores the importance of the ocean twilight zone as a critical biomass resource.

"The twilight zone is vulnerable to overfishing," Thorrold 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."