The team used a combination of acoustic tracking beacons, on-board animal cameras, and their own underwater observations to monitor behavior. They were able to calculate the energy consumption of those who stayed in the channel while surfing and those who left the channel. By dragging and surfing the slope, researchers say the sharks reduced their energy by at least 15%. For an animal that can never stop swimming (because that’s how it gets oxygen), the action of surfing gives it a well-deserved rest.

Fakarava is a famous dive site and home to 500 gray reef sharks. Papastamatiou joined marine biologist and underwater photographer Laurent Ballesta and others on a trip to film the French research documentary 700 Sharks In The Night, which National Geographic then released in an abridged version 700 the Sharks. They were there to document the behavior of sharks after dark when the channel becomes a hunting ground. But it was during the day dives that Papastamatiou realized that many sharks remained in the channel even though they were not actively hunting.

He worked with his longtime collaborator Gil Iossilevskii of Technion – Israel Institute of Technology to use a detailed map built from the multibeam sonar system to predict and model where possible updrafts could appear, based on the direction of the tides. The team then placed tracking receivers along the channel to capture the location of the sharks. Over 40 gray reefs also had special beacons to collect data on their swim activity and depth.

The data confirmed that the sharks stayed in the channel during the day and selected areas of updrafts. To save as much energy as possible, the sharks have also changed the depth to which they descend to surf the slope. On rising tides with strong updrafts, they went further where the current was weaker. During the ebb tides, when there is more turbulence, they have moved closer to the surface for a smoother ride.

“This study is a beautiful demonstration of energetic seascapes, a spatial representation of the amount of energy it takes for an animal to move through an environment,” said Papastamatiou. “Marine environments are much more dynamic because of water currents, which are much less predictable. They can change seasonally, throughout the day, and even minute by minute. Ultimately, the energetic seascape helps explain why these animals are in this channel during the day. Now we have an answer.

These results could also apply to other coastal areas and explain why there may be more sharks in some places. It might even help predict why sharks may prefer one area over another.

The results were published in Journal of Animal Ecology.

Papastamatiou is Assistant Professor of Biological Sciences and Researcher at the CRF Environmental Institute. His research focuses on the physiology and behavior of predators, animal movement and foraging ecology, and the ecology of deep coral reefs in low-light environments in the tropics and subtropics.