Shark power nap

Hang ten! Sharks can't throw a shaka or shred the gnar, but they're still some of the ocean's most skilled surfers. 

But, unlike humans, they aren't after an adrenaline rush or deep connection with nature when they hit the waves. Instead, they ride the sea's powerful currents to conserve energy. As originally published in the Journal of Animal Ecology, marine scientist Yannis Papastamatiou and an international team of researchers have recently discovered hundreds of reef sharks in French Polynesia's southern channel of Fakarava Atoll displaying their own version of surfing by riding the updrafts from currents.

This phenomenon was first noticed during a diving trip, as researchers observed sharks swimming against the current. But, surprisingly, they were barely moving their tails! In fact, the sharks were somehow remaining nearly motionless, even in a strong current.

Upon closer inspection, the sharks seemed to have developed a queue or conveyer-belt-like system. When one shark reached the front of the line, it allowed the current to carry it back to its starting point. Then the next shark in line did the same, and so did its successor. Scientists were intrigued.

To further study this fascinating and never before seen behaviour, the team used a combination of acoustic tracking tags, shark-mounted cameras, and firsthand underwater observations. They were then able to calculate the energy usage of sharks that stayed in the channel 'surfing' and those that left the channel and were free-swimming. Researchers estimate the sharks cut their energy by hanging out and riding the current by at least 15 per cent. And, for an animal that never stops swimming (as it's vital to shark respiration), catching the occasional wave provides some much-needed rest.

Fakarava is a legendary dive site, home to hundreds of reef sharks. Recently, this remote area has gained notoriety thanks to its exposure in the National Geographic film 700 Sharks, a version of a French documentary focused on shark behaviour. Researchers were visiting the channel in hopes of documenting the sharks' dramatic after-dark activities, as the area becomes a hunting ground by night. But, it was during daytime dives when Papastamatiou and the team observed many of the sharks remaining in the channel, although they weren't actively hunting.

Scientists used a detailed map constructed using a multibeam sonar system to predict and model where possible updrafts and oceanic currents might appear, based on the direction of the tides. Then, they installed tracking receivers along the channel to capture the sharks' location. More than 40 individual sharks also had tracking tags put in place to collect information on their activity and swimming depth.

Data confirmed the sharks stayed in the channel during the day, carefully selecting updraft areas to swim in. And, for extra energy savings, the sharks also changed the depth at which they surfed. During incoming tides with powerful updrafts, they ventured deeper to where the current was weaker. Then, during outgoing tides, when there is more turbulence underwater, they moved toward the surface for a smoother ride.

Papastamatiou and fellow researchers hope to apply these findings to other coastal areas and use them to research why there may be larger populations of sharks in certain places. It could even help predict why some shark species may prefer one area over another!