'Invisibility cloak' could save surfers from shark attacks

Scientists have discovered that equipping surfboards with bright LED lights could prevent great white shark attacks on surfers and swimmers. A recent study published in the journal Current Biology found that lighting up the underside of surfboards disrupts the silhouette perceived by sharks - so they are less likely to mistake humans for their usual prey.

Researchers have previously discovered that great whites rely on their eyes to locate prey near the surface - typically a seal - and lunge upwards to take it in their jaws.

In related studies, the researchers have also found that, like most sharks, great whites are probably completely colour-blind and have poor vision. However, the great white shark is really good at detecting a silhouette, and surfers and swimmers may have a similar profile to prey.

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This reliance on seeing the silhouette of their prey also opens up an opportunity. What if a silhouette on the surface of the water was disguised using lights so that the shark - blind to colour and unable to detect detail - would not see it as food?

Dr Laura Ryan, a biologist at Macquarie University in Sydney, Australia, said: "There was a big spate of shark bites and as a really keen surfer I was spending a lot of time thinking about sharks' eyesight.

She began pondering how her research on shark vision could be used to protect people from shark attacks. "I started to think what if what I was learning could potentially protect surfers?" she added.

To test out this counterillumination strategy Dr Ryan spent months, in multiple trips over six years, in one of the world’s notorious hot spots for great white sharks - Mossel Bay in South Africa. She and her colleagues towed 1.2m long, seal-shaped foam decoys on a 20m line behind a boat and saw the decoys regularly attacked by great whites. But they also experimented with using LED lights, in different configurations, to break up the silhouette of the decoys.

Covering the whole of the underside of the seal decoy in bright lights was found to work very well in deterring sharks. But using so many lights was not energy efficient and it would be expensive and impractical to apply to surfboards.

They also tried flashing lights, which strobed four times a second, but that proved ineffective. After much experimentation, they found the ideal pattern to fool the sharks was to place the lights in stripes across the bodies of the seal decoys, perpendicular to the direction they were being towed through the water.

Will the system work to protect surfers? The study did not specifically test the effectiveness of lights mounted under surfboards because the South African authorities, understandably, wouldn’t agree to using surfboard-shaped decoys to lure great white sharks to attack.

The sharks still saw the decoy, but its shape was broken up and the great whites stopped attacking.

“It’s like an invisibility cloak but with the exception that we are splitting the object, the visual silhouette, into smaller bits,” says Professor Hart. “It’s a complex interaction with the shark’s behaviour. The lights have to be a certain pattern, a certain brightness.”

The team also tried longitudinal stripes of LED lights, but these were less effective. “When you have the lighting going along the body, you still have a long, narrow silhouette left over, which is going to be similar to what a seal produces,” says Dr Ryan.