Lighting the underside of surfboards, kayaks, and wetsuits could prevent the majority of great white shark attacks on humans.

It has long been known that sharks often attack humans, mistaking their silhouettes on the water for prey such as seals. Now, researchers have conducted an experiment to see what happens when a decoy seal's silhouette is distorted by illuminating its underside to disguise its shape.

laura ryan Researchers from Macquarie University in Sydney spent nearly 500 hours towing seal-shaped decoys around Mossel Bay in South Africa's Western Cape region, where great white sharks (carcharodon carcharius) gather in large numbers to hunt.

The team tested multiple light treatments. I covered the underside of the decoy with dim, medium, and bright LED and strobe lighting, as well as horizontal and vertical strip lighting. Each time we towed with any of the light treatments, we immediately towed the control decoy seal without using any underside lighting. As an additional experiment, we conducted a paired test in which an unlit control dummy was towed 3 meters away from a lit dummy.

Unlit decoys were more likely to be attacked or chased by sharks than lit decoys. The brightest light appeared to be the most effective, with zero predator incidents when the decoy illumination was at its brightest.

Vertical strip lighting was less effective than horizontal strips, Ryan said, perhaps because it broke up the silhouette into long sections that could be identified as seals.

Strobe lighting was less effective than continuous lighting, likely because the sharks could still see the silhouette of their likely prey between flashes.

Ryan said the team expected it would be important to match the decoy's lighting with the background light and make sure it was no brighter than the surrounding underwater conditions, but that wasn't the case.

“The most important thing was that the brightness of the decoy had to be brighter than the background light,” she says. “As long as the lighting doesn't make the silhouette look black, it seems to work.”

The research team has now developed a prototype lighting array to be used as a deterrent for great white sharks. “We are now moving from research to providing protection for swimmers and surfers,” Ryan says. “We took an approach that involved understanding the sensory systems of these animals, how they see the world, and their behavior.”

Ryan cautioned that lighting deterrents have not been tested against other species known to attack humans, such as tiger sharks (Galeocerdo Cuvier) and bull sharks (Porgy whale), each with a different hunting strategy.

david booth The University of Technology Sydney conducts a number of field studies off the Sydney coastline, where sharks, including great whites, are common. Based on these findings, he says he will definitely order a counter-illuminated wetsuit when it becomes available.

“I was surprised to find that ‘destructive camouflage’ was more effective, as I thought low lighting to match the background was most effective,” he says.

“These results apply only to great white sharks, and only to this feeding mode, so it is unclear at this point how broadly applicable they are.”