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Imagine a Great White Shark, lurking beneath its prey. As it gazes upward, a shadowy shape glides through the water. Then suddenly, the shark swerves upward in the water column, grabbing the unsuspecting creature as a meal.
Scientists have long known that white sharks rely on their eyes to hunt—despite having middling visual acuity. They mostly depend on their eyes to quickly detect the silhouettes of potential prey, many of which swim near the surface, backdropped by the light sky. Seals top the dinner list for some shark species, for example, and when seen from below, the silhouette of a human is not too different from that of the seal. (Encounters with humans are known to happen more frequently when the waters are turbid, further reducing sharks’ visual sensitivity.)
“They patrol for seals from about 15 meters below, in general,” says Laura Ryan, a biologist at Macquarie University in Australia. Ryan, who is also an avid surfer, began to wonder: “If we change the silhouette, would they change their behavior?”
Scientists have been looking for ways to deter shark attacks for a century at least.
Over the course of five years, she was able to test the theory. Ryan and her team created 4-foot-long seal-shaped foam decoys. They dragged the decoys behind a boat at dawn and dusk—prime shark feeding time—in Mossel Bay, South Africa, an area known both for seal and white shark activity. The decoy seals were regularly attacked. But they also experimented with lighting up the decoys with LED lights in differing configurations, disrupting the silhouette and creating a kind of camouflage.
The team tested a few different ways to light up the seals. When the whole decoy was covered in lights the sharks stayed away—but the lights and battery packs were heavy. Strobing lights that flickered on and off every few seconds didn’t work as well. But creating stripes of light across the chest, belly, and hips of the seal decoy was efficient and seemed to be a useful pattern to prevent attacks, Ryan says. While the sharks followed or jumped at the control decoys 12 times over 34 hours of testing, they left the striped light decoys alone entirely over that same period: No follows and no jumps. The work was published this month in the journal Current Biology.
Ryan says she could see embedding stripes of light into the bottoms of surfboards to disrupt the sharks’ visual systems and deter them from attack. It’s actually a strategy animals have developed in the wild to confuse prey: Some fish, like the juvenile plainfin midshipman fish, feature cells called photospores on their undersides that disrupt the shapes of their silhouettes, a means of diverting predators. Small sharks like the velvetbelly lanternshark also have photospores on their bellies to help protect against attacks from larger predators.
Scientists have been looking for ways to deter shark attacks for a century at least. They have tested surfactants that can be squirted into sharks’ mouths, electric prods that can zap them when close, and magnets that disrupt sharks’ electro-receptors, which allow them to sense electrical fields in the ocean. But these strategies have been hit or miss.
If we change the silhouette, would they change their behavior?
Gavin Naylor, a biologist who directs the Florida Program for Shark Research, says the new study was “simple, straightforward, and rigorously executed.” He adds that the approach is elegant and sensible, “based on the premise that if we understand how a particular species of shark perceives its environment, and identifies its prey, we can develop protocols that interrupt the flow of this information.”
Naylor points out that while Ryan’s lights have been shown to be effective for white sharks in South Africa, it isn’t yet clear that it will work with other species in other places. For example, the method will likely not work for other species of sharks that do not use silhouette profiling to identify their prey. Where he works in Florida, shark bites on surfers mainly come from black-tip reef sharks, which target schooling fishes—they are not looking for seal-shaped silhouettes.
Ryan is still playing with her seal decoys to see what works, she says. She’s looking at how other shark species respond to the lights. She’s also testing how the lights would impact a seal decoy that is not moving, but just stationary in the water. “As a sensory biologist, I’m also interested in the other senses that sharks have,” she says, such as electroreception and lateral lines, tiny pores along the sharks’ sides and head which allow them to detect vibrations and pressure changes in the water.
Says Ryan, “There’s still so much we don’t know about the sensory systems and how these guys experience the world.” 
Lead art: Jsegalexplore / Shutterstock
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