Sharks that prey on surfers or bathers have such poor eyesight that scientists have concluded that they are probably mistaking them for their usual prey, according to a study published on Wednesday.
"From the point of view of a white shark, neither the movement nor the form allow an unequivocal visual distinction between pinnipeds and humans", write the authors of the article published in the scientific journal
Journal of the Royal Society Interface
.
They conclude that their work “supports the theory of misidentification to explain certain bites”.
“This is the first study to test this theory from the visual point of view of a white shark,” says lead author Laura Ryan, a researcher in the Department of Biological Sciences at Macquarie University of Australia.
Shark attacks remain rare (less than sixty worldwide in 2020), according to a specialized department at the University of Florida.
A climate of "disproportionate" fear
But they maintain, according to the study, a climate of “disproportionate” fear, associated with ignorance of the animal's motivations. Sometimes the consequence is hunting campaigns which also harm other species. Most often incriminated, white sharks, tigers and bulldogs, mostly attack surfers. If the white shark is known to detect sounds and smells at great distance, up close it is assumed that it mainly trusts its sight to spot and aim for prey.
However, the shark's visual system is almost insensitive to color and has a very poor ability to distinguish the details of a shape.
Its resolving power, up to six times lower than that of a human, is even lower in young white sharks, which pose the greatest risk of fatal bites to surfers, according to the study.
To test the misidentification theory, Macquarie's team made "videos taken from a shark's point of view, and processed them with a program to mimic the shark's visual system," and specifically its ability to distinguish a shape and its movement, explains the scientist.
Researchers want to find a solution
To do this, the researchers recorded from the bottom of a basin the images and videos of a sea lion and a fur seal, delicacies of choice for the shark, which would pass near the surface, a few meters away. above a shark. They then compared their signals to those of swimmers and surfers paddling with their arms, and with or without kicks, on the three major types of surfboards (longboard, shortboard and hybrid).
From a young white shark's perspective, the movement signals of a swimmer like those of a surfer paddling his board are almost indistinguishable from those of a pinniped - a marine mammal - according to the study. . A fortiori in seawater, where visibility would be less than in the basin used for the experiment. As for shape, a pinniped with folded fins looks more like a swimmer or surfer on his shortboard than a pinniped with outstretched fins.
“Longboards look less like a sea lion,” says Laura Ryan, who notes that “there have been incidents of biting on longboards”.
The researchers will now try to determine if a "change in the visual signals of potential prey would be an effective technique of protection against white sharks", continues the scientist.
With the imperative of solutions which “not only prevent shark bites, but which do not endanger other marine species”.
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