Why sharks mistake surfers for seals (and how to stop them)

It is called the mistaken identity theory and it has long been offered as the reason large sharks attack humans — especially those on surfboards.

Australia’s surfers have suffered a 300 per cent increase in shark attacks in the past 15 years. Now, research from a zoo in Sydney is showing how sharks see —from below — their preferred meal of seal and how they see a surfer. It goes some way to explaining why they seem to confuse their quarry.

Researchers at Taronga zoo have embarked on a series of experiments this month to help understanding of what causes a shark to attack.

They are mimicking what sharks see underwater by filming the movements of the zoo’s seals and sea lions, and comparing this with people on surfboards. They use computer software to interpret the images as sharks — which are visually impaired compared with humans — would see them.

The researchers hope to devise new ways to repel sharks, such as equipping surfboards with lights, a feature they will test on South Africa’s white pointer shark population later this year.

“We know their visual system isn’t as good as ours,” said Nathan Hart, a neuroscientist at the University of Western Australia and the lead researcher. Sharks are colour blind but have very sensitive eyes. This makes them accomplished at finding objects in low contrast but they also have poor spatial acuity, which means that their vision is more blurred than that of humans.

“If you now imagine blurring those images, you can see how there’d be even more similarity between them because the details of the arms and the legs get hidden,” Dr Hart said. “You can see quite easily how that mistaken identity might come about.”

The study is part of a project funded by the Western Australian government looking at shark attack deterrents. Data published in the Australian Shark Attack File has found that most people are surfing when attacked by a shark. “If we can come up with a directed solution for surfers in particular, hopefully we can make an impact,” Dr Hart said.

Researchers believe a large part of the surge in shark attacks is because the number of recreational swimmers, surfers and scuba divers is increasing, thanks to Australia’s expanding population and a flood of retiring but very active baby-boomers.