Monday, April 6, 2015

Do sharks really mistake humans for seals?

Do sharks really mistake humans for seals? that is the headline of a report in "The Sydney Morning Herald"

The article says that Researchers test 'mistaken identity' theory  by conducting some studies on sharks in a pool.

Photo: Taronga Zoo  

"You can see quite easily how that mistaken identity might come about," Dr Nathan Hart explains. 

Look at these images and it's easy to imagine how a shark might mistake a swimmer or surfer for a seal.

At least that's the premise behind the "mistaken identity" theory that tries to explain why sharks sometimes attack people.

While the idea seems reasonable, even logical, it has never been tested until now.

What? Never been tested? Ever heard of Dr. Peter Klimley, Scott Anderson and many others.

Photo: Taronga Zoo

At Taronga Zoo, researchers have this month run a series of experiments to understand what drives a shark to attack by mimicking what they see and hear underwater.

With this information, they hope to develop specific shark repellents, such as making surfboards less attractive with lights; a feature they'll test on South Africa's white pointer population later this year.

"We know their visual system isn't as good as ours," said lead researcher Nathan Hart, a neuroscientist at the University of Western Australia. 

Sharks are colour blind but they have very sensitive eyes, making them good at detecting objects in low contrast. However, they also have poor spatial acuity, which essentially means their vision is more blurred than humans.

Wait, their visual system isn't as good as ours? They are color blind? Their vision is more blurred? I guess Dr. Hart has never heard of Dr. Gruber's research published way back in 1985! 

Thanks to decades of careful, dedicated work by Samuel Gruber and his co-workers, we now know that many sharks see in color, too

In a revealing 1985 paper, shark biologist Samuel Gruber and anatomist Joel Cohen studied the retina of the White Shark. Gruber and Cohen demonstrated that the Great White retina has both rods and cones, but at a significantly different ratio from most sharks. The small, moderately deep-dwelling Spiny Dogfish has a rod-to-cone ratio of about 50:1, while in the larger, more shallow-water Sandbar Shark (Carcharhinus plumbeus) the rod-to-cone ratio is about 13:1. But in the White Shark, the rod-to-cone ratio is about 4:1 - roughly the same as in human beings. From these results, Gruber and Cohen concluded that the White Shark has the retinal mechanisms necessary for acute, bright-light, color vision. source

I would expect a researcher to 1. not make a general statement about shark's vision, when different species have very different eyesight and 2. know about the fact that white sharks can see color, which has been known for 30 years now.
I can see you! In color :-)

Maybe, if Dr. Hart had done a little research on what is already known about sharks, he would not make statements like these 

"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," he said.

The article states that  The study forms part of a broader project funded by the Western Australian government to assess shark attack deterrents.

Well, knowing what kind of "science" the Western Australian Government used to justify their drum line program, I'm not really surprised that they are funding these kinds of researchers.

Link to the article here

Martin Graf
CEO Shark Diver

About Shark Diver. As a global leader in commercial shark diving and conservation initiatives Shark Diver has spent the past decade engaged for sharks around the world. Our blog highlights all aspects of both of these dynamic and shifting worlds. You can reach us directly at

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