A bracelet against sharks?

DW: Have you ever been bitten by a shark?

Erich Ritter: Yes, I have - several times. One of these accidents was a bit worse than the others. But most of the time, I experienced only harmless nips, which didn't even bleed much.

Why do accidents with sharks happen?

Eighty to 90 percent of all bites are only the shark's way of checking out what we are. It takes, for example, a hand into its mouth and presses it shut only slightly. The really bad wounds originate when you try to pull out your hand from the shark's mouth.

Can a magnetic shark repellent like the "Sharkbanz" protect you against shark bites?

Absolutely not! These Sharkbanz act only as a psychological safeguard - they have no effect at all. Last year, on 27 December, a 14-year old surfer in Florida had an accident with a shark - despite wearing such a Sharkbanz. The usefulness of these bracelet has never been verified, because they have never been tested scientifically.

But Sharkbanz video adverts make it look as though they work.

That's right. But such things are easy to manipulate. You can make the shark swim away [for reasons other than the bracelet].

Sharkbanz have a small magnet inside. How is that supposed to prevent a shark attack - in theory?

Sharks can detect bioelectrical fields using a special sensing organ called the ampullae of Lorenzini. Living things generate electromagnetic waves when they contract muscles. A shark can detect these waves. The magnets in Sharkbanz are said to change the bioelectrical field around a swimmer or surfer. And this, it is claimed, interferes with the shark's electroreceptors, with the result that it goes away. But to make that work you would need a giant magnet to generate a huge magnetic field. The field that the Sharkbanz triggers is much too weak.

What happened in the accident you mentioned earlier - with the young surfer wearing a Sharkbanz?

It was a typical surfing accident - the sort that Sharkbanz claims to be able to prevent. The boy was bitten on the arm. 

 

The problem with these bracelets is their low efficiency. A shark has to come as close as 10 centimeters (4 inches) to detect the magnetic field the bracelet generates. The boy wore the band on his leg and was bitten on the arm. The deterrent simply couldn't work.

Would it help, then, to wear several of these bracelets?

No. The field of protection would still be too weak. You would have to wear kilos of magnets, anything less wouldn't work. Sharkbanz are not the first of their kind. Another such technology is Sharkshield which works electronically. All of them try to do the same thing - to meddle with the shark's electroreceptors. But none of these technologies are well thought through.

So there's no efficient protection against sharks?  

No. Sharkshield is electronic and is the only technology that stood a chance of working. But it's also got a problem. Sharkshield's batteries are much too small to generate an efficient electrical field. The best protection is to deal with the animal.

What do you mean by that?

You have to establish contact with the animal. Look at it and try to understand what mood it's in and why it's swimming towards you. You have to try to understand the factors influencing the situation. If you manage that, you can be in the water with any shark - even with a great white.

Erich Ritter is behavioural scientists at the University of West Florida. He studies shark accidents and tests shark deterrent systems.