When Shark Shields and Great Whites Collide

After a spate of shark attacks in the New South Wales region of Australia a few years ago, sales of the electronic repellant device known as Shark Shield skyrocketed all over Australia ( they're sold online at www.sharkshield.com ). Shark Shields generates an electrical field, or a "shark-safe zone," 25 feet in diameter around the diver. Electrodes generate a pulsing sensation detected by the shark through its sensory receptors, and create muscular spasms that send the shark fleeing but cause no lasting effect. The latest iteration is Shark Shield Freedom7, which differs from previous models in that the electrodes trail behind the leg of the diver, instead of one electrode placed on the tank and the other on the diver's ankle. As a result, the electric field source of is located behind the diver instead of being centered on him.

Scientists from the South Australian Research and Development Institute wanted to find out whether the different location and configuration changed the Shark Shield's efficiency. They chose white sharks as the test group, as they are responsible for the most unprovoked attacks, and they have shifting swimming patterns based on their habitats and hunting strategies.

In the first experiment, the researchers used "static bait," fish oil and chopped bluefin, behind the stern of their anchored boat, to bring out the sharks in the Neptune Islands, a popular cage-diving site in South Australia. The Shark Shield was placed five feet away from the bait. During 116 trials, the amount of bait the sharks took was not affected. However, the electric field typically increased the time it took them to eat the bait, and the number of their interactions per approach.

In the second experiment, they used a seal decoy towed behind a moving boat (with the Shark Shield streaming seven feet behind the seal) near Seal Island in South Africa, where sharks love to hunt, often breaching in the process. Ninety-eight of the 189 tows were performed with the Shark Shield turned off, and 91 with it turned on. When it was not turned on, great whites made 16 breaches and 27 surface interactions with then. When it was turned on, there were no breaches, and only two surface interactions were observed during the tows.

The researchers think that even though white sharks are still able to get bait located close to a strong electric field, it can make them hesitate and take longer to eat it. However, responses varied by individual sharks, with some less affected than others. The reason why is still unknown -- motivation, different feeding histories, dominance hierarchies or individual experiences may play roles. And while tests of the electric fields have produced a behavioral reaction in some sharks, the responses vary across species. Because Shark Shield hasn't been scientifically tested on all shark species, it can't be relied on to prevent shark attacks in all situations.

C. Huveneers , P.J. Rogers , J.M. Semmens , C. Beckmann, A.A. Kock AA, et al.; Effects of an Electric Field on White Sharks: In Situ Testing of an Electric Deterrent. PLoS ONE (2013) 8(5): e62730. doi:10.1371/journal.pone.0062730