“Shark magnets” are devices developed to reduce interactions between sharks and humans in marine environments. These tools aim to deter sharks by interfering with their natural senses, specifically their ability to detect electrical fields. This technology provides a way to enhance safety for swimmers, surfers, and divers.
Sharks’ Electroreception
Sharks possess a unique sensory ability known as electroreception, allowing them to detect weak electrical fields in the water. This sense relies on specialized organs called the ampullae of Lorenzini, which are a network of jelly-filled pores located primarily on a shark’s snout and head.
When an electrical field is present, it creates a voltage difference across the ampullae, triggering a signal that is sent to the shark’s brain. This enables sharks to detect the faint bioelectric fields generated by the muscle contractions and movements of potential prey, even when hidden in sand or murky water. The ampullae of Lorenzini are incredibly sensitive, capable of detecting electrical currents as low as 5 nanovolts per centimeter. This sensitivity also aids sharks in navigation by sensing the Earth’s geomagnetic field.
How Magnetic Deterrents Function
Magnetic deterrents operate by leveraging the highly sensitive electroreception system of these animals. These devices, often containing powerful permanent magnets, generate a strong magnetic field in the surrounding water. As a shark approaches, this intense magnetic field is designed to overstimulate the ampullae of Lorenzini, creating a sudden and unpleasant sensation.
The magnetic field produced by these deterrents is significantly stronger than the faint electrical signals sharks typically encounter from prey or the natural environment. This overwhelming sensory input is thought to be disorienting or irritating to the shark, prompting it to turn away. Some devices, like Sharkbanz, utilize specific types of permanent magnets, such as Neodymium-Iron-Boride or Barium-Ferrite, which are designed to interact with the ampullae of Lorenzini. These permanent magnets do not require batteries or external power, making them continuously active when in the water.
Effectiveness and Practical Applications
The effectiveness of magnetic shark deterrents in real-world scenarios varies across scientific studies and shark species. While some laboratory experiments demonstrate that sharks exhibit avoidance behavior when exposed to strong magnetic fields, the range of deterrence is often limited to within a few meters, sometimes as little as 0.3 meters from the magnet. For instance, studies have shown that permanent magnets can elicit avoidance in species like Galapagos and hammerhead sharks. However, research on white sharks, for example, has indicated that some commercially available magnetic deterrents had limited or no measurable effect on their behavior or on reducing bait taken.
The deterrent effect can also be influenced by factors such as the strength and type of magnet used, with rare-earth magnets like Neodymium-Iron-Boride potentially offering a more powerful deterrent than ferrite magnets due to higher surface field strengths. There is also evidence suggesting that the effectiveness can vary individually among sharks. Despite these limitations, magnetic deterrents are employed in practical applications for recreational users, such as divers, surfers, and swimmers, often integrated into wearable devices like bracelets or leashes. They are also explored for bycatch reduction in fisheries, aiming to selectively deter sharks and rays without affecting bony fish.