With the ability to track electromagnetic signals, and a sense of smell that can detect a drop of blood in the water from up to 0.4 kilometers away, the shark is nothing less then the perfect hunter. It is built to search and destroy, and surely nothing can hope to deter it once it has its target in sight.
Yet this is what scientists have been trying to achieve for decades, ever since the USS Indianapolis tragedy of 1945 alerted the United States Navy to the threat posed by roving sharks. The ship was sunk by two torpedoes from a Japanese submarine, and of the 900 sailors who escaped into the wreckage, only 317 were left alive by the time rescue arrived four days later.
Though most of those deaths were due to the effects of exposure and thirst, horrific accounts of sailors screaming and flailing helplessly as circling sharks closed in and dragged them down into the depths shocked US Navy officials, and prompted significant investment in the research and development of shark-repelling technologies.
The need for such technologies was re-emphasized during the Cold War, when sharks would occasionally attack the underwater listening devices employed by US submarines for the purpose of spying on Soviet activity. The Navy also needed a way to prevent them from prematurely detonating underwater explosives.
Over time, research has turned up a number of potential methods for repelling sharks, either by emulating the chemical signals they associate with danger, disrupting the electric currents they use to track their prey, or through visual trickery that exploits the shark's color-blindness.
Some of these methods have proven effective in clearing an area of sharks, while others serve to keep them at bay or divert them, and while there is as yet no guaranteed form of protection that can realistically be utilized by civilians, progress is being made.
Sharks are not easily intimidated, but researchers have discovered the one thing capable of sending them into flight: the smell of dead shark.
Certain fish, when attacked, release chemical signals that cause the rest of the school to scatter, and it seems that sharks possess a similar defense mechanism. The response is so strong that even sharks in a state of tonic immobility (temporary paralysis that occurs when the shark is turned upside down) are spurred into action.
Attempts to develop a repellent by isolating the chemical responsible for triggering this response have shown promise. Researchers conducted field tests in the Bahamas by dropping a mixture into water teeming with sharks, causing them to promptly flee the area.
The tactic has proven effective against six different species, and even sharks in the midst of a feeding frenzy are unable to ignore this call to retreat. What's more, the substance is non-toxic, and appears to have no effect on other fish in the vicinity.
A shark's snout and lower jaws contain open pores called ampullae of Lorenzini, through which they can respond to electrical currents in much the same way as we do to sound. This, combined with the electroconductive capabilities of salt water, enables them to track the electric signals generated by muscle movement.
Research has demonstrated that these signals have significant influence on the shark's prioritization of its targets. They use their sense of smell to track down their prey, but resort to electroreception as they close in for the kill, and sudden changes in electromagnetism can cause them to alter their course. For example, a shark with a dead fish and an electrically charged rod in its path will initially go for the fish, but suddenly veer off towards the rod at the last second.
So although this powerful sixth sense is part of what makes them such efficient hunters, it can be used against them. Utilizing magnets is one way to achieve this, as researchers have found that combining charged metals with salt water creates a powerful electrical field that overwhelms the shark's senses, though it would require multiple magnets arranged precisely on the body to achieve sufficient protection.
The problem with the electromagnetic approach is that it relies on disorientating the shark, which may not be enough to deter them once they have smelt blood in the water. Furthermore, technologies that utilize this method, such as Shark Shield, are not proven to be effective in all situations.
Nonetheless, research into potential applications are underway, with the possibility of substituting shark nets with more effective and environmentally-friendly magnetic fences being one example.
Contrary to what some may believe, the shark's highly attuned sense of smell does not preclude the need for vision, which plays an important role when closing in on prey. While the quality of their eyesight varies amongst species, in general their eyes are well adapted to the low-light conditions of the ocean.
But while their eyes are sensitive to light, they are not so effective at perceiving color. They rely on the contrast between an object and its background rather then the color of the object itself, a fact that can be used to our advantage.
Australian company Shark Attack Mitigation Systems (SAMS) has designed a set of wetsuits with the purpose of repelling sharks through misdirection. One of these wetsuits, named the “Elude”, sports a blue pattern design that enables the wearer to blend in with the surrounding ocean. The other, named the “Diverter”, adopts a black and white striped design that emulates the patterns usually found on poisonous fish, thereby encouraging the shark to seek more edible prey elsewhere.
Tests conducted in Western Australian waters have produced promising results, with sharks appearing to swim past dummies wearing the specially designed wetsuits while attempting to eat those wearing standard black wetsuits. Nonetheless, SAMS admits that the wetsuits cannot yet be deemed a fail-safe form a protection against shark attacks.
Sharks are the ultimate hunter-killers, and are not easily put off their dinner. Yet many fish species clearly possess the means to do so, whether it be through chemical secretions – as seen with the Red sea Moses sole, or through stealth. By borrowing some ideas from nature, we can hope to devise methods for keeping them at a safe distance, something that will be of significant benefit to both humans and sharks.