Kinder, Gentler Shark Deterrents

Until fairly recently, sharks were widely perceived as dangerous vermin. Therefore, in protecting ourselves from sharks, no one much cared if our deterrent methods killed these potential man-eaters. Collectively, tens of thousands of sharks were drowned, shot, inflated, exploded, or otherwise dispatched with ruthless efficiency. But in these more ecologically enlightened times, people are gradually coming to appreciate that sharks - while remaining potentially dangerous - are wildlife, too, and thus not necessarily deserving of an unconditional death sentence whenever we work or play in their environment. Our new-found appreciation of the intrinsic value and sensory sophistication of sharks has spawned a kinder, gentler, non-lethal approach to shark deterrence. Two of the more recent and interesting of these approaches are discussed below, one chemical and the other electrical.

Since the earliest days of shark deterrence, a favored approach has remained chemical repellents. In 1974, ichthyologist Eugenie Clark noticed that the delicate Moses Sole (Pardachirus marmoratus) was easy to catch and appeared to secrete a milky, astringent substance from the base of its dorsal and anal fin spines. Suspecting that the little fish was protected by a toxin of some kind, Clark collected several specimens for study. She found that the Moses Sole did indeed secrete a toxin she named 'pardaxin', which caused red blood cells to rupture and - most intriguingly - to repelled sharks. Tests by Clark in the laboratory and open sea revealed that at least four species of sharks were repelled by pardaxin for 10 hours or longer. Further laboratory work by Israeli zoologist Eliahu Zlotkin and other researchers revealed that pardaxin is a rather large, clunky protein (162 amino acids long) that is very heat sensitive and thus not stable for storage at room temperature; the only form in which pardaxin proved stable is freeze-dried, but in that form was only 30% as effective as fresh.

Zlotkin noted that pardaxin frothed in water and reduced its surface tension by 60%, much like a soap. Zlotkin teamed with shark biologist Samuel Gruber to test a hunch: could commercially available soaps repel sharks? Zlotkin and Gruber tested two inexpensive commercial soap components, sodium and lithium lauryl sulfate (SLS and LLS, respectively - SLS, incidentally, is a common ingredient in shampoos), on young Lemon Sharks (Negaprion brevirostris). They found that both compounds were even more effective than pardaxin at repelling captive Lemon Sharks: starved Lemon Sharks rejected bait and those under tonic immobility (limp, trance-like state induced in sharks while inverted) revived and flipped over spectacularly. Gruber and Zlotkin then teamed with shark behaviorist Donald Nelson to test a related compound, sodium dodecyl sulfate (SDS), on Blue Sharks (Prionace glauca) in the wild. Nelson devised a pressurized cylinder that could deliver precise squirts of SDS directly into a shark's face or mouth. At a concentration of 15%, SDS caused each squirted Blue Shark to reject the bait, close their nictitating ('winking') membranes, and depart the area rapidly. Tagged individual Blue Sharks squirted with SDS never returned, while those squirted with ordinary water were startled into temporary retreat only. Clearly, these inexpensive, readily available soaps repelled sharks.

How does soap repel a shark? Laboratory work by Zlotkin, Gruber, and - most recently - Larry Smith (a former student of Don Nelson's) have revealed that SLS, SDS, and other soaps attack the fatty molecules of cell membranes in a shark's delicate gill filaments, causing a massive influx of sodium ions from the surrounding seawater. This ionic influx is clearly highly distressing to many sharks. A 1991 paper by Smith reported that shark reactions to SLS vary with concentration of soap used and from species to species.

In 1996, Donald Nelson and yet another former student, Rocky Strong, tested the repellent effect of SDS on large, tagged White Sharks off South Australia. Nelson and Strong found that a dilute solution SDS, squirted directly into the mouth of a feeding White Shark, induced prompt, accelerated departure from the area. A 14-foot (4.3-metre) White Shark squirted with 125 millilitres (about a pint) of 10% SDS darted away and was not resighted at the bait for 12 minutes. Two other White Sharks - 9.5 feet (2.9 metres) and 11 feet (3.4 metres) in length, reacted in much the same way when squirted with 250 millilitres (about a quart) of 10% SDS - but were not resighted near the bait for 144 and 322 days, respectively. Apparently not even the mighty Great White can tolerate having its mouth washed out with soap.

A newer approach to shark repellency is electricity. It had been known that sharks are highly sensitive to electric fields since the early 1960's. In a 1963 popular article, engineer John Hicks demonstrated that an underwater electric current too small to be felt by humans would reliably repel captive Lemon and Tiger (Galeocerdo cuvier) Sharks at distances of up to 50 feet (15 metres). In a 1974 paper, South African physicist E.D. Smith reported that captive juvenile Dusky Sharks (Carcharhinus obscurus) refused to cross a row of electrodes energized by 7 to 10 volts of alternating current; field tests of this electric barrier were intriguing but inconclusive. In the early 1990's, South African inventor Norman Starkey demonstrated that a wire loop immersed in a shark tank and energized by a 12-volt direct current (DC), would invariably cause Dusky and Bull (Carcharhinus leucas) Sharks to flinch and dart away, apparently in a highly agitated state.

Word of this discovery quickly reached the Natal Sharks Board (NSB), a governmentally-funded organization dedicated to protecting bathers, swimmers, and surfers from shark attack. For over 30 years, the NSB had been culling the local shark population via a network of mesh nets set off popular recreational beaches. The nets proved highly effective in reducing the number of attacks at meshed beaches, but the vast majority of sharks caught were small or otherwise harmless and the nets also caught many inoffensive animals - such as dolphins and sea turtles. By the late 1980's, concerned about the ecological impacts of its operations, the NSB was eagerly searching for new methods of repelling sharks without killing them. Starkey's DC-electrified wire seemed to offer a promising alternative. But would this technology reliably repel White Sharks?

By 1991, NSB had devised a prototype of Norman Starkey's electric shark repeller designed to be worn by scuba divers. The device consisted of two electrodes - one to be worn on an arm, the other on a leg - powered by a 48-volt battery and controlled by a wand-like mercury switch that, when inverted, would complete the circuit and allow a protective electric field to form around the wearer. This 'shark shocker' was tested against baited White Sharks off Dyer Island, South Africa, by famed Australian underwater cinematographers Ron and Valerie Taylor. Valerie wore the shark shocker while Ron filmed the experiment. Valerie waited until a White Shark approached to within about 5 or 6 feet (1.5 or 2 metres), then switched the device 'ON'. Time and again, each White Shark reacted by slamming shut its gill slits, strongly depressing its pectoral fins, and accelerating away rapidly. Within a few hours, all the White Sharks in the area learned to be very wary of Valerie and kept a respectful distance from her. Extensive tests (completed in 1997) at Dyer Island by NSB shark biologist Sheldon Dudley demonstrated that - in over 200 approaches - White Sharks were unable to access baits protected by the shark shocker. The device seemed to work like magic.

NSB produced a second-generation version of the shark shocker, designed to be mass marketed under the name SharkPOD (for Protective Oceanic Device). The SharkPOD consists of an electrode that is strapped to one of a diver's fins and a small unit that contains a second electrode and the rechargeable battery (good for 90 minutes of continuous use); this unit is strapped to the diver's air tank. Several SharkPODs were snapped up by documentary film-makers, eager to test the amazing device for themselves. In these filmed tests, the SharkPOD achieved various degrees of success in repelling Bull, Tiger, Lemon, Blue, and Caribbean Reef (Carcharhinus perezi) Sharks. In 1997, underwater cinematographer John McKenney and Australian aquarist Ian Gordon decided to test the SharkPOD on a White Shark off South Australia. McKenney & Gordon had been continually chumming for days, and had lured quite a few White Sharks. To obtain closer shots of the circling Great Whites and set a target tuna to which an activated SharkPOD had been strapped, McKenney and Gordon exited the shark cage. Almost immediately, a 13-foot (4-metre) White Shark keyed on the divers and zoomed in, causing McKenney and Gordon to scramble for the safety of the cage. As the shark began passing the cage, it suddenly swooped down and calmly ate the tuna with activated SharkPOD attached. At least in this case, the SharkPOD did not 'protect' the tuna.

The White Sharks in McKenney and Gordon's failed SharkPOD experiment were stimulated by a great deal of bait and had even been encouraged to feed on pieces of tuna before the experiment was conducted. The SharkPOD was never intended to prevent a feeding shark from continuing to feed, merely to keep a potentially dangerous shark at bay. In addition, the SharkPOD was not intended to be switched on suddenly to 'zap' approaching sharks - which probably, of itself, causes the strong flinch behavior noted in the Taylors' White Shark trials. Rather, the SharkPOD was intended to be activated at all times during a dive in the company of several large, potentially dangerous sharks. Used properly, the SharkPOD generates a shark-repelling electrical field that extends for a radius of 12 to 21 feet (4 to 6 metres). But the device is expensive ($600 to $800), and most recreational divers are very unlikely to find themselves in situations that warrant using a shark repelling device.

As a shark behavioral ecologist and former diving instructor, my biggest concern is that the SharkPOD may grant inexperienced divers a false sense of security, perhaps inspiring them to take foolish risks in the company of baited sharks. Left in peace to go about their own business, the vast majority of sharks under most diving conditions do not pose a significant risk to humans. But there is no device so powerful that it can protect the user from his or her own stupidity.


Important New Book on Shark Attacks!

The culmination of four decades' of research by my friend and colleague Ralph Collier, President of the Shark Research Committee, this book is the first scientific study of every verified shark attack that occurred along the Pacific Coast of North America during the 20th Century.  Vivid accounts of attacks by survivors, rescuers, and witnesses are punctuated with chilling, never-before published photos.  Patterns in shark attacks are identified, possible motivations for attacks are discussed, and activity-specific safety guidelines for swimmers, divers, surfers, and sea kayakers are offered.  The individual case histories are fascinating; the general conclusions and safety guidelines are applicable word-wide.  If you are interested in Great Whites or shark attacks, this is a Must Have book.


ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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