“Unlocking the Mystery: Can Shark Repellent Truly Keep These Apex Predators at Bay?”

In one of the more memorable scenes of the 1966 camp masterpiece Batman: the Movie, the Caped Crusader ascends a rope ladder to the Batcopter when a large – and very rubbery – shark clamps its jaws around his leg. Unable to dislodge the fiendish fish, Batman calls up to Robin in the Batcopter and tells him to bring down the Shark Repellent Bat Spray. Wasting no time, Boy Wonder nimbly descends the ladder, Batman sprays the shark in the face, and the sinister squalus lets go and falls back into the water – only to promptly explode. Holy cinema, Batman! But while Batman’s spray is obviously meant as a joke, does real shark repellent actually exist – and if so, does it actually work? This is the surprisingly fascinating history of shark repellent, which includes work done by famed chef extraordinaire, Julia Child, among countless others, all to try to keep your limbs attached while foolishly deciding to dip your toes into the big blue.

The quest to develop the first shark repellent was launched in the early 1940s by Henry Field, anthropologist to U.S. President Franklin D. Roosevelt. Up until this point, the U.S. Navy had seen little use for such a substance, as shark attacks were exceedingly rare and considered an insignificant threat to naval personnel. In 1941, however, Field learned of an incident in which a navy aircraft crashed in the Pacific Ocean after running out of fuel. The pilot swam for hours to reach the nearest land, dragging the body of his dead copilot behind him. Throughout the journey, he was followed by hungry sharks, who slowly tore his comrade’s body to pieces. While objectively a freak occurrence, the story nonetheless spread like wildfire through the Navy’s ranks. Fearing a serious drop in morale, Field urged Roosevelt to approve research into shark repellent.

Roosevelt agreed, and in 1942, the task of developing an effective shark repellent was given to the newly-formed Office of Strategic Services or OSS – the precursor to the CIA. Initial research was conducted at the Woods Hole Oceanographic Institute in Massachusetts under the leadership of Stewart Springer, an ichthyologist and commercial fisherman and a leading expert on sharks. As large, potentially man-eating sharks are rather difficult to work with, Springer conducted his experiments on metre-long dogfish sharks. The sharks were kept in large tanks and conditioned to eat fresh bait from small wire cages. By withholding food for certain periods of time, Springer was able to adjust the sharks’ hunger level and thus gauge the effectiveness of whatever repellent substance was being tested.

Unfortunately, at the time relatively little was known about the physiology and behaviour of sharks, leaving Springer with little to go on except bits of often dubious “shark lore” accumulated by generations of fishermen. So Springer simply decided to test any substance that might have a repellent effect. He first tried rotenone, a powerful toxin known to kill fish at low concentrations. Like cyanide, rotenone works by interrupting cellular respiration, preventing fish from absorbing and using oxygen and causing them to suffocate. Alas, as Springer quickly discovered, the concentrations required to kill even small sharks would also kill a human. So Springer tried other well-known poisons, such as sodium cyanide and the sedative quinacrine, but these did little to deter the sharks; they simply ate the poisoned bait and died later. Dozens of other substances produced similarly mixed results.

Running out of ideas, Springer turned to some traditional shark lore. Knowing that fishermen along the Florida coast hung shark carcasses from their boats to keep sharks away from their catches, Springer analyzed the fluid extracted from decomposing sharks in order to pinpoint the active ingredients. He discovered that the two most potent compounds were ammonium acetate and acetic acid – both common and inexpensive chemicals. Knowing also that copper sulphate was widely used by sailors and fishermen to deter marine growth like barnacles, Springer combined this with acetic acid to form the compound copper acetate.

Among the dozens of personnel involved in the development of this shark repellent was 30-year-old Julia McWilliams of Pasadena, California. Rejected by both the Army and Navy’s women’s divisions due to her height – she stood 6’ 2” – McWilliams was readily accepted by the OSS and deployed to Ceylon – today Sri Lanka – and then China. Her first assignment was typing up thousands of file cards of government officials, but she soon grew bored of this and directly lobbied OSS founder and director William ‘Wild Bill” Donovan for more challenging work – including standardizing Springer’s shark repellent formula. As it happened, this would not be her last recipe, for after the war she would marry fellow OSS officer Paul Cushing Child, discover a love for French cooking, and forever change the American culinary and television landscape as the one and only Julia Child.

But while copper acetate performed well in laboratory testing, there was no substitute for actual field trials. So Springer’s team relocated to St. Augustine, Florida, a busy fishing port known to be frequented by several varieties of sharks. Here, tests were conducted by hanging two bags of bait off the stern of a slow-moving boat: one untreated to act as a control, and the other fitted with a slowly-dissolving packet of copper acetate. Initial results were remarkable; while 50 sharks of different types were caught on the unprotected bait, the bait protected with copper acetate was only attacked twice.

But, there was a catch: while copper acetate worked well against individual sharks, it was far less effective against large groups engaged in a “feeding frenzy.” As most shark attack scenarios were likely to involve large groups of personnel entering the water at once, this was something of a problem. Serendipitously, however, around this time Springer’s team was joined by chemists from the Naval Research Laboratory, who were experimenting with soluble fluorescent dye packs to help rescue aircraft spot servicemen lost at sea. Inspired by this research, Springer wondered whether adding an opaque dye to the shark repellent might increase its effectiveness, obscuring the user’s body like the ink secreted by octopuses and squid. A suitably dense black dye was soon found in the form of Nigrosine, produced by the Calco Chemical Division of the American Cyanamid Company. Incredibly, in double bait-line testing, this dye proved nearly as effective as the copper acetate – even at night when the dye was not even visible to sharks. Further tests conducted in the Gulf of Guayaquil in Ecuador and Biloxi, Mississippi in December 1942 produced similar results; the team finally had a winner. Springer mixed the Nigrosine with the copper acetate, pressed it into a 3-inch disk-shaped cake, and packed it in a waterproof pouch that could be attached to a sailor or airman’s life preserver. The resulting repellent was dubbed “Shark Chaser.”

However, the Navy remained skeptical, with one lead scientist pointing out in May 1943 that:

“…none of us expected that the chemical would really function when the animals were stirred up in a mob behavior pattern.”

And while Shark Chaser was mainly intended as a morale booster, some worried that issuing it to servicemen would have the opposite effect, amplifying already overblown fears of man-eating sharks. In the end, however, it was decided that some protection was better than none, and Shark Chaser was issued to sailors and airmen along with an illustrated pamphlet titled Shark Sense dispelling some of the more common myths regarding shark attacks.

Distribution of Shark Chaser remained modest until July 30, 1945, when the U.S. Navy suffered the worst shark attack incident in its history. On that day, the heavy cruiser U.S.S. Indianapolis, fresh from delivering parts of the Little Boy atomic bomb dropped on Hiroshima to Tinian island, was torpedoed and sunk by the Japanese submarine I-58. Some 900 sailors found themselves adrift at sea for four days; so secret was the Indianapolis’s mission that her sinking was not immediately reported. In addition to dehydration and hypothermia, the survivors were soon set upon by dozens of oceanic whitetip and tiger sharks, which are thought to have killed up to 150 men – and if this sounds strangely familiar, yes: this is the same disaster Robert Shaw’s character Quint recounts in the infamous monologue scene in Jaws.

In the wake of the Indianapolis disaster, the Navy began handing Shark Chaser like candy. This practice continued for decades, with packets even being issued to the first Mercury and Gemini astronauts in case they had to leave their capsules after splashdown. But while the Navy claimed that Shark Chaser was 70% effective, experiments conducted by the aptly-named biologist Albert Tester at Enewetak Atoll in 1959 confirmed that many had already suspected for years: Shark Chaser was effectively a placebo with negligible repellent effect. By the 1970s, Shark Chaser was no longer being issued.

At around the same time as Shark Chaser was being phased out, researchers discovered that pardaxin, a toxin secreted by a fish called the Red Sea Moses Sole, is highly effective at repelling sharks. However, while extensive efforts have been made to turn pardaxin into an effective shark repellent, these have proven unsuccessful. Not only is the chemical highly perishable and difficult to store, it is only effective at high concentrations, meaning it would have to be injected directly into a shark’s mouth. Another line of research has focused on soap-like surfactant compounds like sodium lauryl sulfate, which can repel sharks at concentrations of 100 parts per million. However, most soluble substances released into open seawater drop below such concentrations within minutes, making this yet another dead end.

A more exotic approach to repelling sharks has been investigated by the U.S. Naval Surface Warfare Center in Panama City, Florida. In 2017, researchers at the center announced that they had successfully created a synthetic version of hagfish slime. A primitive, eel-like creature with no scales, bones, or jaws, when attacked, hagfish secrete a special mixture of proteins that, on contact with water, extend to over 30 centimeters in length, forming a tough, viscous gel that can ensnare and suffocate the attacker. To produce the slime in large quantities without having to harvest hagfish, the NSWC genetically modified E.Coli bacteria to manufacture the required proteins – similarly to how insulin is produced commercially. In addition to repelling sharks, the Navy foresees using the synthetic slime for protecting ships against torpedoes, mines, and other explosive weapons.

Another effective means of repelling sharks is to use electricity. Shark’s noses are dotted with hundreds of small organs called Ampulae of Lorenzini, which detect the electric impulses generated by their prey. Sharks are thus highly sensitive to electromagnetism, and can be deterred by a powerful enough field. Indeed, several companies including Sharkbanz and SharkDefense Technologies LLC of West Milford, New Jersey, already produce small electromagnetic devices that can be strapped to a diver’s wrist or attached to their equipment to help ward off sharks. SharkDefense is also experimenting with the use of synthetic semiochemicals – i.e. shark pheromones – as a potential chemical repellent – a technology that has shown much promise. And in 2014, the KwaZulu-Natal Sharks Board of South Africa announced that it would be installing a system of submerged electrified cables under popular beaches to help keep sharks at bay.

Interestingly, the approach that inspired the development of Shark Chaser – mimicking the odour of dead shark – might just make a comeback. In 2004, researchers Samuel Gruber and Eric Stroud, working at the Shark Lab on the island of Bimini, announced that they had developed a highly effective repellent called A2, derived from extracts of decomposing sharks. Unlike paradaxin, A2 is non-perishable and effective at very small concentrations, with only a few milliliters per minute needed to repel sharks indefinitely from a piece of submerged bait.

Yet despite these advancements, the greatest defense against shark attack is sheer probability. Every year there are an average of 100 shark attacks worldwide, only around 5 of which are fatal. This gives the average person a less than 0.00000001% chance of being attacked by a shark – far less than being struck by lightning or being killed by a falling vending machine. By contrast, some 50 million sharks are accidentally caught each year by commercial fishing boats, while countless more are actively hunted for sport, food, or to protect those very same fisheries. For this reason, many newer shark repellent technologies are being developed not to protect people from sharks, but the other way around.

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