The result is a yellow, "semiochemical mixture consisting of nucleotides, biogenic amines, and fatty compounds," which works as a chemical warning messenger, said the researchers in a press release.
Called A-2 because it was the second concoction tried, the repellent has proved effective on six species of sharks. It has yet to be tested on species such as the shortfin mako, the great hammerhead and the great white shark, responsible for the most human deaths.
In one of the tests off Bimini island in the Bahamas, the researchers dropped a "cloud" of the mixture from a boat into a chum-filled sea of feeding sharks.
"Populations of up to twelve Caribbean reef sharks were reduced to zero within 180 seconds when 200mL doses of the semiochemical mixture was introduced into the feeding zone ... . It works very, very well," Stroud said.
The sharks responded to even minute proportions of the chemical and the smaller fish in the area were totally unaffected by the repellent.
Though nontoxic, the mixture is so powerful that it can even revive sharks from the natural state of paralysis that occurs when they are turned belly-up, the researchers said.
"This state is called tonic, and it can last for up to 15 minutes," Stroud said.
The hypnotic state is so deep that surgery without anesthetic can be performed during this immobility. Tests showed that the A-2 repellent caused immobile sharks to wake up and flee — right away.
The search for an effective shark repellent begun in World War II, when the U.S. Navy created a chemical repellent called Shark Chaser to protect sailors and downed pilots.
Made of copper acetate, which scientists thought would smell like a decayed shark (ammonium acetate was found to be the major constituent in decomposing shark flesh) and mixed with black dye, the repellent turned out to be ineffective.
Since then, researchers have tried everything from chemicals and cages to electric fields, with only limited success.
A promising finding came in the early 1970s, when marine biologist Eugenie Clark discovered that milky fluid secretions of a Red Sea fish, the Moses sole, acted as a shark repellent.
However, the product proved to be rather impractical as it had to be delivered directly into the shark's mouth to be effective.
Stroud and colleagues hope to use the repellent in bracelets or sunblock lotions. Sharks too would benefit from the product, since they would not be accidentally killed by fishermen seeking tuna and swordfish.
The researchers estimated that, with a shark repellent placed in baits and fishing nets, most of the 100 million sharks that are killed every day would be saved.
