Researchers say they have finally found a potent repellent to drive away sharks – a goal that has eluded scientists for decades.

If proven effective, the repellent, tested off Bimini island in the Bahamas, one day might protect divers, surfers and swimmers.

That, scientists say, would need much more study, but first they hope it can actually protect sharks – in decline worldwide due to overfishing – by reducing the numbers caught needlessly by long-line commercial fishermen.

“You introduce this chemical, and they all leave,” said lead researcher Eric Stroud, a 30-year-old chemical engineer from Oak Ridge, New Jersey. “It works very, very well.”

The repellent, called A-2 because it was the second recipe tried, is derived from extracts of dead sharks that Stroud gathered at New Jersey fish markets and piers. Fishermen and scientists have long noted sharks stay away if they smell a dead shark.

“We have something that really works, but research remains to be done,” said Samuel Gruber, a University of Miami marine biologist and shark expert who is helping conduct tests at the Bimini Biological Field Station.

Tests have found the repellent effective on four species: the Caribbean reef, blacknose, nurse and lemon sharks. Studies are needed on other species such as the great white, mako and oceanic whitetip.

Gruber said the repellent seems to carry a chemical messenger that triggers a flight reaction. He says more studies are needed to pinpoint the active molecule among a dozen or so.

A dose of four fluid ounces is enough to scare away feeding sharks, Stroud says, keeping them away from a fish head for two hours with just a few drops a minute. In contrast, sharks did not respond to a red dye in control tests.

The researchers presented their work in May during a meeting of the American Society of Ichthyologists and Herpetologists in Norman, Oklahoma. Films of their tests captured images of sharks splashing the surface as they turn to flee.

They hope to make a slow-dissolving repellent for use in baits and fishing nets, and to guard equipment on submarines and oil exploration vessels that sharks have damaged in the past.

The repellent, though non-toxic, is apparently so disagreeable to sharks it can revive them from semiconsciousness. Some species slip into a hypnotic state if turned belly-up, and tests found the repellent brought captive sharks out of that trance.

Repellent research began in the Second World War, when the US Navy created “Shark Chaser” for sailors and downed pilots. Mixed with black dye, it was made of copper acetate, which scientists thought would smell like a rotting shark. Studies later showed it was not that effective.

A promising find came in 1972, when University of Maryland shark expert Eugenie Clark discovered that a Red Sea fish, the Moses sole, secreted a milky substance that repelled sharks.

The finding caused a stir, and soon the makers of Coppertone suntan lotion contacted Clark, hoping to market it. She said she discouraged them, saying initial research could not back up such a use.

Years of study followed by Gruber and others. In the end, though, the repellent derived from the sole was not practical because it had to be squirted into a shark’s mouth to be effective.

Clark – who at 82 still works at Mote Marine Laboratory in Sarasota, Florida - said the latest findings could be a welcome way to reduce accidental killing of sharks, though she is sceptical of human use, saying few would be carrying the repellent at the rare moment it’s needed.

“I’d be happy to see somebody work it out, but I don’t see it as a practical solution,” she said.

Stroud got the idea to pursue a repellent after several 2001 shark attacks drew widespread attention, including one that nearly killed an eight-year-old boy near Pensacola, Florida.

Stroud and engineer Mike Herrmann do lab work in a New Jersey warehouse, relying on donations of less than €454,000 from two private benefactors.

They have a patent pending and are starting a company, Shark Defence Inc, to eventually market the repellent.