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SAN JUAN, Puerto Rico - Excited by the scent of blood, a dozen sharks dart about in a frenzy as a researcher dips a pole in the sea and squirts out a clear, yellowish substance. Within seconds, the sharks jerk their snouts away and vanish.
Researchers say they finally have found a potent repellent to drive away sharks, after testing off Bimini island in the Bahamas. It's a goal that has eluded scientists for decades.
If proven effective, the repellent might protect divers, surfers and swimmers. But researchers say that would require much more study. First they hope it can protect sharks - in decline worldwide due to overfishing - by reducing the numbers caught by long-line commercial fishermen.
"You introduce this chemical, and they all leave," said lead researcher Eric Stroud. "It works very, very well."
The repellent 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," said Samuel Gruber, a University of Miami marine biologist and shark expert who is helping conduct tests.
Tests have found the repellent effective on four species: the Caribbean reef, blacknose, nurse and lemon sharks. Studies are needed on other species.
Gruber said the repellent seems to carry a chemical messenger that triggers a flight reaction. He said more studies are needed to pinpoint the active molecule.
A dose of 4 fluid ounces is enough to scare away feeding sharks, Stroud said, keeping them away from a fish head for two hours with just a few drops per minute. Sharks didn't 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, Okla. 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 nontoxic, 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.
Eugenie Clark, who discovered a natural repellant in the 1970s that was not practical for human use, said the latest findings could be a welcome way to reduce accidental killing of sharks, though she is skeptical 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," said Clark, who now works at Mote Marine Laboratory in Sarasota.