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’s eluded scientists for decades.

If proven effective, the repellent one day 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 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, N.J. “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,” 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 seemed to carry a chemical messenger that triggered a flight reaction. He said more studies were needed to pinpoint the active molecule among a dozen or so.