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Another long holiday weekend is starting, and tourists are flocking by ferry and small plane to Vieques, a tiny island off the coast of Puerto Rico that's famed for bright turquoise waters, lush mangrove forests and picturesque free-roaming horses.

In an empty lot near the $500-a-night W Retreat & Spa, a man with a gun is stalking some of those mares. He slowly walks toward a group of brown and white horses, raises a pistol and fires. A brown mare kicks her hind legs and sprints away.

Richard LaDez, director of security for The Humane Society of the United States, picks up a contraceptive dart that fell from the horse's rump and declares, "We're good!" He gives a thumb's up to a team waging an unusual fight to control a tourist attraction that's become something close to a plague on the island, best known as the site of a former U.S. military bombing range.

First imported by Spanish colonists, horses are used by many of Vieques' 9,000-odd residents for running errands, taking children to school, transporting fishermen to their boats, competing in informal races between teenage boys and delivering late-night drinkers back home. They're adored by tourists, who love taking pictures of them eating mangos and frolicking on the beaches.

Many locals keep their horses in open fields near the sea, where they graze until they're needed next. Feeding and sheltering a confined horse on an island with a median income of less than $20,000 a year is out of reach for many. Some horses are branded, many are not and a few just run wild.

Officials say that as a result, it's nearly impossible to control the horse population and hold owners accountable when trouble occurs. The population has grown to an estimated 2,000 animals that break water pipes to quench their thirst, knock over garbage cans in search of food and die in car crashes that have increased as tourists flock to Vieques, which grew in popularity after the U.S. Navy shuttered military operations in the early 2000s.

"There are more and more horses all the time," said Vieques Mayor Victor Emeric. "It's not that easy to solve this problem."

Desperate, Emeric called the Humane Society, which agreed to launch a five-year program of dispatching teams to Vieques armed with compressed-air rifles, pistols and hundreds of darts loaded with the animal contraceptive PZP.

The program began in November and picked up speed with a two-day push by about a dozen volunteers and Humane Society employees over the Martin Luther King Day weekend. More than 160 mares have been darted and Humane Society officials say they expect to inject virtually all the island's mares with contraceptives by the end of the year.

The program will cost up to $200,000 a year to run and is funded entirely through donations.

Stephanie Boyles Griffin, the Humane Society's senior director of wildlife fertility control programs, said she expects the horses to be healthier and live longer, noting that the oldest horses on Vieques are 7 to 10 years old, while the average lifespan for wild horses is 15 to 20 years.

On a recent weekend morning, members of one darting team peered through their binoculars as they waited for an opening to shoot a mare that was due for a booster shot.

"It's hard to confirm who's who," said team leader Kali Pereira as she flipped through a binder containing files identifying all the horses that had already received their first shot.

The logistics of darting are tricky. Wildlife experts have to identify hundreds of mares (sometimes by the direction in which their mane falls), register their GPS coordinates, photograph them, assign them a number and give them their first shot. Then they have to find them again several weeks later to give them a second injection that they'll receive annually.

Pereira squinted and took her shot. The dart whizzed through the air and landed on the rump of a horse that kicked up its hind legs and ran off. The rest of the herd joined her, their manes flying out of sight. The team sighed. They still had to dart several more mares in that group.

Many locals have embraced the darting program and took dozens of their horses to a Humane Society-sponsored event for contraceptive shots and deworming. Among them was 19-year-old Jesus Miranda, who said his family owns six to eight horses that he rides several times a week.

"It's in our blood, you understand?" said Miranda, who knows his horses' birthdays by heart.

He brought two horses to the event, Wifi and Burro Fly, which mingled with other horses including one named Gringo, which had pale blue eyes.

Helping organizers was Juan Feliciano, a fisherman who said he owns 23 horses but only keeps nine of them at his house.

"Almost everybody here grew up on a horse," he said. "Nobody taught me how to ride a horse. They taught me how to ride a beast. They threw me atop and told me, 'Learn.'"

Emeric, the Vieques mayor, said he believes the vaccination program will keep the horses healthy and attract more tourists.

It was welcome news for Juan Angel Santos, a 54-year-old construction worker who said he couldn't imagine Vieques without horses.

"I prefer to ride horses because I never learned how to drive," he said. "It's best to be on a horse."

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Danica Coto on Twitter: www.twitter.com/danicacoto