While villagers chose new livelihoods, the fish population boomed, and big predators such as tiger and bull sharks, marlin, tuna, wahoo, snapper, grouper and sailfish also thrived. The predators caused smaller species to reproduce more quickly, strengthening the entire marine ecosystem.

The Cabo Pulmo reefs hold 11 of the 14 species of coral found in the Sea of Cortez, which also is known as the Gulf of California. Its regular inhabitants or visitors include five of the world's seven endangered species of sea turtles.

Oceanographers report amazement at what they see. Aburto-Oropeza said he flew in an ultra-light aircraft earlier this year and spotted a congregation of sharks in the reserve.

"There were up to 200 sharks in a small part of the reef. It was unbelievable. In my 20 years of diving in the gulf, I hadn't seen anything like that," he said.

On a recent dive, he said, he witnessed "a group of about 20 large fish, groupers and snappers, eating a bunch of grunts that were between 50 and 70 centimeters," a foot and a half to more than 2 feet long. "It was an incredible spectacle."

The marine park, which was named a UNESCO World Heritage Site in 2005, was clearly one of the reasons that Hansa Urbana, a development company based in Alicante, Spain, chose in 2007 to plan a major tourism development in Baja California.

Cyclone fencing surrounds much of its 15.4-square-mile site, and no ground has yet been broken. The firm's website and statements by company officers pledge three golf courses, a 490-slip marina, a jetport, 15 hotels and numerous condos, equivalent to 30,692 hotel rooms or 10,230 three-bedroom condos or houses. Construction would extend over three or four decades. For comparison, Cancun, Mexico's vast tourist development on the Caribbean, has more than 32,000 hotel rooms.

"Cabo Cortes will provide select visitors with the ultimate vacation experience of Mexico, and, for a fortunate few, second homes equal to anything the world can offer," the website says.

Residents in the nearby port of La Ribera, population 3,000, generally support the Cabo Cortes project, enticed by promises that it will create 19,000 direct and indirect jobs.

"Imagine how the merchandise will fly off the shelves around here," said Jose Leal, a fisherman. "You have to be in favor of development as long as it pulls you along, too."

Leal said local fishermen had been promised berths in the marina for their fishing boats, and that he'd receive a concession to provide ice for boaters.

Among leading marine scientists, the mood is far less cheery. National Geographic explorer-in-residence Sylvia Earle and 21 other U.S., Canadian, Mexican and Costa Rican scientists wrote to UNESCO headquarters in Paris in May to say the Cabo Cortes project "could cause irreversible harm to this unique and vulnerable reef" with its "overwhelming pollution."

Echoing that opinion, residents in Cabo Pulmo say the region's aquifer can't sustain such a large project, golf courses will expel chemicals into the sea and the building of the marina (where no natural bay exists) will send sediment southward.

"The dredging for the marina will create turbidity and hurt the reef. Fish will flee," said Judith Castro, one of the community's most vocal activists.