Where once the waters here were almost barren of life, there are now vast schools of randy, spawning big-eyed jacks. The grouper lurking in the thriving coral have reached the size of overindulged toddlers. The tiger sharks are big enough to star in scary movies. The nesting turtles, devil rays and sea lions have returned, too, alongside a few tourists wearing flippers and masks.

Marine scientists express pure astonishment at what has happened in the 16 years since Cabo Pulmo National Marine Park was created at the southern tip of Mexico’s Baja California state, 60 miles north of Cabo San Lucas.

In August, researchers reported that the biomass of fish in the no-take marine reserve had increased by an unprecedented 463 percent in 10 years, offering hope that, if just left alone for a little while, the planet’s depleted seas can rebound.

But all is not well in Cabo Pulmo.

These days, the talk in the little solar-powered village of 200 Mexicans and expats is not the vigor of the reefs but, of all things, the European debt crisis.

On an empty spit of shadeless sand just a few miles north of the marine reserve, an ailing Spanish development conglomerate called Hansa Urbana plans to erect an all-inclusive mega-resort, a city of sun and fun the size of Cancun, according to its Web site, with 30,000 hotel beds, three golf courses, an airport capable of serving small jets, a major yacht marina, a shopping mall, a desalination facility and a wastewater treatment plant. A sister city would inevitably arise in the desert to house an equal or greater number of waiters, caddies and maids to serve the resort, called Cabo Cortes.

U.S. and Mexican environmental groups are appalled at the prospect of another Cancun here, so close to one of the northernmost coral reefs in the world. Greenpeace labeled it the worst kind of “predatory tourism.” A U.N. delegation is coming this month to investigate what is happening to its World Heritage Site. A letter-writing campaign — with crayoned cards by elementary-school children — has begun, imploring President Felipe Calderon, who bills himself a leader on sustainable tourism and climate change, to intercede to halt the project or scale it way down.

As for the developers, Omar Vidal — the director general of the World Wildlife Fund in Mexico, here to snorkel the reef with his daughter — said environmental activists are not even sure who controls the resort project anymore.

Ripples from Europe’s crisis

Hansa bought the Cabo Cortes property in 2007 during the global real estate boom. A Spanish savings bank, Caja de Ahorros del Mediterraneo (CAM), lent Hansa millions. But the bubble burst, and the bank failed a stress test in July. The government-controlled Bank of Spain was forced to bail out CAM with $3.8 billion.

CAM had avoided the subprime mortgage sludge in the United States, but it was up to its eyeballs in tanking European real estate. To stay afloat as the economy worsened, Hansa gave CAM the assets in Mexico, and, in return, the bank wiped out $114 million of Hansa’s debt. The Bank of Spain was planning to auction the floundering CAM to the highest bidder but has delayed the sale until after the Spanish elections this month.

“Calderon has given a message to the world, that he is a leader on the environment. And this project is not right; nobody wants it,” Vidal said. “This is the chance for our president to make his legacy, to say, ‘Here, in Mexico, we will do things differently.’ ”

Neither CAM nor Hansa responded to telephone calls and e-mails seeking comment.

“Maybe we will be saved by the economic crisis in Europe,” said Judith Castro, a village activist working to protect the Cabo Pulmo reef. Castro’s family settled here in 1900, and her grandfather, and then her father and uncles, dived for pearls in the gin-clear waters, until the pearls were all gone, and fished the reefs, until they were depleted, too.

“They knew they had a beautiful garden in the ocean, but each year, less fish, smaller fish, farther away,” she said. “The university came to study the reef and said, ‘Hey, do you know what you have here?’ We saw the damage, a lot of damage, and we knew we had to change, and we supported the creation of the national park, and we think we did the right thing.”

The Castro family now makes its living running a dive shop and other concessions, and the local community zealously guards the reef.

“The place is astounding, and I’ve dove in the most pristine waters in the world — in the Galapagos, in Antarctica. And I have never seen anything like this,” said Soames Summerhays, the award-winning documentary-filmmaker. “This” being the health and biodiversity of Cabo Pulmo reefs.

“I dove the reef in 1993, before the park was established, and to be honest, it was not very impressive,” said Exequiel Ezcurra, director of the University of California’s Institute for Mexico and the United States. “But now nobody can believe how it has rebounded.”

‘Nobody’s for it’

The scientists say that the resort development could flush its sewage and silt into the ocean, whose polluted currents may flow south and smother Cabo Pulmo. They say there is not enough water to sustain the resort and that, in Mexico’s laissez-faire way, the project will spur more development.

“Nobody’s for it. It will hurt us all — reef, fish, local businesses,” said Mike O’Dell, a master angler who has caught (and mostly released) more than 1,300 marlin and runs a local RV park.

Hector Flores, who builds houses in the area, said: “We need some more work around here, true, but I agree, the project is just too big. The permits were shoved through by the thieves in Mexico City,” meaning the government.

Hold on, said Juan Rafael Elvira Quesada, the cabinet-level secretary of Mexico’s department of natural resources and the environment, which is overseeing permissions.

“Without a doubt, this is the most important project of this administration,” Elvira said, “and our priority is to protect the coral reefs, which are the most precious in the entire Sea of Cortez.”

Elvira said that although the resort city has been given preliminary approval, the government is demanding that the developer prove that the ocean currents will not flow south loaded with silt, salt or sewage.

No construction has begun, he said. “I realize the company is upset. They are not happy because, they say, you give us a permit but you don’t allow us to do things.”

The Calderon government is consulting with expert marine biologists and U.N. officials.

The deadline for submission of data from the Spanish developer, or whoever ends up owning the property, is January 2013 — a month after Calderon leaves office.

Such timing makes environmentalists nervous.

“You could say that it is very fishy,” said Vidal of the World Wildlife Fund, referring to the permitting process. “But it is not funny. The project should be stopped now.”