AHÍA DE LOBOS, Mexico, April 5 — The fishermen set out before dawn from this dirt-poor town to the Gulf of California. They returned with seven crabs and a baby shark.

"There just aren't any fish anymore," said Teresa López, 39, a villager. "Less and less every year for many years. Now we haven't enough to eat."

Greed and corruption are draining the gulf, also known as the Sea of Cortés. It is not dead yet, but it is exhausted.

American and Japanese ships were the first to exploit it. Now fleets of Mexican fishermen, mostly unlicensed and ungoverned, are taking whatever they can, as fast as they can, for American and Asian markets. Every important species of fish in the sea is in sharp decline, fishermen and marine scientists say.

"Too many fishermen and not enough fish," said Pedro Álvarez, pulling tiny mullet from his net near the city of Guaymas.

Overfishing is a global problem. People are taking marine life faster than it can reproduce. The world's catch peaked at 86 million tons in 1989, up fourfold in 50 years.

But many governments, including the United States, Mexico, the European Union, Japan and China, kept on pouring subsidies into commercial fishing fleets to keep them afloat.

Crucial fisheries have collapsed worldwide.

"We have an endowment in the bank, and we're spending it all instead of living off the interest," said Juan Manuel García Caudillo, a Guaymas conservationist trying to protect the Gulf of California.

This, the world's youngest sea, created when the San Andreas fault split Mexico millions of years ago and let the Pacific pour in, is home to 875 species of fish and 30 species of marine mammals. They have been killed indiscriminately for years.

"The philosophy is: get it now; grab it — if I don't, the next guy will," said Juan Pablo Gallo, a marine biologist in Guaymas who has recorded steep declines in sea lion populations and has found DDT residues among dolphins in the gulf.

Some say the trouble began when the United States started damming and diverting the Colorado River in the 1930's. The river that carved the Grand Canyon became a bare trickle at its Mexican mouth, turning the gulf's biggest estuary, a bountiful breeding ground, into a dried-up delta.

Before and after World War II, American ships took every school of tuna and every swarm of sardines they could, along with sea lions for pet food and sharks to use the livers to remedy iron-poor "tired blood." The Japanese came too, "destroying the ecological balance of the whole region," John Steinbeck wrote in 1940.

The foreign boats, many buying permits and government concessions with bribes, worked the gulf hard until the catch started plummeting about a decade ago.

Then the great divide between Mexico's laws and its law enforcement began taking its toll. In 1992, President Carlos Salinas, fighting for the free-trade agreement with the United States, essentially deregulated Mexican commercial fishing without creating an effective system of licensing and permits.

"The political mistakes of past governments had a terrible effect," said Otto Clausen, the federal environmental protection officer for the state of Sonora, which is bordered on the west by the gulf. "The economic development strategy was wrong. It broke all authority over fishing."

Jerónimo Ramos, the national fisheries commissioner, is based in Mexico City. He said about 1,200 permits existed for boats in the Gulf of California and the Pacific Ocean, estimating that 20 to 30 percent of the catch was being taken illegally.

León Tissot of Mexico's National Fishing Industry Council says that "there is an illegal traffic in permits." Fishermen say permits are bought under the table, sold and resold, ignored with impunity.

"The laws are totally clear, and their application is totally cloudy," said Felipe Rodríguez, a scientist working with the fishermen of the Seri Indian tribe.

Fishermen, businessmen, scientists and even some federal officials say at least 12,000 unregulated fishing boats, probably more, now are at large in the gulf, a number that doubled in the last decade.

"It's the law of the jungle out there," said Luis Bourillón, a marine biologist in Guaymas. "You can do anything you want."

The unregulated boats, whose crews include thousands of men who came to the coast in the 1990's looking for a living, set gill nets, nylon webs banned by many nations as a barbaric and indiscriminate form of fishing, but not in Mexico. More than 1,000 miles of gill nets were sold in Sonora last year.

Gill nets trap everything: endangered sea turtles, sea lions, even the vaquita, a rare porpoise on the edge of extinction. They take so many sailfish, tuna and marlins that the rich American sports fishermen who considered the gulf a paradise are staying home — another drain on the local economy.

A gill net fleet backed by unknown financiers appeared seven years ago in Sonora. Fishermen and scientists say it slaughters thousands of sharks solely for their fins, which when dried sell for as much as $300 a pound in Asian markets.

The fishing boats also play out long lines, each with hundreds of baited hooks, reaching for miles. The long-liners land as much as 20 tons a day of dorado, sold as mahi-mahi, in the port of Guaymas alone, along with unrecorded illegal catches like sea turtles, which can sell for as much as $200 apiece in Mexican black markets.

The high price of turtle meat and shark fins, founded on male folklore long predating Viagra, spurs the fleet.

The shrimp fleet wreaks its own separate havoc. Shrimping throughout the world uses bottom-scraping dragnets that haul up 10 pounds of life — often young fish too small to sell — for every pound of shrimp, like gathering wild mushrooms with a bulldozer.

Underwater, "one day there's all kinds of fish, crab, octopus, maybe a turtle, and the next day it's empty, nothing but rocks and a sandy bottom," said Feliza Ríos, a scuba diving instructor in San Carlos who has seen the effect at first hand. "It takes years, many years, to come back."

Discarded shrimp nets do more damage: one strangled three whales last week.

A pound of Mexican shrimp sells for $16 or more in American markets, and though Mexico no longer directly subsidizes shrimp boats, it underwrites the fleet through a quasi-governmental, California-based corporation called Ocean Gardens, which buys half of its catch. So the shrimpers work the sea floor as hard as they can.

Recognition is now dawning that if nothing changes, "in a few years, you could end up without any fish in the sea," said Víctor Lichtinger, Mexico's environmental minister.

Change may begin with the Mexican government observing its own laws.

"We have to prevent the sales of permits under the table," said Juan Carlos Barrera, the World Wildlife Fund's representative in Sonora. "The answer is zero tolerance for corruption. If we do nothing, we'll kill the sea."

The Mexican Navy has standing orders to police the outlaws by chasing them from the sea. If not, fishing villages may take the law into their own hands. The Seri Indians, Mexico's smallest tribe, already have.

At gunpoint, they guard the waters around their fishing village, Punta Chueca, barring outsiders or demanding tribute in the form of shrimp or money. The other afternoon, their boats came back with a bushel of scallops, a peck of crabs and a giant manta ray, a haul that meant the difference between subsistence and the near-starvation stalking Bahía de Lobos.

The sea can revive if overfished areas are given a rest. But as Josefina Molina, a 45-year-old Seri woman in Punta Chueca, said, "If the sea takes a siesta, how are the people going to eat?"