BAHIA DE COHANA, Bolivia (AP) — Gulls swept down to feast on hundreds of dead and dying giant frogs floating in the rancid waters along a southeastern shore of Lake Titicaca, where the algae-choked shallows reek of rotten eggs.

The die-off was the most striking sign yet of the deteriorating state of South America’s largest body of fresh water.

Local fishermen have a harder time finding anything to catch, while farmers who work the land along the shores complain that tainted water is stunting crops.

As human and industrial waste from nearby cities increasingly contaminate the famed lake that straddles the border between Bolivian and Peru, the native Aymara people who rely on it for food and income say action must be taken before their livelihoods, like the frogs, die off.

“We used to live off of fishing,” said Juan Quispe, a local villager. “But now we have nothing to sustain us.” The fish have moved farther and farther from shore.

On a recent Saturday, the 78-year-old Quispe joined a cleanup brigade to remove dead dogs, tires and other refuse from the shore of Cohana Bay where the lake meets the Katari River.

Near-shore fishing was good until about 2000, when locals began to notice that the crystal azure waters periodically would turn a murky green, Quispe said.

Most pollution on the Bolivian side, including such toxic heavy metals as lead and arsenic, originates in El Alto, a fast-growing city of 1 million people near La Paz that sits 600 feet (200 meters) above the lake and just 25 miles (40 kilometers) away.

Seventy percent of El Alto’s 130 factories operate illegally and are not monitored for pollution, says Bolivia’s environment ministry. Runoff from mining exacerbates matters.

A study by the binational Lake Titicaca Authority found elevated levels of iron, lead, arsenic and barium in the water, the worst at the mouth of the Katari, which flows from El Alto.

Contamination is most serious in the shallow waters of Cohana Bay, near the popular tourist spot of Copacabana and, on the Peruvian side, Puno Bay and near the Ramis and Coata rivers. The latter flows from the city of Juliaca.

Urban runoff is not solely to blame. More than half the people living along the shores lack plumbing and existing local water-treatment plants are badly overtaxed, the lake authority says.

To date, the only true remediation has been sporadic algae cleanups, authority president Alfredo Mamani said. “It’s like cleaning a pus-oozing wound without attacking the cause.”

Bolivia and Peru created the authority to manage the body of water but have given it few resources to do so, he said. While Mamani declined to disclose the authority’s budget, he said it has 30 employees and no money for equipment or projects.

During a meeting Tuesday in Peru, the presidents of the two countries agreed to strengthen the authority and to form a binational commission that, over the next six months, will come up with a plan to help the lake and finance the effort.

Long before the frog deaths in April, the authority asked Peru and Bolivia for permanent monitoring efforts and laboratories to measure contaminants entering the lake. Even without such resources, the signs of contamination are obvious.

Mamani blames the frog kill on untreated sewage and other waste that distill into a hydrogen-sulfite cocktail that chokes the life out of near-shore aquatic habitats.

“It is time that urgent and coordinated measures be taken,” Mamani said.

While only a small portion of Titicaca’s waters are polluted, the affected areas are along shores where more than a half-million Aymara people live, he said.

Trout farms and nearby agriculture also have suffered.

Quispe said the potatoes he grows near the shoreline have shrunken over time, a change he blames on contamination of the lake waters, which partially cover his fields before each growing season.

Locals fear the tourism industry is next. Each year, some 750,000 tourists visit the 12,470-foot-high (3,800 meter-high) Lake Titicaca for its reed boats, pre-Columbian ruins and majestic views of snowcapped Andean peaks.

Villagers from Puerto Perez, who paddle tourists on the lake on weekends, showered Bolivia’s environment minister, Alejandra Moreira, with complaints at a meeting in a nearby village in May.

She suggested the 46 communities on Titicaca’s shores and islands, which are among the poorest in the two Andean nations, pool funds for expanded sewage systems and treatment plants.

The village secretary, Guillermo Vallejos, called her response worse than inadequate.

“When we have problems, officials come, take pictures and leave,” he said. “Rarely do they return. We must do everything ourselves to save the lake.”

Deputy Environment Minister Ruben Mendez later said that the Bolivian government plans to raise tens of millions of dollars to build waste treatment plants along Titicaca’s shores.

Details about the plan, however, have yet to be provided.

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Associated Press writer Frank Bajak in Lima, Peru, contributed to this report.