First came the dead fish, then invasive plants. A year later and Lake Suchitlán’s pollution remains a mystery

By August 2025, nearly 70% of the lake’s 135 sq km (33,000 acres) surface was carpeted with an invasive species, water lettuce (Pistia stratiotes). In the following months, plastic waste accumulated along the shoreline, dead fish became more frequent and residents who rely on fishing the lake for income reported that their livelihoods were deteriorating.

Nearly a year later, no explanations have been provided; the Salvadorian authorities, under President Nayib Bukele’s authoritarian government, remain silent, and questions about responsibility go unanswered.

The die-off did not occur in a remote ecosystem. Lake Suchitlán is a Ramsar site – a wetland home to 12 of El Salvador’s 14 native fish species, as well as cougars and ocelots at risk of extinction. It is also a reservoir feeding the Cerrón Grande dam, which supplies roughly 28% of the country’s hydroelectric power.

If Lake Suchitlán collapses, at stake is the nation’s food security, power generation and public health, say experts.

For fishers, who earn about $15 (£11) a day, the lake’s 2025 collapse forced them to join clean-up crews and rely on relatives’ income or crops. The military was also mobilised to assist with the operation.

The pollution has also affected tourism, one of the few alternative sources of income around the lake. Local guides say visitors who once came for boat tours, birdwatching and lakeside restaurants stopped coming as the water turned opaque and foul-smelling.

Alberto Castillo, a boat operator in the nearby town of Suchitoto, says: “The clean-up seemed impossible.

“People are starting to come back very slowly, but during these months we had to take different jobs, getting only 30% of what we were making before.”

Scientists and local environmental organisations had warned for years that untreated sewage, agricultural runoff and weak water-quality enforcement were pushing the lake towards collapse.

Gabriel Cerén, a biologist, says the nutrient overload in the water was severe. “What facilitates the reproduction [of the water lettuce] is the high amount of nutrients that the Lempa River gets from fertilisers that end up in the lake and concentrates a high amount of nitrogen and sulphates,” he says.

Under such conditions, invasive plants such as water lettuce flourish, depleting the oxygen that fish and other aquatic life need to survive. Fish died, mosquito numbers surged, and persistent foul smells rose from the water, marking a turning point that transformed what had long been a gradual deterioration into an ecosystem breakdown.

If there was an official explanation for what happened, it never reached the communities that depend on the lake. In the weeks after the die-off, researchers from the University of El Salvador’s toxicology laboratory (Labtox) were asked to analyse the water.

According to the researchers, the request came through institutional channels linked to the courts – an arrangement under which the lab provides technical support without publishing public reports.

Labtox is one of the few institutions in the country equipped to monitor cyanobacteria, organisms that thrive in nutrient-rich water and can release toxins harmful to humans and wildlife. Their work at Suchitlán focused on measuring nitrogen and phosphorus, key indicators of eutrophication, or over-enrichment of water with nutrients.

Yet they do not test for pesticides or herbicides, including paraquat, the extremely poisonous chemical widely known in the region by the trade name Gramoxone.

By the time sampling was carried out, several weeks after the fish had washed ashore, results showed no anomalies. Nutrient levels were within expected ranges, and no active cyanobacterial bloom was detected.

The researchers cautioned against drawing conclusions. During the die-off itself, monitoring was impossible. Dense mats of water lettuce covered the surface, blocking access to sampling points and disrupting standard measurements, they alleged.

For residents, the gap between what they witnessed and what could be proven has deepened mistrust. Fishers and local leaders say the water lettuce appeared suddenly and was cleared just as abruptly. Videos recorded by residents show agriculture-use drones flying low over the lake in the days before the die-off. No authority has acknowledged deploying drones or using chemicals on the water.

A local newspaper reported that residents travelling by boat across the reservoir were told by fishers and community members that poison or some kind of chemical had been sprayed from the air to eliminate the aquatic plants. Others speculated that a vein of sulphur beneath the lake had been disturbed.

None of these claims has been confirmed. What remains consistent is the silence in a country where, according to campaigners, human rights have been ignored by a steadily more repressive regime, and environmental activism is becoming increasingly dangerous.

El Salvador’s environment, agriculture and health ministries have not commented on how the water lettuce was removed or addressed whether chemicals were used.

In Copapayo, a lakeside community of families displaced during El Salvador’s civil war, uncertainty has again become part of daily life. Avalos, who settled there after the conflict ended, says the scenes on the lake would trouble anyone unfamiliar with it.

Despite concerns, many families continue to eat what they catch as fishing provides one of the few available sources of protein. “We eat them out of necessity,” Avalos says. “Our bodies have had to adapt.”

What is unfolding at Suchitlán is not an isolated episode. Across Central America, lakes and reservoirs are showing signs of strain as pollution, rising temperatures, land-use change and weak environmental oversight converge.

Nearly a year after the dead fish washed up, environmental concerns persist around Lake Suchitlán. In June, the government-run newspaper reported a large amount of rubbish swept into the reservoir after a storm.

Earlier this year, authorities were also forced to respond to a large cyanobacterial bloom in Lake Coatepeque, one of El Salvador’s most important freshwater lakes and tourism destinations. Environmental officials attributed the outbreak of the blue-green algae to high temperatures, intense solar radiation and excess nutrients in the water.

Untreated wastewater and expanding development around Lake Atitlán in Guatemala has also led to recurring cyanobacterial blooms. Lake Yojoa in Honduras faces similar stressors, with communities reporting fish deaths and declining water quality tied to agricultural runoff, deforestation and industrial activity.

Suchitlán offers a particularly human-scale view of that crisis. Tourists have reported skin rashes after swimming in the lake. In Copapayo, residents say mosquito populations exploded after the die-off, making it difficult to sleep at night. Public health reports list gastrointestinal and acute respiratory infections among the most common illnesses in communities around the lake.

Residents have been left with a lake that appears normal again; the fish have returned and the water lettuce is gone. But for those who live along the lakeshore, the crisis has exposed years of structural neglect. El Salvador treats only a fraction of its wastewater, and municipalities upstream discharge effluent directly into rivers that feed the reservoir.

Environmental regulations rarely translate into enforcement, and agencies responsible for monitoring water quality remain chronically underfunded.

“The lake needs an urgent study,” Castillo says. “We’ve had fish die before, but nothing like this. First the water lettuce, then the plastic, now the fish – it demands attention.”

For residents such as Avalos, the concern is not only what happened in 2025, but whether the conditions that created the crisis remain today. He steers his boat through narrow channels cut into the vegetation. “This has become the perfect breeding ground for it to happen again and again,” he says. “It’s pure contamination.”