Endangered pink river dolphins face rising mercury threat in the Amazon

Scientists and veterinarians working along the Amazon in Puerto Nariño, Colombia, are finding alarmingly high levels of mercury in endangered pink river dolphins, a trend researchers say reflects growing contamination of waterways from illegal gold mining and forest clearing.

Teams from the Omacha Foundation and partners routinely capture dolphins in carefully planned operations to take blood, tissue and other samples. The animals are kept out of the water for no more than 15 minutes while researchers perform ultrasounds, swab blowholes and genital areas, implant microchips and collect material for mercury testing and disease screening.

Fernando Trujillo, a marine biologist who directs the Omacha Foundation, said mercury levels recorded in dolphins are many times higher than safe thresholds. "The maximum any living being should have is 1 milligram per kilogram," he said, referring to the World Health Organization guideline of about 1 part per million. "Here, we're seeing 20 to 30 times that amount." Past tests by his team found 16 to 18 milligrams per kilogram in dolphins; in Colombia's Orinoco River some dolphins have registered as high as 42 milligrams per kilogram, among the most extreme concentrations documented in the species.

Mercury in the Amazon is primarily linked to artisanal and small-scale gold mining, where miners use mercury to separate gold from sediment and then discard contaminated sludge into rivers. Deforestation and land disturbance also mobilize mercury that naturally occurs in soils, sending it into aquatic food webs where it concentrates in fish and top predators such as river dolphins.

Mercury is a neurotoxin that can damage the brain, kidneys, lungs and immune system, and prenatal exposure is associated with developmental delays and reduced cognitive function. Researchers say mammals, including dolphins and people, can suffer neurological and organ damage when mercury accumulates. Trujillo noted that his own blood tests three years ago showed 36.4 milligrams per kilogram — more than 36 times the safe limit — a level he attributed to decades of work in mercury-affected areas and a fish-heavy diet. With medical assistance, his levels later fell to about 7 milligrams per kilogram.

Scientists use river dolphins as sentinels of river health because they are long-lived, top predators that accumulate contaminants from the food chain. The capture-and-test campaigns rely on experienced fishermen and veterinarians: crews corral dolphins with nets, haul them into boats and onto riverbanks, and perform rapid medical assessments while documenting skin lesions, scars and evidence of disease.

Omacha's work has identified not only elevated mercury but also antimicrobial resistance, respiratory problems and signs of possible emerging infections such as papilloma virus, which could pose risks to both aquatic wildlife and humans. The organization estimates there are 30,000 to 45,000 river dolphins across the Amazon basin but notes that populations in monitored areas have fallen sharply. Trujillo's long-term monitoring showed a 52% decline in pink dolphins and a 34% reduction in gray river dolphins over recent decades. The International Union for Conservation of Nature listed the pink river dolphin as endangered in 2018.

Mercury exposure among Indigenous and riverine communities across the Amazon has also been documented by academic researchers and international watchdogs. Hair and blood samples in multiple countries — Brazil, Peru, Colombia, Suriname and Bolivia — have shown averages well above the WHO safe threshold, with at least one Colombian community recording hair measurements higher than 22 milligrams per kilogram.

Governments across the region have taken varied steps to address mercury pollution and illegal mining. Colombia banned mercury use in mining in 2018, ratified the Minamata Convention on mercury and submitted an action plan in 2024. Authorities in Colombia point to joint operations with Brazil and enforcement sweeps, but watchdogs say illegal mining remains widespread and enforcement uneven. Brazil has conducted raids and moved to restrict satellite communications that sustain illegal mining camps. Peru has seized large quantities of smuggled mercury, and Ecuador, Suriname and Guyana have filed national plans to curb mercury use in small-scale gold mining.

Researchers and conservationists say the recent surge in illegal mining is driven in part by higher global gold prices and the remoteness of affected waterways, which complicates enforcement and monitoring. In the field, the dolphin health assessments are painstaking and dangerous work: captures can conclude with the animals thrashing and delivering stinging blows to handlers, and teams must repair nets and equipment to repeat the operation.

Trujillo said the scientific work is intended to document the scale of contamination and to spur stronger action to protect both wildlife and human communities. "We take blood and tissue samples to assess mercury," he said. "Basically, we're using dolphins as sentinels for the river's health." He warned that if current trends continue, the species and the people who rely on the river could be pushed closer to critical risk: "We are one step away from being critically endangered and then extinct."