Rising Mercury from Gold Mining Threatens Endangered Pink River Dolphins in the Amazon

Scientists and veterinarians working along the Amazon are finding dangerously high mercury levels in endangered pink river dolphins, a sign of widespread contamination driven largely by illegal gold mining and land disturbance. Teams conducting field health assessments say mercury concentrations in some dolphins are tens of times higher than safe limits, raising alarms for wildlife and riverside communities that rely on fish for food.

Researchers from the Omacha Foundation and partner veterinarians carry out painstaking capture-and-release operations to collect blood and tissue samples, ultrasound readings and other health data within a strict 15-minute window while animals are out of the water. The hands-on work is intended to use dolphins as sentinels for the broader health of river ecosystems and the people who depend on them.

Field leader Fernando Trujillo described a routine that blends local fishing expertise, veterinary care and conservation science. Teams enclose pods with mesh nets, haul individuals into small boats and ferry them to riverside sites where researchers check breathing, wet the animals’ skin, perform ultrasounds of lungs and hearts, swab blowholes and genital areas for pathogens, implant microchips and take samples destined for mercury testing. The work also screens for other threats, including respiratory problems, antimicrobial resistance and emerging viruses that could affect both dolphins and people.

Mercury enters Amazonian rivers mainly through artisanal and small-scale gold mining, where miners use mercury to separate gold from sediment and then discard contaminated sludge back into waterways. Forest clearing and soil erosion also mobilize naturally occurring mercury into aquatic systems. Rising global gold prices have helped expand mining activity across remote parts of the basin, researchers say.

Trujillo said international health benchmarks set 1 milligram per kilogram as a reference maximum for living beings, but tests in recent years have revealed levels in dolphins that are 16 to 18 milligrams per kilogram on average and, in some places, far higher. In Colombia’s Orinoco River, researchers reported levels reaching 42 milligrams per kilogram in some animals — among the highest recorded for the species. Trujillo and colleagues have found evidence of mercury-linked neurological and organ damage in mammals and warned that any mammal exposed to very high concentrations can die.

Mercury is a potent neurotoxin that can harm the brain, kidneys, lungs and immune system, with particular danger to pregnant women and young children because prenatal exposure is associated with developmental delays and reduced cognitive function. Several scientific studies and monitoring programs have documented elevated mercury in hair and blood samples of Indigenous and riverside communities across Brazil, Peru, Colombia, Suriname and Bolivia, sometimes far above World Health Organization safety thresholds.

Dolphin populations in parts of the Amazon have also declined markedly. Long-term monitoring by Trujillo’s group shows a roughly 52% drop in pink river dolphins and a 34% decline in gray river dolphins in the monitored area over recent decades. Exact basin-wide totals are uncertain; Omacha estimates 30,000 to 45,000 pink dolphins across the Amazon. The International Union for Conservation of Nature listed the pink river dolphin as endangered in 2018.

Fieldwork is physically demanding and can be stressful for animals. Local specialist José “Mariano” Rangel, a former fisherman who helps lead captures, said enclosing and hauling dolphins — which can weigh up to about 160 kilograms (353 pounds) — requires strength and coordination and sometimes results in the animals thrashing to break free. Teams limit handling time and use measures intended to reduce stress, such as covering the animals’ eyes and constant monitoring of respiration.

Colombia, where many recent captures and tests have occurred, banned mercury use in mining in 2018, ratified the Minamata Convention on mercury and submitted a national action plan in 2024. Authorities say they are conducting enforcement operations, including joint efforts with Brazil, but watchdogs and field researchers say illegal mining and mercury usage persist in many areas. Other Amazon countries have taken steps as well: Brazil has launched raids on illicit mining operations and sought to disrupt logistics for mining camps; Peru reported seizing four tons of smuggled mercury; and Ecuador, Suriname and Guyana have filed action plans aimed at reducing mercury use in small-scale gold mining.

Researchers stress that linking mercury directly to dolphin mortality is complex and requires more study, but the toxin’s well-documented effects on nervous and organ systems make it a likely contributor to disease and population decline. Trujillo described mercury as an “invisible enemy” that accumulates over years until clinical problems emerge; he cited his own past blood tests, which once registered roughly 36 times the safe limit and have since been reduced with medical care and dietary changes.

The capture-and-monitoring program also serves to detect other threats that could compound contamination impacts, including signs of infection and antibiotic-resistant bacteria. Samples taken in Puerto Narino and other field labs are analyzed locally or sent to larger facilities for further testing, and the research teams routinely repair equipment and prepare for repeat expeditions.

Scientists and conservationists say coordinated regional action is needed to curb mercury pollution and address the drivers of illegal mining and deforestation. In the meantime, the health of river dolphins and riverside communities remains an urgent indicator of how contamination is moving through Amazonian food webs and human populations.