Endangered pink river dolphins in Amazon face rising mercury threat from illegal gold mining

PUERTO NARINO, Colombia — Conservationists say pink river dolphins in parts of the Amazon are showing mercury concentrations tens of times higher than safe limits as illegal gold mining and forest clearing push the toxic element into waterways.

Teams led by the Omacha Foundation and local veterinarians regularly net, examine and sample dolphins to measure mercury and other health indicators. The work — carried out in small boats and on sandy riverbanks — is intended to use the animals as sentinels for the river’s health and to document threats that also affect Indigenous and riverine communities.

"Taking a dolphin out of the water, it’s a kind of abduction," said Fernando Trujillo, a marine biologist who directs Omacha and leads the health evaluations. Teams have about 15 minutes to conduct a battery of tests, including blood and tissue sampling for mercury, ultrasonic scans of lungs and heart, bacterial swabs, photographic records of scars and skin, and microchip implantation for individual identification.

Mercury used in small-scale illegal gold mining is the primary contaminant researchers cite. Miners mix mercury with sediment to separate gold, then release the mercury-laden sludge back into rivers. Deforestation and erosion also mobilize mercury naturally present in soils, sending it into aquatic food chains. Fish and the animals that eat them, including dolphins and people, bioaccumulate the toxin.

World Health Organization and U.S. Environmental Protection Agency guidelines indicate that mercury exposure can impair the brain, kidneys, lungs and immune system, and can cause developmental damage in fetuses and young children. Trujillo said studies and field data show widespread elevated mercury in the region, with mercury levels in some dolphins reaching 16 to 18 milligrams per kilogram in previous surveys; in the Orinoco River, he said, some animals have measured as high as 42 milligrams per kilogram. By comparison, he said, the maximum safe concentration for any living being should be about 1 milligram per kilogram.

Researchers emphasize that proving mercury directly kills dolphins is difficult, but they say the concentrations recorded are high enough to cause neurological and organ damage. "Any mammal with a huge amount of mercury will die," Trujillo said. He and colleagues have also documented antimicrobial resistance and respiratory problems in sampled animals, and have identified viruses such as papillomavirus that could pose additional risks to both wildlife and people.

Dolphin populations are also declining from nonchemical threats. Omacha monitoring indicates a 52% drop in pink river dolphins and a 34% decline in gray river dolphins in parts of the Amazon over recent decades. The International Union for Conservation of Nature listed the pink river dolphin as endangered in 2018. Exact basin-wide counts are uncertain; Omacha estimates between 30,000 and 45,000 river dolphins across the Amazon.

Field operations rely on experienced local crews. José "Mariano" Rangel, a former fisherman from Venezuela, leads the capture effort, which requires encircling dolphins with nets and hauling animals that can weigh up to about 160 kilograms into boats. "The most difficult part of the captures is enclosing the dolphins," Rangel said. Once ashore, teams work quickly to keep animals moist and calm, count breaths, perform scans and collect samples before returning the dolphins to the water.

The samples also reveal risks to people. Hair and blood tests in Indigenous communities across Brazil, Peru, Colombia, Suriname and Bolivia have shown mercury levels above World Health Organization thresholds; one Colombian community recorded more than 22 milligrams per kilogram. Trujillo said that when he and colleagues tested their own blood three years ago, his level was 36.4 milligrams per kilogram, about 36 times the safe limit. With medical assistance and changes to diet and exposure, his level later fell to about 7 milligrams per kilogram.

Rising global gold prices have fueled small-scale and illegal gold-mining operations across the Amazon Basin in recent years, observers say. Governments in the region have taken a range of actions to curb mercury use and illegal mining. Colombia banned mercury in mining in 2018, ratified the Minamata Convention on Mercury and submitted a national action plan in 2024. Authorities cite joint operations with Brazil and periodic enforcement sweeps, but watchdogs and scientists say illegal mining and mercury pollution persist across large swaths of the basin.

Brazil has conducted raids and sought to limit the logistics of illegal mining camps by restricting satellite internet services used by some miners. Peru recently seized a record 4 tons of smuggled mercury. Ecuador, Suriname and Guyana have filed action plans aimed at reducing mercury use in artisanal and small-scale gold mining.

Omacha staff process some samples in a riverside laboratory and ship others to larger facilities for analysis. Each capture requires months of planning and collaboration among fishermen, veterinarians, local communities and researchers. The teams also track reproductive status and infection, factors that can compound pollution impacts and inform conservation strategies.

Trujillo said the monitoring and health interventions are part of a larger effort to prevent local extinctions. "We are one step away from being critically endangered and then extinct," he said, summarizing the threat posed by combined pressures including pollution, overfishing, entanglement in nets, boat traffic, habitat loss and prolonged drought.

Scientists and advocates call for stronger enforcement of mining bans, improved monitoring, expanded health screening for affected communities and regional cooperation to curb mercury trade and use. They say the dolphins provide a visible measure of contamination that should prompt more urgent policy and enforcement responses across the Amazon Basin.

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