Study ties Amazon dolphin die-off to lake water near 106°F

Researchers say a 2023 drought and heat wave turned Lake Tefé into a hot-tub-like basin, killing more than 200 river dolphins.

TEFÉ, Brazil — Water in a central Amazon lake where hundreds of river dolphins died in late 2023 climbed to roughly 41°C (105.8°F), hotter than a typical spa bath, according to a study released this week. The extreme heating coincided with a historic drought, leaving carcasses of pink river dolphins and tucuxis along Lake Tefé’s shores as researchers scrambled to understand the cause.

Scientists involved in the investigation say the heat did not just broil the surface; it penetrated down the water column, trapping animals in a shrinking, overheated habitat. The findings arrive as officials review the Amazon’s severe 2023 dry season and as Brazil braces for another year of low rivers and persistent heat. The study’s authors report that at least half of 10 lakes they analyzed exceeded 37°C during daytime peaks, with Lake Tefé hitting the highest mark. Local and federal agencies are weighing what the results mean for endangered species and for communities that depend on the rivers.

In late September and early October 2023, residents along Lake Tefé began reporting unusual numbers of stranded or floating dolphins. By the week of Sept. 23, more than 150 dolphins had been found dead, including at least 130 pink river dolphins and 23 tucuxis. Over the following days, counts rose past 200 as search teams moved through inlets and coves made more isolated by falling water. “It was a scene none of us will forget,” hydrologist Ayan Fleischmann said, describing streaks of hot, still water and animals surfacing slowly near beaches where fish had already perished. Onshore, volunteers worked in the heat to record locations, note conditions and collect tissue, while others ferried live dolphins toward cooler channels when possible.

The new analysis uses on-the-ground measurements, satellite data and hydrodynamic modeling to reconstruct how the 2023 drought and an intense heat wave combined to overheat shallow Amazon lakes. Researchers report that five of the 10 monitored lakes surpassed 37°C during the day — a threshold they describe as unsuitable for most fish and aquatic mammals — with Lake Tefé reaching about 41°C. They found the warming extended below the surface, narrowing refuge for air-breathing mammals that still need cooler water to avoid thermal stress. Oceanographer Miriam Marmontel, who leads an Amazon aquatic mammals group based near Tefé, said water temperatures in the lake typically average about 30–32°C during the hottest months. “In 2023 we recorded values near 40°C. That’s far beyond what these species evolved to endure,” Marmontel said.

Officials say much remains unknown, including the precise role of concurrent stressors such as low dissolved oxygen, algal toxins or disease. But the temperature spike and timing line up with the peak mortality. Records from field teams show dead dolphins concentrated in coves where water levels had dropped sharply, limiting movement to the main channel. Biologists documented thermal lesions on some carcasses and noted that fish kills occurred in many of the same coves. The study’s authors conclude the thermal environment alone could have been lethal in enclosed areas, especially as animals expended energy searching for cooler layers that no longer existed. They add that a return to normal river stages later in the year appeared to slow the die-off.

The Amazon basin experiences seasonal swings, but the 2023 low-water period was among the most severe on record, with long stretches of triple-digit air temperatures and rivers falling to levels that stranded boats and cut off commerce. Regional agencies declared emergencies as isolated towns ran short on fuel, food and medicine. In and around Tefé, municipal workers and nonprofit teams shuttled water and supplies while monitoring wildlife. Previous studies have tied the drought’s intensity mainly to human-driven warming, compounded by El Niño. The new paper adds detail on how that atmospheric heat translated into hazardous lake conditions, offering a physical link between the broader climate signal and the localized mortality event.

Lake Tefé is a seasonally connected floodplain lake off the Solimões River, part of a network of shallow basins that expand and contract with the annual flood pulse. During drought, that pulse weakens, leaving lakes thinner, warmer and more isolated. Scientists say the geometry of Tefé — broad, shallow and wind-sheltered — made it especially vulnerable to rapid heating when skies cleared and river inflows collapsed. Compared with long-term patterns, the study documents warming in Amazon lakes since the 1990s at rates several times the global average for inland waters. Researchers also observed sharp contractions in lake surface area during the drought; they estimate Tefé lost much of its extent, focusing sunlight and accelerating heat buildup.

Authorities from Amazonas state and Brazil’s federal environmental institute opened inquiries after the deaths, while research groups collected necropsy samples and environmental data. Both the pink river dolphin (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) are listed as endangered. Conservation planners say a loss of even a few hundred animals in a localized population is significant. A preliminary review by wildlife officials noted that early carcass counts likely understate the toll because some animals sink or are scavenged before being found. The study’s reconstruction gives investigators a clearer timeline to match with necropsy results and to assess whether lingering effects, such as reduced reproduction, show up in future surveys.

Next steps include continued temperature and oxygen monitoring across key floodplain lakes, briefings with local governments, and seasonal forecasting to flag high-risk periods. State officials said they expect updated field reports later this month, including a comparison of 2024–25 data to the 2023 event. Researchers plan to deploy additional loggers before the next dry season and to test whether shade structures or engineered mixing can create small refuges for stressed wildlife in critical coves. A public summary of the new paper, written for river communities and fishers, is due in the coming weeks from the team’s partner institute.

On the ground, the 2023 die-off unfolded under punishing conditions. Midday air shimmered over exposed lakebeds as boats idled in channels so narrow that operators pushed with poles to avoid grounding. At a lakeside dock, Marmontel described crews lifting sluggish dolphins with slings and wet cloths. “The water felt like a bathtub,” she said. In nearby neighborhoods, residents lined up for barge deliveries as passing storms failed to refill the basin. A field technician who helped with carcass retrieval recalled the silence between outboard motors. “No birds, no splashing — just heat,” he said.

Fleischmann said the goal now is to pair what the team learned at Tefé with real-time tools that can warn when lakes approach dangerous thresholds. “These are living systems that normally breathe with the rise and fall of the river,” he said. “When the pulse weakens and the heat piles in, we see how quickly conditions can turn against the animals.” As of this week, scientists say water levels in parts of the central Amazon remain lower than average, but not at the extreme lows of 2023. They caution that another prolonged dry spell could again push enclosed lakes toward critical temperatures.

Officials plan a technical briefing on the study’s methods and findings early next week, followed by a state committee meeting on wildlife response protocols. Field teams in Tefé are scheduled to update mortality and reproduction estimates by late November. Longer-term, researchers will integrate the new lake-heating model into seasonal outlooks ahead of the 2026 dry season and expand monitoring to neighboring lakes where fish kills also occurred.

Author note: Last updated November 7, 2025.