How fish suffocate in overheated rivers
Report documents how low oxygen levels and high temperatures cause breakdown of river ecosystems.
SANTAREM, NOVEMBER 2024: The unprecedented mass death of fish in a floodplain region of the Amazon River in Pará was caused by low oxygen levels in the water. This lead to "competition" among animals in bodies of water that had high temperatures, drastic volume reduction, high turbidity values, decreased light penetration, and organism decomposition.
This conclusion is from the hydrometeorological monitoring unit of the SEMAS (Secretary of Environment and Sustainability) of the Pará Government, which released a technical report in late November 2024.
The visit occurred after Folha, with support from Rainforest Foundation Norway, published a report detailing the extent of mass fish die-offs in the area.
The MMA (Ministry of Environment and Climate Change) also announced measures following the report’s publication. Minister Marina Silva (Rede) states that an ICMBio research vessel will collect biological data.
About the series
In the fall of 2024, Rainforest Foundation Norway sent investigative journalist Vinicius Sassine and award-winning photographer Lalo de Almeida to document the human and environmental costs of climate change in the Amazon Rainforest.
The stories were originally published by Brazil's largest broadsheet newspaper, Folha de São Paulo.
Caption: Section of Pitomba Creek covered with dead fish in the Igarapé do Costa river community, Santarém (PA). Lalo de Almeida/Folhapress.
"The values are extremely low, indicating hypoxic or anoxic conditions, possibly resulting from greater organic matter decomposition and decreased water circulation due to low volume," the report states. "This led to the death of aerobic aquatic organisms (exclusively dependent on dissolved oxygen in the water)."
An extreme climatic event—a prolonged drought in the region—caused a reduction in lake volume, and low oxygen levels were the main cause of the fish kill, the report concludes. "The reduction of dissolved oxygen is critical for fish and other aerobic organisms."
The map shows the location of the mass fish die-offs and nearby communities. Illustration: Folhapress.
Stagnation and decomposition
The report also notes high turbidity values, reduced light penetration (affecting photosynthesis), shrinking space due to water body drying, and oxygen competition.
"Organism death and decomposition in the water produced substances further damaging already compromised environmental conditions, like pH changes and bacterial food availability that further reduce oxygen levels," the SEMAS report states.
In addition to fish, caimans, turtles, and stingrays died. Foul odor lingered where organic matter accumulated, along with a sea of flies. A huge patch in the remaining stream consisted of dead animals on the surface covering the water.
This was a never-before-seen scenario—not at this scale—in this part of the Brazilian Amazon, known as the Amazon River's floodplain, 30 minutes from Santarém by high-power motorboat.
First, fragile fish like pescada, cujuba, and bacuzinho died. Then, commercially valuable fish, widely consumed regionally, like surubim disappeared. Finally, the strongest and most resilient animals, such as pirapitinga and the giant pirarucu, a symbol of sustainable Amazon management died off.
Community members rescued agonizing turtles from lifeless water and transferred them to pools still resilient against harsh heat and lack of rain. This transfer continued for consecutive days.
In Igarapé do Costa, there’s no longer a lake, and the stream has turned into a trickle. The same happened to two other critical waterways, vital for the subsistence of approximately 500 families from seven communities: Pitomba Creek and Aramanaí Channel.
Community leader Erick Penna Ribeiro paddles through part of Pitomba Creek covered with dead fish, located in the Amazon River floodplain area, Santarém (PA). Lalo de Almeida/Folhapress.
Fishermen raced against time to catch still-living animals in Igarapé do Costa, like pirarucus. Days earlier, they dragged tonnes of dead fish, arranging this organic matter in heaps along the shore and setting it on fire afterward.
The inevitable feeling that the fish death would continue, with new waves of massive losses of fish and other animals, loomed for anyone observing the water. Fish sought surface oxygen; bubbles popped across the remaining water horizon.
Santarém's region has been under dense smoke from forest fires for days. The long drought has worsened the issue with its absence of rain.
