More than 100 dolphins and thousands of fish have been found dead in the Amazon as record-breaking temperatures soared to 39C.
A drought gripping the Brazilian region is making it increasingly difficult for wildlife to survive.
The carcasses of 120 dolphins have been discovered floating on a northern tributary of the Amazon River.
Several thousand more fish have died recently due to a lack of oxygen in the water.
Amid the stench of decomposing remains, biologists and other experts donned in white personal protective clothing and masks worked on Monday to conduct post-mortems on each carcass to determine a cause of death.
The scientists do not know for certain drought and heat are to blame for the spike in mortality but are working to rule out other causes, such as a bacterial infection that could have killed the dolphins on a lake formed by the Tefé River before it runs into the Amazon.
Scientists at Mamirauá Institute, the Brazilian government-funded research facility, told CNN: “It’s still early to determine the cause of this extreme event, but according to our experts, it is certainly connected to the drought period and high temperatures in the Tefé, in which some points are exceeding 39C.”
Experts fear there could be further dolphin and wildlife fatalities with the severe drought conditions expected to continue into the next couple of weeks.
Environmental activists have blamed the unusual conditions on climate change, which makes droughts and heat waves more likely.
Global warming’s role in the current Amazon drought is unclear, with other factors such as El Nino at play.
“We have documented 120 carcasses in the last week,” said Miriam Marmontel, a researcher at Mamirauá.
She said roughly eight of every 10 carcasses are pink dolphins, called “botos” in Brazil, which could represent 10 per cent of their estimated population in the Tefé.
The Chico Mendes Institute for Biodiversity Conservation has rushed veterinarians and aquatic mammal experts to rescue dolphins that are still alive in the lake, but they cannot be moved to cooler river waters until researchers rule out a bacteriological cause of the deaths.
The drought has led water levels to fall, making it more difficult for people to access food and drinking supplies.
Officials said the dead fish has rotted and contaminated water supplies, affecting more than 110,000 people.
In the city of Manacapuru, there were reports of fish seen jumping in an attempt to escape the scorching, shallow waters.
