Extreme heat in Brazil's Amazon River is killing a rare species of dolphins at an alarming rate.
At least 125 dead dolphins have been found in the Amazon tributary of Tefe Lake over the course of a week, NPR reported. Thousands of fish have also died.
Experts say high water temperatures combined with recent extreme droughts are likely to blame. The lake has soared to temperatures of 102 degrees Fahrenheit in recent weeks -- about nine degrees higher than normal, according to the New York Times. Such a drastic, prolonged change in temperature can leave the dolphins disoriented and unable to dive.
"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 Lake Tefé," the Mamiraua Institute for Sustainable Development, which is investigating the die-off, said in comments to CNN Brasil.
Eight out of every ten carcasses were pink Amazon river dolphins, while the rest were grey river dolphins, according to the Weather Channel. Both are listed as a threatened species by the International Union for Conservation of Nature.
Miriam Marmontel, a researcher from the Mamiraua Institute, said the deaths represent roughly 10% of the river dolphin population in the lake.
"Ten percent is a very high percentage of loss, and the possibility that it will increase could threaten the survival of the species in Lake Tefe," Marmontel told the Weather Channel.
Rescue crews are working to help the surviving dolphins, but the animals can't be transferred to cooler waters unless they're first tested for disease.
Making matters worse, more severe droughts are expected in the coming weeks, CNN reported, which could be even more deadly for dolphins