At least 120 bottlenose dolphins that live in rivers, were found dead in a tributary of the Amazon in recent days, in conditions that lead experts to suspect that their deaths are due to drought and heat.

Due to low river levels during a severe drought, the water warmed up to temperatures that are unbearable for the dolphins, researchers believe. Thousands of fish have recently died in Amazonian rivers due to lack of oxygen in their waters.

Amazonian dolphins, many of which are strikingly pink in color, are a freshwater species that lives only in rivers in South America. Due to their slow reproductive cycle their population is very vulnerable to environmental threats.

Biologists and other experts, wearing special protective suits and masks because of the unbearable stench, today began autopsies on the dead dolphins to determine the causes of their deaths. Scientists do not know for sure that drought and heat are responsible for the high mortality of these mammals. They are currently trying to rule out other causes, such as a bacterial infection that could have killed them in a lake that forms in the Tefe River before it joins the Amazon. At least 70 dead animals surfaced on Thursday when the water temperature in Lake Tefe reached 39 degrees Celsius, more than 10 degrees above average for this time of year.

The water temperature dropped in the following days but on Sunday it rose again to 37 degrees.

“We counted 120 bodies last week,” said Miriam Marmodel, a researcher at the Mamirawa Environmental Institute. Almost 8 out of 10 carcasses belong to pink dolphins, which are called “botos” in Brazil. This means that 10% of their population died in Lake Tefe.

Botos and gray river dolphins, called “tukuxi”, are on the International Union for Conservation of Nature’s endangered species list. “10% is a very high percentage and if it increases further it will threaten the survival of the species in Lake Tefe,” Marmodel said.

The Chico Mendes Biodiversity Conservation Institute (ICMBio) mobilized veterinarians and marine mammal experts to save the lake’s dolphins that are still alive. But it is not possible to transfer them to cooler waters until the researchers rule out the possibility of a bacterial infection.