More than 100 freshwater dolphins have been found dead in the Brazilian Amazon, probably as a result of the heat recorded in the region, dpa reports.

The bodies of the mammals were discovered last week in the waters of Lake Tefe, about 520 kilometers west of the city of Manaus, the Mamiraua Institute for Sustainable Development said on Friday, according to Agerpres.

The exact cause of death has not yet been established. However, it is believed to be related to the current heat and drought, phenomena that have caused the lake’s water temperature to exceed 39 degrees Celsius.

Many rivers in the Amazon basin carry much less water than the average in recent years. The usual dry season is now being exacerbated by the El Niño climate phenomenon.

Amazonian dolphins are the largest river dolphins, reaching approximately 2-2.5 meters and weighing between 85 and 185 kilograms.

“Amazon dolphins are exposed to numerous threats, such as the impact of hydroelectric power plants, mercury pollution and conflicts with humans,” said Mariana Pasholini Frias of the environmental organization WWF.

“Freshwater dolphins are now even more affected by the effects of climate change,” Frias added.