Over 100 dolphins have been found dead in a tributary of Amazon river in Brazil this past week. Experts suspects the deaths may have been caused by severe drought and rising heat.

At least 70 of the remains were found floating on Thursday when the temperature of Lake Tefe's water reached 39 degrees Celsius (102 degrees Fahrenheit) — more than 10 degrees higher than the usual average for this time of the year.

Warming water a concern

Low river levels during a severe drought have heated water in stretches to temperatures that are intolerable for the dolphins, experts believe.

Following a decline for a few days, the water temperature again soared to 37 degrees Celsius (99 Fahrenheit) on Sunday.

The region around the lake is a key habitat for mammals and other aquatic species. Large amounts of fish have also died according to local media reports.

The scientists are working to rule out other causes like bacterial infections as they do not know with certainty that drought and heat are to blame for the rise in dolphin mortality.

"We have around 900 river dolphins and 500 Tucuxis (in the Tefe Lake) and in one week we have already lost around 120 animals between the two of them, which could represent 5% to 10% of the population," Miriam Marmontel, a researcher from the Mamiraua environmental institute, said.

A dead dolphin is seen at the Tefe lake. Deutsche Welle

Threatened species

The Amazon river dolphins — some pink in color — are a unique freshwater species found only in the rivers of South America.

A slow reproductive cycle makes their populations especially prone to threats.

The freshwater dolphins, or "Tucuxis," are on the International Union for Conservation of Nature's red list of threatened species.

"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 Tefé," Marmontel warned.

Researchers on Monday were still recovering dead dolphins in the region where dry rivers have also impacted impoverished riverside communities, beaching their boats on the sand.