MANAUS – The carcasses of 120 river dolphins have been found floating on an Amazon tributary since last week in conditions experts suspect are caused by severe drought and heat.
Low river levels during a severe drought have warmed parts of the water to temperatures unbearable for the dolphins, researchers believe. Thousands of fish have recently died in the Amazon rivers due to a lack of oxygen in the water.
The Amazon river dolphins, many of which have a striking pink color, are unique freshwater species found only in the rivers of South America and are one of the few species of freshwater dolphins left in the world. Slow reproductive cycles make their populations particularly vulnerable to threats.
Scientists have not said with complete certainty that drought and heat are responsible for the spike in dolphin deaths. Researchers are trying 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 flows into the Amazon.
At least 70 of the carcasses surfaced Thursday when Lake Tefé water temperatures reached 39 degrees Celsius (102 degrees Fahrenheit), more than 10 degrees higher than the average for this time of year.
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Environmentalists have blamed the unusual conditions on climate change, which is making droughts and heat waves more likely. The role of global warming in the current Amazon drought is unclear; other factors, such as El Nino, also play a role.
“We have documented 120 carcasses in the last week,” said Miriam Marmontel, a researcher at the Mamirauá environmental institute that focuses on the middle of the Solimões river basin.
She said about eight in 10 carcasses are pink dolphins, called ‘botos’ in Brazil, which could represent 10% of their estimated population in Lake Tefé.
The boto and the gray river dolphin, called the “tucuxi”, are on the International Union for Conservation of Nature’s Red List of Threatened Species
“10% is a very high rate of loss, and the possibility that this will increase could endanger the survival of the species in Lake Tefé,” Marmontel said.
Brazil’s Chico Mendes Institute for Biodiversity Conservation (ICMBio) has rushed veterinarians and aquatic mammal experts to save dolphins still living in the lake, but they cannot be moved to cooler river water until researchers determine a bacteriological cause of the deaths ruled out.
To this end, experts performed an autopsy on each carcass.