Rare Amazon river dolphins studied for climate change impact

MANAUS, Amazonas — A team of biologists, vets and fishermen captured rare freshwater dolphins this week on a lake in the Amazon to study their health in the hopes of avoiding hundreds of deaths of the mammals due to a severe drought in 2023.

The dolphins were brought ashore for blood tests and other exams and returned to Lake Tefe as soon as the researchers had finished their work, which included inserting a microchip to follow their behaviour via satellite.

Fishermen were careful not to hurt an adult female dolphin during capture and kept her close to her offspring.

"She relaxed and we could do the tests. She appeared in good health," said project head Miriam Marmontel, of the Mamiraua Institute of Sustainable Development, which mounted the expedition planning to capture up to 20 dolphins.

The work included removing a sample for a biopsy and the placing of the microchip on her back, which will allow researchers to follow her movements, the depths she swims at and even know the water temperatures remotely.

In a grim fallout from the longest drought in the rainforest's recorded history in 2023, the carcasses of more than 200 river dolphins were found floating on the lake formed by a tributary of the Amazon River.

Low river levels during the drought heated the water to temperatures that were intolerable for the dolphins, researchers say. Thousands of fish also died on Amazon rivers due to a lack of oxygen in the water.

The Amazon river dolphins, many of a striking pink colour, are a unique freshwater species found only in the rivers of South America and are one of a handful of freshwater dolphin species left in the world. Slow reproductive cycles make their populations especially vulnerable to threats.

"We study the health of the dolphins before the dry season sets in, so we can understand how they react to the heat," said Marmontel.

She said 300 dolphins died in 2023, most of them in Lake Tefe, a 45km-wide expanse of water where the dolphins like to be located, just off the Solimoes River.

The lake's water reached 40.9 deg C during the 2023 drought, more than 10 degrees higher than the average for that time of the year. The water is currently at 30 deg C, said Ayan Fleischmann, a geosciences researcher at the Mamiraua Institute.

Environmental activists have blamed the unusual conditions on climate change, which makes droughts and heat waves more likely and severe.

Global warming's role in the 2023 Amazon drought is unclear, with other factors such as El Nino at play.

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