Satellite images uncover mystery of 275 rare dolphin deaths

There were extinction fears after rare pink dolphins began to die in lakes on the Amazon River. Experts in satellite analysis were determined to work out why.

A spectral view of Lake Tefé in September 6.
A spectral view of Lake Tefé shows water levels reasonably high on September 6. Source: Landsat-8 satellite. Animation: EOS Data Analytics

Satellite images have been used to investigate the cause of a tragedy that resulted in hundreds of dolphin deaths. Analysis has revealed several likely human and weather related events contributed to the disaster.

As the incident began to unfold a year ago along the Amazon River Yahoo News spoke regularly with Sea Shepherd Brazil as it documented the deaths on the ground. Its president Nathalie Gil was sure humans had played a role in the catastrophe, but she was unsure how.

At least 275 dolphins were reported dead, including 178 rare pink river dolphins — roughly 10 per cent of the entire species. “Before this stress happened we worried they probably could disappear in three decades, so with this threat on top it could be faster,” she told Yahoo in October.

Responding to concern from conservationists, experts from EOS Data Analytics, a company that specialises in satellite data interpretation, used niche software ordinarily used to monitor crops and forestry to determine what killed the dolphins, and released a report this week.

It used the tools to examine images of the basins of Lake Tefé and Lake Coari where the majority of deaths occurred, looking at dramatic changes to the water.

An image from October 24, 2023 shows the lake had shrunk.
By October 24, 2023 the lake had shrunk. Spectral imaging was used in EOSDA LandViewer software to reveal the dynamics of its drying. Source: Landsat-8 satellite. Animation: EOS Data Analytics

EOS Data Analytics found the perimeter of Lake Tefé had dramatically shrunk and become shallower because of drought, and an algal bloom was also detected. Around 170 km southeast at Lake Coari the problem was similar, except the algae was so severe the vegetation reading matched those of nearby forests. Sadly the problem persisted even after the lakes began to fill again.

On top of the drying of the lakes the temperature was also a problem. Analysis found the waters were a “hostile environment” for the dolphins, chiefly because of lower humidity and warmer conditions.

A gif showing changes in Lake Tefé’s water colour on September 6, September 22, October 8, and October 24, 2023.
Changes in Lake Tefé’s water colour can be seen in images taken on September 6, September 22, October 8, and October 24, 2023. Source: Landsat-8 satellite. Animation: EOS Data Analytics

The results have helped refine two theories about factors likely contributing to the dolphin deaths. The first is that temperature stressed the dolphins' physiological limits.

The other factor is that an algal bloom, although not directly harmful to the dolphins themselves, may have poisoned their main food source — fish. Consuming them could have resulted in the dolphins dying.

Three images of a dead pink dolphin at Coari.
When this image was taken in October, a total of 30 dead dolphins had been found at Coari. Source: Sea Shepherd Brasil

A third contributing issue has been chronicled on the ground, rather than using satellite data. And that's the likely impact of fishermen.

They accidentally kill dolphins with nets and also intentionally target them to butcher their flesh for use as bait. Analysis of the bodies found 10 per cent had signs of mutilation, harpoon marks, or net entanglement.

A gif created using LandViewer software shows the algae bloom in Lake Coari. It tracks the changes from September 5, October 5, October 25, and November 14, 2023, and collected in EOSDA.
This visualisation using LandViewer software shows the algae bloom in Lake Coari. It tracks the changes from September 5, October 5, October 25, and November 14, 2023, and collected in EOSDA. Source: Sentinel-2 satellite. Animation: EOS Data Analytics

EOS Data Analytics spokesperson Viktoria Troiian said ongoing monitoring will be crucial for helping to understand and mitigate the problems impacting the environment.

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