Around 10% of the river dolphins in Lake Tefé died in one week

Posted on October, 18 2023

Over 150 endangered pink and tucuxi river dolphins have died as drought intensifies in the Amazon
Researchers from the Mamirauá Sustainable Development Institute (IDSM) and other organisations are racing against time to mitigate the impacts of the environmental emergency in and around Lake Tefé in the Brazilian state of Amazonas.

Since September 23, as the drought has worsened and water temperatures have risen, 153 river dolphins have been found dead in the region: 130 pink river dolphins and 23 tucuxi. In one week, the loss was around 10% of the local river dolphin population. On the 28th of September alone, when the water temperature exceeded 39ºC, 70 river dolphin carcasses were recorded, in addition to hundreds of fish.

The crisis, however, goes far beyond the loss of river dolphins. There is an increase in mortality of fish species in the region, which are essential for the food security and livelihoods of local communities. The drought is also impacted water supplies and transportation, isolating some communities. Overall, 500,000 people have already been impacted.

“What is happening at Lake Tefé is terrifying. The impact of the loss of these animals is enormous and affects the entire local ecosystem”, warns Mariana Paschoalini Frias, Conservation analyst at WWF-Brazil. “River dolphins are considered ‘sentinels’. In other words: they are indicative of the health of the environment where they live. What happens to them is reflected in the other species that live around them, including humans.”

"In our studies on Amazonian dolphins, we found that they suffer from several pressures, such as fishing, mercury contamination and the impact of hydropower plants. But these events in Tefé show that more research needs to be carried out on how they will be affected by worsening climate change," she added.

The deaths highlight the urgent need to scale up efforts to conserve the world's river dolphins - and the importance of the Global Declaration for River Dolphins, which will be signed by South American and Asian range states in Colombia on October 24th.

The operation was divided into different fronts. One of them, called Alive Operation Sector, monitors groups of pink and tucuxi river dolphins along Lake Tefé, an isolated environment that is home to a large population of these two endangered species. When the team finds an individual with signs of abnormality, they are able to rescue them and take them to the Rehabilitation Float. The Operation Dead Sector, in turn, aims to identify and search for carcasses in the region and perform necropsies to collect samples for laboratory analysis. And the Environmental Operation Sector works to monitor water, fish and phytoplankton, organisms composed of microalgae and photosynthetic bacteria.

Of all the variables analysed so far by experts, the one that has shown discrepant changes is water temperature, reinforcing that river dolphin mortality is related to climate change, the effects of El Niño and extreme drought. The water temperature of Lake Tefé reached close to 40°C, says oceanographer Miriam Marmontel, leader of the Amazon Aquatic Mammals research group at IDSM, when the maximum average over time has been 32 degrees, a fact that certainly generated thermal stress in animals.

Members of the environmental consultancy Aqua Viridi also identified in one of the lake's points an unusual number of alga Euglena sanguinea, which produces a toxin that can cause mortality in fish. The assessment carried out on the river dolphins, however, did not confirm that the animals have been affected by possible toxins produced by these organisms. Other analyses are underway to help understand the algae's possible role in the current environmental and health emergency.

Since the beginning of the crisis, 104 river dolphins have been necropsied and tissue and organ samples have been sent to specialised laboratories. Seventeen individuals have already been assessed and, to date, there is no evidence of an infectious agent as the primary cause of deaths. Molecular diagnostics of 18 individuals also tested negative for infectious agents, such as viruses and bacteria, associated with mass deaths.

In Lake Tefé, there is a stretch called Enseada do Papucu, which has been critical for animals due to the water temperature. Even so, many dolphins continue to frequent the area due to the abundance of fish, their basic diet. To prevent further deaths, the area is being isolated with a physical barrier called “pari”, which is made of wooden stakes and is based on traditional riverside knowledge. Subsequently, the species will be moved to deeper, less hot areas.

WWF-Brazil has been working in partnership with the Mamirauá Institute for Sustainable Development, which is leading the rescue of dolphins in Lake Tefé, providing fuel, Personal Protective Equipment (PPE), veterinary supplies and logistical support for the movement of volunteers.

WWF-Brazil is also in contact with local partners and has mobilised to support them in respond to the humanitarian crisis caused by the drought in the Amazon region, as the consequences are especially dramatic for the most vulnerable populations, such as indigenous people, quilombolas, extractivists and riverside communities. Currently, the main area of activity is supplying food to communities impacted by shortages.
Studying dead Amazon river dolphin in Lake Tefe in Brazil
© Miguel Monteiro
Photographing dead river dolphins in Lake Tefe, Brazil Amazon, during drought in 2023
© Miguel Monteiro