- The construction of the Balbina hydroelectric dam in the Brazilian Amazon led to the loss of seven endemic fish species in the Uatumã River, researchers have found.
- The hydroelectric dam has transformed the Uatumã River’s fast-flowing habitats into static environments, making them unsuitable for certain fish species.
- Researchers call for the investigation of unaffected tributaries, such as the Jatapu River, as possible refuges for the missing species and future conservation efforts.
- The study underscores the broader threat of hydropower dams and other environmental stressors, like industrial fishing and climate change, to Amazonian fish populations.
The presence of the Balbina hydroelectric dam in the Brazilian Amazon led to the local extinction of several endemic fish species in the Uatumã River, according to research from the National Institute for Amazonian Research (INPA) and its partners. Thirty-five years after the dam’s construction, the study found that rheophilic species — those adapted to fast-flowing waters — are now absent from the impacted area.
“This study may be the first to confirm a local extinction directly linked to a hydroelectric dam in the Amazon,” Lúcia Rapp Py-Daniel, an INPA researcher involved in the study, told Mongabay.
Fieldwork carried out since 2023 and reported this year confirmed the disappearance of seven fish species in the region, highlighting a significant loss of aquatic biodiversity in the Uatumã River, a major tributary of the Amazon River in the state of Amazonas. The study, funded by the Amazonas State Research Support Foundation, was led by Rapp Py-Daniel and postdoctoral fellow Douglas Bastos, in collaboration with the State University of Amazonas and the Federal University of Pará.
The researchers spent 20 hours diving across five days in the Uatumã River to investigate the rock crevices near the Balbina dam, collecting 945 specimens that resulted in 247 tissue samples. The researchers found no trace of the endemic fish in the Uatumã River previously seen in the region prior to the dam’s construction.
“These observations made it clear to us, and we can confidently state, that the fish species have disappeared from Uatumã,” Bastos told Mongabay. “The underwater environment there is heavily impacted, with almost no life remaining. There are very few fish, if any at all.”
Silvia Santana Zanatta, a conservation specialist from WWF-Brasil who was not part of this study, told Mongabay that the disappearance of fish species in areas like the Uatumã River triggers far-reaching ecological consequences, disrupting both habitats and local livelihoods. Fish are crucial for maintaining aquatic balance through roles like population control, nutrient cycling and seed dispersal.
“When endemic species vanish, these processes break down, causing imbalances that ripple through the entire ecosystem, affecting life both in and out of the water,” Zanatta said.
In addition to the seven endemic species, other species specialized for fast-flowing waters have also vanished. One example is the badi, or giant pacu (Mylesinus schomburgkii). Measuring 30-40 centimeters (11.8-15.7 inches) long, this fish once thrived in Uatumã River’s rapids during the 1980s and was commonly eaten by the local Indigenous group, the Waimiri-Atroari (whose people call themselves the Kinja). However, according to Bastos, local reports from the community confirmed that this species disappeared after the dam was constructed.
Locally, the loss of fish species poses serious threats not to just food security but the economic stability and social fabric of traditional and Indigenous populations. “Fish are not just a food source,” Zanatta said. “They are deeply woven into the culture, rituals and traditions of these communities, forming an essential part of their identity.”
The team also speculates that another nearby hydroelectric dam, Pitinga, may be contributing to the decline of local species, though further research is needed to confirm this hypothesis. Researchers are now focusing their efforts on tributaries of the Uatumã River that remain unaffected by the construction of hydroelectric dams, like the Abacate and Jatapu rivers, where the conditions appear to be suitable for the survival of the endemic rheophilic species.
During a field collection to Jatapu at the end of 2024, the researchers made an encouraging discovery on their first dive: They found some of the missing species, including the badi and two others listed as threatened with extinction: Apteronotus lindalvae, an electric fish, and Harttia uatumensis, a type of catfish.
The difference in biodiversity between Uatumã and Jatapu underscores the link between the impact of hydroelectric power projects and environmental degradation. These findings also indicate that while the species may no longer exist in the altered areas, they might have survived in other parts of the basin.
“The results of the research generate a sense of hope, as in some of the tributaries of the Uatumã that remain free and healthy, those species wiped out by the Balbina dam’s construction continue to survive and reproduce steadily,” Flávio Montiel, manager of nonprofit International Rivers in Brazil who was not part of this study, told Mongabay. “Life persists in these rivers.”
A ‘particularly cruel’ environment
Rheophilic species depend on high oxygen levels and specific conditions found in fast-flowing waters such as rivers, rapids and waterfalls. When flowing water is transformed into a still lake, everything changes: depth, temperature, oxygen levels and light penetration. These drastic shifts fundamentally alter the ecosystem, forcing animals to either relocate or face local extinction.
“For rheophilic species that rely on currents, it’s particularly cruel,” Bastos said. “These species need flowing water to breathe and attach to rocks. Without it, they are effectively stranded. Being in still water is unnatural for them; they don’t know how to survive in such conditions.”
Currently, very little is known about these endemic fish, with limited information available on their behavior and population size, and no published list of the aquatic species in the Uatumã Basin. The lack of taxonomic knowledge of the region is so significant that recent expeditions led researchers to discover potentially 15 species previously unknown to science: five from the Uatumã River and 10 from the Jatapu River.
“Diving allows you to observe certain species that are otherwise difficult to encounter and cannot easily be captured with nets,” Rapp Py-Daniel said. “The fact that these species were identified and collected, some of which had never been found before, is remarkable. It highlights that there is still an underestimated diversity out there.”
The researchers describe this study as a crucial first step in assessing the aquatic biodiversity of the basin and understanding what has been lost. It lays the foundation for defining conservation efforts and mapping the ecological and human impacts of the species’ disappearance.
“This is a region impacted by various factors, such as hydroelectric projects, mining, deforestation and the presence of Indigenous lands,” Rapp Py-Daniel said. “It presents a unique opportunity to build comprehensive knowledge about a wide area of Amazonian drainage systems.”
A legacy of harm
The problems associated with the Balbina dam were highlighted early in its construction. In 1989, Philip M. Fearnside, ecologist and senior researcher at INPA, published a paper criticizing the project, citing several environmental costs, including forest loss, biodiversity fragmentation, methane emissions, disruption of turtle migration and fish mortality, particularly when the dam is closed.
A study by Alexandre Kemenes, a researcher at Brazil’s agricultural research company Embrapa, found that Balbina emits 10 times more greenhouse gases per megawatt produced than a thermal power plant.
“Therefore, we can affirm that the Balbina hydroelectric power plant contributes significantly to atmospheric pollution and global warming,” Montiel said.
The impact of Balbina on Indigenous livelihoods was recently acknowledged when a Federal Court ruled in November 2024 that Eletronorte (the owner of Balbina), Brazil’s Indigenous agency, Funai, and the Brazilian government must compensate the Waimiri-Atroari for lands flooded by the Balbina hydroelectric plant in the 1980s. The ruling recognized the dam’s severe cultural and environmental harm, with the compensation amount to be determined later.
The Waimiri-Atroari have long resisted large-scale infrastructure projects on their land, including the BR-174 Highway, mining operations and the more recent Tucuruí transmission line, which cut through 122 kilometers (75.8 miles) of their territory. In 2019, Brazil’s Indigenous Missionary Council lamented their hardships, saying in a statement that the Waimiri-Atroari “carry deep in their memory what it means to face violence and the usurpation of their territories, treated by the State as second-class citizens, doomed to disappear.”
Eletronorte told Mongabay that Balbina complies with environmental regulations and holds a certification for responsible management. Without addressing the study’s findings on missing endemic fish, it highlighted its conservation efforts, including a scientific breeding center for aquatic mammals and turtles, and the release of thousands of hatchlings and rescued manatees. The company also emphasized its decades-long relationship with the Waimiri-Atroari through a program supporting health, education, and environmental protection since 1987.
Funai told Mongabay it is overseeing an environmental impact assessment of the Balbina hydroelectric power plant, using an interdisciplinary approach that integrates Indigenous and technical knowledge. While steps are being taken to address the plant’s effects, formal recognition and completion of the Indigenous impact study remain necessary, the agency said.
Brazil’s environmental agency, IBAMA, told Mongabay that the case concerns state-level licensing and is therefore outside its jurisdiction.
A long line of degradation
While hydroelectric power plants are often labeled as renewable energy sources, experts, such as Montiel and Zanatta, told Mongabay that they should not be considered clean energy due to their significant environmental and social impacts. For decades, hydroelectric projects in the Amazon have posed a threat to fish populations.
In the Madeira Basin, for example, experts claim that the Santo Antônio and the Jirau hydroelectric plants in the state of Rondônia have disrupted the downstream water cycle, altering migratory patterns of fish and reducing annual catches in the Humaitá municipality by 39%, Mongabay reported in July 2024.
A 2024 WWF-Brasil study found that environmental and social costs of planned dams for the Tapajós River far outweigh the energy benefits, potentially resulting in a social value loss of at least 11.8 billion reais ($2 billion), although estimates suggest up to 34 billion reais ($5.9 billion), Zanatta said. An alternative, she suggested, is leveraging solar and wind energy, which provides energy generation that causes less environmental harm and can coexist with local biodiversity.
In the unaffected tributaries, like the Jatapu River, implementing strategies for conserving fish species is crucial, experts say. These include creating and strengthening conservation units, establishing continuous monitoring programs to track fish populations and threats, and preserving river connectivity by preventing new dams and reducing habitat fragmentation.
Balbina dam is a prime example of poor planning, Rapp Py-Daniel said, but also serves as an important lesson for future projects. “It didn’t succeed in terms of electricity production or environmental preservation,” she said. “However, it serves as a warning of what should and shouldn’t be done.”
Banner image: A 2023 expedition along the Uatumã River documented biodiversity loss linked to the Balbina dam. Image courtesyvey of the undammed Jatapu River revealed a region teeming with life. Image courtesy of the National Institute for Amazonian Research (INPA).
Citations:
Fearnside, P. M. (1989). Brazil’s Balbina Dam: Environment versus the legacy of the pharaohs in Amazonia. Environmental Management 13(4): 401-423. philip.inpa.gov.br/publ_livres/preprints/1989/balbina-eng2.pdf
Hydropower plants disrupt fishers’ lives in Amazon’s most biodiverse river basin
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