MANAUS, Brazil — In September 2024, the landscape in the Middle Solimões region of the Brazilian Amazon lay in stark contrast to its usual exuberance of lush greenery. “When we were arriving in Tefé and the plane approached to land, I was shocked to see everything very dry, with sandbanks multiplying in the waters,” says biologist Susana Braz-Mota.

Lake Tefé is close to the Mamirauá Sustainable Development Reserve, a 1.12-million-hectare (2.77-million-acre) area of forests flooded by the Solimões and Japurá rivers. The entire region is usually a mosaic of waters of high ecological importance.

During flooding, the Solimões River brings sediments and nutrients that fertilize the flooded lands, sustaining all wildlife. The abundance of resources in these areas, regulated by natural flood cycles, favors the diversity of fish in the reserve: 541 species have been cataloged here, or 20% of the entire diversity of the Amazon Basin.

But the historic drought of 2023, followed by a weak flood in 2024, drastically altered the environment, forcing the fish to survive in waters that were shallower and warmer than usual.

Braz-Mota traveled from Manaus, capital of Amazonas state, to Tefé with fellow biologist Rafael Duarte to investigate the impacts of the drought on Amazonian fish. A native of Amazonas, she’s used to the immensity of the region’s rivers, but was now hearing an increasingly common refrain from fishers: “We have seen droughts before, but never as severe as this one.”

The historically low levels of the rivers exposed the fish to multiple risks — from the effects of the heat, to the impacts on their reproduction and the food chain in their habitats. While these pressures affect all species, scientists are concerned that fish of importance to Amazonian communities are among the most sensitive to changes in the flood pulses.

Lake Tefé: Before and after

The Amazon has warmed over the past 40 years by 0.6-0.7° Celsius (1.1-1.3° Fahrenheit), according to monitoring conducted during this period. With the planet heading toward 2°C (3.6°F) of warming above preindustrial levels, temperatures in the region could rise even further this century, by 3-4°C (5.4-7.2°F), according to the Intergovernmental Panel on Climate Change (IPCC).

High temperatures are already altering the rain cycle in the Amazon. In 2023, the basin faced a combination of intense drought and unprecedented heat waves, resulting in the loss of 3.33 million hectares (8.23 million acres) of water surface area — an area 22 times the size of São Paulo, Brazil’s largest city.

Lake Tefé has become a striking example of such change. Between September and October 2023, its area shrank by 75%, from 379 to 95 square kilometers (146 to 37 square miles), and at its shallowest point was only 0.5 meters (1.6 feet) deep. At the same time, the water temperature peaked at 41°C (105.8°F), and remained above 37°C (98.6°F) for several consecutive days — higher than the usual average for that time of year, which is 29-30°C (84-86°F).

In the last two years, low river levels have been the focus of the research and meeting agenda of Ayan Fleischmann, a water dynamics expert at the Mamirauá Institute, a government-funded research institute, and a resident of Tefé. According to Fleischmann, riverine communities often complain of extreme drought being followed by extreme flooding. “Indeed, in January, the river started to rise a lot. But then in February, it stagnated,” he says.

June usually sees a peak in floods. But in June 2024, river levels were below average, leaving many streams unflooded in Lake Tefé. On June 30, the section of the Solimões River running through Coari municipality reached a depth of 16.14 m (52.95 ft), well below the 17.83 m (58.50 ft) and 18.8 m (61.7 ft) recorded on the same dates in 2023 and 2022, respectively.

According to the 2022 census, Tefé is the largest municipality in the Middle Solimões region, with a population of 73,669. Like its wildlife, its local communities depend on the rivers’ flood pulses for fishing and transportation. With the drought, this balance was broken, resulting in food insecurity and difficulty accessing basic services.

Does water temperature threaten fish biodiversity?

“Understanding the thermal limit of fish is essential to predict which species are most resistant to rising temperatures,” says Duarte, who has studied Amazonian fish for more than 20 years.

Unlike humans, fish depend on external heat sources to regulate their body temperature. So when environmental temperatures change, their bodies must adjust to survive the more adverse conditions.

“In warmer waters, fish expend more energy to sustain basic functions such as feeding, reproducing and swimming,” Braz-Mota says. “This extra effort makes them more vulnerable to problems such as parasites, and it may even damage their genetic material.”

Climate change could make conditions in the Amazon Basin even more extreme. High temperatures reduce the amount of oxygen dissolved in the water, while also increasing fish’s need for more oxygen, making breathing difficult for them and compromising their metabolism. This may slow the growth of juveniles and increase physiological stress.

At the height of the 2023 drought, Braz-Mota was at Lake Janauacá, in the Greater Manaus area, where she recorded oxygen levels of just 0.5 milligrams per liter — dangerously close to zero. In such conditions, extreme heat and low oxygen levels become lethal for aquatic life, as occurred in Tefé and Janauacá, where communities reported high fish mortality.

Different species of fish react differently to temperature variations in the environment. Studies by Braz-Mota and Duarte in Tefé show that aracu (Schizodon fasciatus), a commercially important species, is particularly vulnerable. When exposed to temperatures close to its limit, the species suffers stress that alters its cell structure, potentially damaging its DNA and causing mutations.

Tambaqui (Colossoma Macropomum), of which Amazonian communities consume some 60,000 metric tons per year, has been shown to tolerate temperatures above 40°C (104°F) in experimental tests. However, this resistance doesn’t spare it from heat stress: prolonged exposure to heat may cause harmful effects even before death.

When exposed to temperatures of 29-30°C, juvenile tambaquis exhibit restricted growth due to difficulty converting food into energy. In tests with temperatures of up to 38°C (100°F), the fish required more oxygen to sustain basic functions — cause for concern during extreme droughts, when oxygen in the water is also scarcer.

In light of this evidence, research on various fish species’ thermal tolerance has predictive potential: in a future with higher temperatures or more droughts, it’s possible to assume that more sedentary species will have a better chance of surviving, as they expend less energy to sustain their basic functions under adverse conditions.

But even if this prediction is confirmed, the question remains: under what conditions will they survive? Known effects of prolonged heat exposure suggest a scenario of smaller fish that are more vulnerable to diseases.

Recurrent droughts like the one in 2023 pose a serious risk to fish biodiversity in the Amazon. These species have evolved over millions of years across vast areas, with abundant resources and stable temperatures.

Extreme events, however, drastically alter these conditions, potentially leading to selection of some species and reduction of the more than 2,300 ones already described. “In the Amazon, less biodiversity also means lower quality of life for those who live here,” Braz-Mota says.

Duarte says extrapolating findings derived from a controlled laboratory environment to the natural environment is a challenge. After all, several risk factors in a habitat may influence the fish’s response to temperature. “In Tefé, for example, there is heavy boat traffic, and we are investigating whether contamination by engine oil adversely affects their thermal capacity,” he says.

Shallower, warmer lakes: A worldwide trend

What’s happening in the Amazon has caught the attention of researchers around the world. One of them is Priit Zingel from Estonia. In 2023, he visited the region for the third time to conduct studies on two lakes: Janauacá and Lago do Prato. The latter is located in Anavilhanas National Park, a nine-hour boat ride from Manaus along the Rio Negro.

“Witnessing that drought had a huge impact on me,” Zingel recalls. “When I first visited the Amazon, the vastness of the waters gave me a sense of freedom, of infinite possibilities. We knew in advance that the situation was extreme, but seeing it up close was depressing.”

Zingel’s focus is on shallow lakes around the world. Like many Amazonian lakes, these ecosystems are usually up to 5 m (16.4 ft) deep — enough for sunlight to reach the bottom and allow aquatic plants to grow.

In the Amazon, lakes that are connected to rivers, such as Lake Tefé, also enable fish to migrate between habitats. During the dry season, they provide shelter and food for animals, due to the abundance of aquatic plants. However, intense variations in water levels can transform the habitat, affecting the availability of nutrients and competition for resources.

Other lakes around the world face similar challenges. Lake Chad in Africa shrank from 25,000 km2 (9,650 mi2) in the 1960s to a tenth of that size by the 1980s. But it has been recovering since then. In Bolivia, Lake Poopó dried up completely on two occasions, in 2015 and 2021.

In light of these challenges, Zingel is looking into the long-term impacts of water level fluctuations on fish, comparing the relationship between size and weight for 10 different species. He collected specimens during two Amazonian droughts: a typical one in 2019, and the extreme one in 2023. The results were striking: with severe drought, fish size and weight decreased, both for sedentary fish and for more active ones such as piranhas.

Even though some species tolerate warmer waters, variations in lake levels pose challenges for fish in general. “All of these factors affect population dynamics. With events like this becoming frequent, it is reasonable to assume that the most resilient species will become dominant,” Zingel says. “However, if many species are lost, the remaining populations become more vulnerable and the ecosystem as a whole loses resilience.”

Fishing under threat

Three factors are crucial to understanding the impact of extreme droughts on fish: their mortality, growth and reproduction. The combination of high water temperatures and low oxygenation already poses an imminent risk on these three fronts.

As research indicates, even fish that survive this altered environment don’t have ideal conditions to grow in. Additionally, thermal stress may compromise reproduction, affecting sperm quality and larval development. Finally, loss of connectivity between lakes and rivers prevents migration, an essential process for the reproductive cycles of many species.

Carlos Edvar de Freitas is an environmental engineer and a fisheries expert at the Federal University of Amazonas. He warns that all fish with reproductive cycles synchronized with river pulses are at risk. That includes species of importance to local fisheries, such as long-distance migratory fish like piraíba and dourado, and fish that spawn in lakes at the beginning of the flood, such as tambaqui and pacu.

“There is a word that is key for those who work in fishing: recruitment. It is the moment when young fish join the adult stock,” Freitas says. “Extreme droughts can delay spawning for many Amazonian species. In other cases, spawning does occur, but the larvae become easy prey in the riverbed, unable to reach the lakes. All of this reduces recruitment, an impact that will only be felt in the next fishing season.”

That’s why it takes time to fully understand the impact of a drought on the fish population. A study published in 2024 analyzed fishing data in the Lower Amazon and confirmed that productivity is directly influenced by climate events that alter habitats. However, the research used data collected between 1993 and 2005, highlighting the gap between field research and the publication of the results, which don’t include the impacts of the extreme droughts this century.

There are few studies relating fishing productivity to environmental and physiological variables. As Freitas points out, most of the existing research is conducted by universities with targeted funding and isn’t part of a regional public monitoring policy.

“There is a lack of data on catches to model fish stocks. But this is not a task that researchers can perform alone,” Freitas says. “Statistics should be conducted by state governments, with support from the federal government. This needs to be state information.”

More frequent extreme events: What now?

Freitas says ignoring these risks means setting the ground for a future crisis. In the Amazon, small-scale fishing is essential for food security and poverty reduction, generating revenue of approximately $465 million per year and creating more than 160,000 jobs. In a region where the average person consumes 130-180 kilograms (290-400 pounds) of fish annually, fishing supports thousands of families and the local economy.

Since returning from Tefé, Braz-Mota has been attending conferences to warn about the severity of the drought and the risk of its recurrence. However, even with the necessary patience to explain a crucial problem over and over again, her disappointment is evident.

“Extreme events like the one we are experiencing now are not isolated. They tend to repeat themselves with increasingly serious consequences,” she says. “Most of the discussions and mitigation strategies are still concentrated in the south and southeast regions” of Brazil, which falls outside the country’s Amazonian region.

The latter, she says, “plays a crucial role in global climate balance, [but] remains underrepresented.”

“We need policies and investments that consider the peculiarities of the region and the urgency of the situation.”

Carlos Freitas is part of a group that released a policy brief with recommendations for policymakers on mitigating the drought. In the short term, the brief proposes changes to the closed fishing season, adjusting the periods for certain species when severe droughts are anticipated. It also suggests including new species on the ban if their vulnerability to extreme events is proven. The brief also stresses the need to intensify monitoring against illegal fishing, preventing the capture of immature fish before their recruitment.

In the medium term, it recommends investing in faster data collection and improving tools for predicting impacts on fish stocks. In a scenario of increasingly frequent extreme weather events, information is the best form of prevention. “In addition, the Amazon needs communication channels between universities, decision-makers and riverine communities,” Freitas says.

Banner image: Fish carcass on the dry bed of Lake Tefé during the 2024 drought. Image courtesy of Alessandro Falco/INCT Adapta.

Citations:

Hercos, A. P., Oliveira, J. A., Oliveira, J. C., Rodrigues, E. K., Barbosa, R. L., & Queiroz, H. L. (2021). Checklist of the ichthyofauna of Mamirauá Sustainable Development Reserve, Middle Solimões, Amazonas, Brazil: High richness in a large protected area of western-central Amazonia. Biota Neotropica, 21(4). doi:10.1590/1676-0611-BN-2021-1207

Marengo, J. A., Souza, C. M., Thonicke, K., Burton, C., Halladay, K., Betts, R. A., … Soares, W. R. (2018). Changes in climate and land use over the Amazon region: Current and future variability and trends. Frontiers in Earth Science, 6. doi:10.3389/feart.2018.00228

Espinoza, J., Jimenez, J. C., Marengo, J. A., Schongart, J., Ronchail, J., Lavado-Casimiro, W., & Ribeiro, J. V. (2024). The new record of drought and warmth in the Amazon in 2023 related to regional and global climatic features. Scientific Reports, 14(1). doi:10.1038/s41598-024-58782-5

Braz-Mota, S., & Val, A. L. (2024). Fish mortality in the Amazonian drought of 2023: The role of experimental biology in our response to climate change. Journal of Experimental Biology, 227(17). doi:10.1242/jeb.247255

Amanajás, R. D., & Val, A. L. (2022). Thermal biology of tambaqui (Colossoma macropomum): General insights for aquaculture in a changing world. Reviews in Aquaculture, 15(2), 480-490. doi:10.1111/raq.12732

Zingel, P., Tuvikene, A., Zingel, T., Val, A. L., & Agasild, H. (2025). How do extreme fluctuations in water level affect fish condition in Amazonian floodplain lakes? Journal of Fish Biology. doi:10.1111/jfb.16063

Furtado, M. D., Queiroz, J. C., Bentes, B., Gouveia, N. D., De Lima, M. J., Ruffino, M. L., & Isaac, V. (2023). How does climate change affect small scale fisheries? A case study of the Lower Amazon in Brazil. Fisheries Management and Ecology, 31(1). doi:10.1111/fme.12654

 
This story was first published here in Portuguese on May 2, 2025.