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Climate change makes its presence felt in the Amazon’s shrinking fish

  • Studies show that the effects of climate change can already be seen in Amazonian fish, which are growing smaller and less abundant in wetlands and streams; female fish are also reproducing at a younger age.
  • It’s estimated that half of all threatened fish species in the Brazilian Amazon are sensitive to the impacts of climate change.
  • A project by the National Institute for Amazonian Research (INPA) is documenting these impacts through the perspective of Indigenous communities, and has found that they match what the scientific data already show.
  • Indigenous communities like the Ticuna and the Kokama in the Upper Solimões region are reporting the disappearance of large fish, the need to travel longer distances to find places to fish, and warmer waters in rivers and streams.

“Everything is happening at the wrong time,” says Myrian Pereira Vasques. “The trees are blossoming at the wrong time, the soil isn’t the same, the weather is too hot and the fish are dying in the dry season. People who fish for a living are feeling it.”

Myrian lives in the Amazonian village of Filadélfia, home to the Indigenous Ticuna people of the Upper Solimões River region, near Brazil’s border with Colombia and Peru.

“When the rains come, they are extremely heavy. This also impacts farmers and people who fish,” she says. “These days, fishermen sometimes come home empty-handed. The fish they catch are so small that it makes them sad.”

Myrian’s father, João Almeida Vasques, is one of these fishermen. He tells his daughter in their native Ticuna language that he can no longer sustain their family by selling what he catches; there are fewer fish today than there once were. “I don’t catch big fish anymore, or manatees, there aren’t any big pirarucus or tambaquis, or any big maparás. We don’t even find caimans anymore. This affects us a lot,” Myrian translates for her father.

Myrian studies agricultural engineering with the help of a scholarship from Projeto Climas (The Climate Project), an initiative of the National Institute for Amazonian Research (INPA). Since November 2021, the project has worked to identify the effects of climate change through the perspective of Indigenous communities.

The changes reported by the Indigenous people interviewed are in line with the data collected by the scientific community. A 2016 study by Brazilian and U.S. researchers found that half the threatened fish species in the Brazilian Amazon are considered sensitive to the impacts of climate change. Other studies have shown that, aside from diminishing numbers of their populations, some species are already physically manifesting the impacts of climate change: many are smaller in size, and females of some species are reproducing at an earlier age.

A pirarucu (Arapaima gigas), one of the world’s biggest freshwater fish, is common to the wetlands of the Amazon. Indigenous communities that catch the fish have noticed that their size is diminishing. Image by Cliff via Wikimedia Commons (CC BY 2.0).

Stronger swimmers get ahead in Amazonian streams

“The eastern part of the Amazon, over there in Pará [state] where they’re feeling the most effect from the Arc of Deforestation, is where things will worsen with time because of extreme climate events, mostly extreme droughts,” says Jansen Zuanon, a co-author of the 2016 study who has researched Amazonian fish for 40 years. “In contrast, in the west, along the border with Peru and Colombia, we are expecting the opposite — a disproportionate increase in rainfall.”

Not only does climate change affect different geographical regions of the Amazon differently; its effects on the native fish also vary depending on their habitats.

“The number of bodies of water is staggering. It really looks like a blood circulation system in a living organism,” Zuanon says, referring to the igarapés, the cobweb of small streams running hidden beneath the forest canopy, occasionally reflecting rays of sunlight through openings in the trees. “Every square kilometer of rainforest holds 2-4 linear kilometers [1.2-2.5 miles] of igarapés. They meander through the forest in curvy beds; igarapés never run in a straight line, but rather in a path of very sharp curves. We think of the Amazon as being flat, like a plain, but it is, in fact, a wrinkled plain.”

Igarapés comprise 80% of the Amazon’s watershed and have a very stable temperature of 23-25° Celsius (73-77° Fahrenheit). Changes in climate associated with deforestation are expected to increase the temperature of the environment and, consequently, of these waters.

“Many of the igarapé fish won’t be able to deal with this change physiologically,” Zuanon says. “They evolved in a very stable environment, so any abrupt change affects the biology of these organisms, interrupts their reproduction and movement patterns. They will probably suffer quite a bit.”

A 2020 study that Zuanon co-authored, carried out in the central Amazon, showed that increased rainfall in recent decades had already affected fish in the igarapés. After analyzing different species over an 18-year period, the study found that small fish with large fins and a fusiform shape — much thicker in the middle than at the ends — increased in number. These characteristics suggest that strong swimmers with a greater capacity for maneuvering during more frequent flooding have better chances of survival.

The doublespot acara (Aequidens pallidus), one of the species whose numbers have fallen in the igarapés of the central Amazon. Image by David J. Stang via Wikimedia Commons (CC BY-SA 4.0).

Female wetland fish are reproducing earlier

Meanwhile, the rainforest’s wetland fish — those living in large rivers that spill into floodplains sometimes stretching 100 kilometers into the rainforest — are more accustomed to water temperature variation and have the capacity to spend long periods of time in warm-water lakes with low oxygen levels.

“The wetland fish are much better adapted to extreme variations in temperature and oxygen levels than the igarapé species,” Zuanon says. “But this doesn’t mean the wetland fish don’t suffer as well.”

A 2022 study that Zuanon also co-authored, this one from Lago Catalão, a wetland region at the confluence of the Amazon and Negro rivers near the city of Manaus, was based on water level data collected over 113 years and fish biology data collected over 19 years. The study found that changes in water flow are already affecting the 16 most common species of wetland fish in the central Amazon.

The main changes noted were fewer large adult females, smaller females at sexual maturity, and smaller mature females in general.

“We found that, as time passed, fish numbers dropped. Their size at the age of reproduction also reduced. They are reproducing earlier and growing less,” Zuanon says. “As time passes, the tendency is to reinforce this pattern. In other words, fish will continue to reduce in size. This is because it’s advantageous: you manage to survive by shortening your life cycle. To be successful, you have to leave offspring. It’s the law of natural selection.”

Yet another of Zuanon’s studies in the region, based on 65 years of water variation data and published in 2017, showed that severe droughts like one that occurred in 2005 have repercussions for years to come, causing changes in fish behavior.

“If we were to have two or three droughts in a row, or two or three heavy flood seasons in a row, we could see results in the fish communities and fishing communities that last for many years,” Zuanon says.

The Indigenous Kokama village of Santo Antônio during the flood season. Image courtesy of Projeto Climas.

The Indigenous communities’ perspective

“If they used to be able to catch fish in front of their houses, today they have to travel much farther,” says Thatyla Farago, coordinator of Projeto Climas, referring to the Indigenous communities of the Upper Solimões region. “When we speak with the elders, they say it’s difficult to predict when the river will rise or dry up, and this is affecting their fishing and farming. The species they are used to catching are hard to find and smaller than usual when they do find them, and less abundant.”

In addition to tracking climate change through the perspective of the Indigenous communities in the Upper Solimões, with a focus on the municipalities of Benjamin Constant and Tabatinga, Projeto Climas also trains Indigenous women to carry out climate change mitigation projects themselves.

Because they’re dependent on the availability of natural resources, Indigenous people in the region are considered quite vulnerable to climate change — and the women especially so because of the many roles they fill.

Projeto Climas researcher Myrian Pereira Vasques measures the water level on a bridge over the Santo Antônio igarapé, which connects Indigenous communities to the municipality of Benjamin Constant. Image courtesy of Projeto Climas.

The Indigenous residents of both the Ticuna village Filadélfia and the Kokama village of Santo Antônio have long realized that the fish are shrinking in size, are now found far from their usual fishing grounds, and come in numbers much too small to feed their families and sell. In one survey, many residents mentioned warmer waters in the rivers, igarapés and lakes than in previous years. They also said it was impossible to predict the rains and that igarapés that had previously never run out of water were now dry.

“Scientific data on climate change is very sparse in the Upper Solimões region,” Farago says. “Since we have no historical data, we work together with traditional knowledge, recovery and memory to try and understand exactly what’s happening in the region.”

Fishing, which benefits millions of families by providing food, income and a means of sustenance, is one of the services offered by freshwater fish. But the ways in which fish contribute to a functioning ecosystem is much broader and in part unknown, as a 2022 study shows (also co-authored by Jansen Zuanon). In aquatic environments, fish are part of the food chain, work to recycle nutrients, and help conserve forests by dispersing seeds, among other functions.

In Brazil, freshwater fish are so abundant that a new species is described almost every three days. “On any given expedition that we take a certain distance from the large towns, we find new fish species,” Zuanon says. “This has been a constant. We describe new freshwater fish species at the rate of more or less 100 new species per year. It’s a lot of new animals.”



Frederico, R. G., Olden, J. D., & Zuanon, J. (2016). Climate change sensitivity of threatened, and largely unprotected, Amazonian fishes. Aquatic Conservation: Marine and Freshwater Ecosystems26(S1), 91-102. doi:10.1002/aqc.2658

Borba, G. C., Costa, F. R., Espírito‐Santo, H. M., Leitão, R. P., Dias, M. S., & Zuanon, J. (2020). Temporal changes in rainfall affect taxonomic and functional composition of stream fish assemblages in central Amazonia. Freshwater Biology66(4), 753-764. doi:10.1111/fwb.13675

Röpke, C., Pires, T. H., Zuchi, N., Zuanon, J., & Amadio, S. (2022). Effects of climate‐driven hydrological changes in the reproduction of Amazonian floodplain fishes. Journal of Applied Ecology59(4), 1134-1145. doi:10.1111/1365-2664.14126

Röpke, C. P., Amadio, S., Zuanon, J., Ferreira, E. J., Deus, C. P., Pires, T. H., & Winemiller, K. O. (2017). Simultaneous abrupt shifts in hydrology and fish assemblage structure in a floodplain lake in the central Amazon. Scientific Reports7(1). doi:10.1038/srep40170

Pelicice, F. M., Agostinho, A. A., Azevedo-Santos, V. M., Bessa, E., Casatti, L., Garrone-Neto, D., … Zuanon, J. (2022). Ecosystem services generated by neotropical freshwater fishes. Hydrobiologia. doi:10.1007/s10750-022-04986-7

Banner image: A catch of tambaqui, a commonly eaten fish from the piranha family. Image by Wenderson Araujo/Trilux via Flickr (CC BY-NC-SA 2.0).

This story was reported by Mongabay’s Brazil team and first published here on our Brazil site on Jan. 30, 2023.

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