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Past and future tropical dams devastating to fish the world over

Most research on the ecological impacts of tropical dams does so one dam project at a time. But a new landmark study attempts to connect the dots globally by analyzing tropical dam impacts on freshwater river fish around the world.The research assembled data on the geographic range of 10,000 fish species, and checked those tropical species against the location of 40,000 existing dams and 3,700 dams that are either being built or planned for the near future.Scientists found that biodiversity hotspots including the Amazon, Congo, Salween and Mekong watersheds are likely to be hard hit, with river fragmentation potentially averaging between 25% and 40% due to hydropower expansion underway in the tropics.Dams harm fish ecology via river fragmentation, species migration prevention, reservoir and downstream deoxygenation, seasonal flow disruption, and blockage of nurturing sediments. Drastic sudden fish losses due to dams can also destroy the commercial and subsistence livelihoods of indigenous and traditional peoples.

Hydropower dams are rising on rivers throughout the tropics, their energy promoted as vital to development, or hyped under the banner of renewable energy. But old dams have been having, and new dams are likely to have, disastrous impacts on river fish, according to a new global assessment by researchers at Radboud University, the PBL Netherlands Environmental Assessment Agency, and the Stanford Natural Capital Project.

The study, recently published in the PNAS journal, mapped the existing and projected impacts of current and future tropical river dams on thousands of fish species, and showed that dam construction will increase habitat fragmentation along rivers like the Amazon, Niger, Congo, Salween and Mekong by a quarter or more.

“Understanding the impact of fish habitat fragmentation due to dams is key to start quantifying these [ecological] tradeoffs,” says Valerio Barbarossa, a researcher at the PBL Netherlands Environmental Assessment Agency and lead author on the paper.

While researchers have long suspected that global development of dams was becoming a severe threat to river habitat, there were no global studies attempting to quantify those effects. Dams are currently most prevalent — and habitat fragmentation correspondingly highest — in the U.S., Europe, South Africa, India and China. But with hydropower development shifting rapidly to the tropics in recent years, ecologists have been sounding the alarm.

Barbossa’s interest in the topic led him to begin synthesizing available knowledge on the subject as part of his PhD research. He collected and assembled data on the geographic range of 10,000 fish species, and checked those species against the location of 40,000 existing dams.

He also looked beyond existing dams to the future impacts of 3,700 dams that are either being built or planned for the near future. Barbossa found that biodiversity hotspots like the Amazon, Congo and Mekong watersheds — home to charismatic giants like freshwater stingrays, and a huge array of smaller species — were likely to be hard hit.

Habitat “Fragmentation might be as high as 40% on average due to the current hydropower expansion that is [underway] in the tropics,” Barbarossa told Mongabay. “[F]or instance, the completion of one planned dam on the Purari River in Papua New Guinea might have very high impacts on fish that migrate to and from the ocean during their lifecycle.” He noted that this particular dam could cut freshwater fish habitat connectivity by about 80%.

Shattering river connectivity and altering ecology

Habitat fragmentation occurs when previously large swaths of landscape (or riverscape in this case) are broken up by development — by roads, plantation agriculture, pipelines, or dams — isolating genetic populations of animals and dwindling their available territory.

“The role of dams in blocking fish migrations is an impact that reduces or eliminates the reproduction of these species, reduces their ranges and breaks populations into isolated groups,” said Philip Fearnside, a professor at Brazil’s National Institute for Research in Amazonia who was not involved with the study.

In some cases, he noted, dams can also have an opposite but still adverse effect, with fish ladders aiding species in bypassing natural barriers such as river rapids removed by dams, allowing those species to invade areas where they were not native.

Also ecologically detrimental: dams convert formerly fast moving streams into still-water reservoirs, with the water at the bottom becoming oxygen-poor, potentially wiping out bottom-dwellers — not only within the manmade lakes, but also downriver. “The water released from the turbines and spillways usually comes from depths in the reservoir where there is little or no oxygen, thus killing fish downstream,” said Fearnside.

The adverse biodiversity impacts of tropical dam construction can be seen in the severe depletion of the Mekong giant catfish and Amazon giant catfish, as well as in the recent extinction of the Chinese paddlefish, which had survived millions of years, but whose numbers were greatly reduced by dams within the species’ habitat.

Species losses don’t only diminish diversity: tropical river fish are essential to the commercial and subsistence livelihoods of indigenous and traditional peoples, Fearnside said. When dams eliminate fish, they also end those livelihoods, sometimes with catastrophic economic and social impacts, forcing sustainable communities into cash economies for which they may be little prepared.

Detrimental downstream impacts

Dams have other potent downstream effects. In addition to blocking migrations and lowering downstream oxygen content, dams release water primarily when there’s a need for power, making river flows unpredictable. This thwarts the natural rising and falling seasonal cycles of river levels downstream of dams, eliminating an important signal for fish behavior which can be key to healthy riparian ecology.

In Amazonian floodplain lakes, for example, where many fish reproduce — including important commercial species — the natural seasonal peak flood pulse brings nutrient-rich water and sediments into rainforest lakes, supporting the growth of newly hatched fish in critical “nursery” habitats. But once dams are built, seasonal pulses merely back up behind them.

“Dams trap sediments in their reservoirs, thus reducing nutrient content and fish production in the downstream river stretches,” Fearnside explained. “Particularly ironic are the major dams that Brazil plans to build in Peru and Bolivia, which will trap sediments and thus reduce fish production in Brazil,” harming that nation’s ecology and fisheries.

“We knew that future river infrastructure development [would] impact fish species, but in most places with acute development pressure, there were very few comprehensive data [points] to evaluate potential impacts,” said researcher Rafael Schmitt, of the Stanford Natural Capital Project and a study co-author. The scientists suggest that their data may help developers plan dams more strategically to avoid devastating impacts.

Fearnside praised the new study’s scope, noting that most such research occurs “one dam at a time,” and that the team’s work was an important indicator of the global scale of the problem.

However, he took issue with other aspects of the research which pointed toward win-win outcomes: the idea that the study can aid in picking sites for hydropower expansion or in creating river bypass construction that will be less damaging to fish populations.

“My reading of the meaning of the results is not so favorable to supposed ‘win-win’ outcomes,” he said. “To me the results indicate that, with rare exceptions, we should simply stop building big dams… It is important to keep focus on the decision to build or not build dams, rather than assuming that dams are inevitable and that only modest add-ons to soften impacts are open to discussion.”


Barbarossa, V., Schmitt R., Huijbregts, Mark., Zarfl, C., King, H., Schipper, A. (2020.) Impacts of current and future large dams on the geographic range connectivity of freshwater fish worldwide. Proceedings of the National Academy of Sciences 117 (7) 3648-3655. DOI: 10.1073/pnas.1912776117.

Banner image caption: An Arapaima, among the world’s largest freshwater fish, at the Cologne Zoological Garden. These Amazonian fish depend on flood pulses, an aspect of river ecology that dams restrict.  Image by Superbass, copyright CC BY-SA 3.0.

Source: News Mongabay

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