LIMA—Once upon a time, thousands of dorados, a giant among catfish, would swim more than 3000 kilometers from the mouth of the Amazon River to spawn during the austral autumn in Bolivia's Mamoré River, in the foothills of the Andes. But the dorado, which can grow to more than 2 meters in length, is disappearing from those waters, and scientists blame two hydropower dams that Brazil erected a decade ago on the Madeira River.
"The dams are blocking the fish," says Michael Goulding, a Wildlife Conservation Society aquatic ecologist in Gainesville, Florida, who has been studying the dorado since the 1970s. They are "probably on their way to extinction" in Peru and Bolivia.
Most Amazon dams are in Brazil, where scientists have raised concerns about the displacement of local communities and emissions of greenhouse gases such as carbon dioxide and methane from large reservoirs. But as countries seek new energy sources to drive economic growth, a surge in dam construction on the eastern flank of the Andes could further threaten fish migration and sediment flows, Elizabeth Anderson, a conservation ecologist at Florida International University in Miami, and colleagues warn today in Science Advances.
For the ecology of the western Amazon Basin, where the mountains meet the lowlands, the main consequence of proliferating dams is habitat fragmentation. Interference with spawning is one facet. Another is that dams hold back sediments and nutrients that nourish the Amazon Basin, Anderson says. Her team documented 142 hydropower dams that are operating or under construction on headwaters in the western Amazon Basin, and another 160 that are under consideration. If even a fraction of the planned projects is completed, the habitat disruption could have a cascade of ecosystem effects with devastating consequences, scientists say.
The disappearance of the dorado (Brachyplatystoma rousseauxii) from the Mamoré River suggests fragmentation is already taking a toll. And that's despite features of the dams that are meant to mitigate their impact. The Madeira dams, for example, are designed to allow fish to pass: The lower dam has a bypass channel and the upper dam has an enclosure in which fish are captured. They are then trucked upstream for release into the reservoir. Perhaps because of variations in currents or water chemistry, dorados are not using the channel, says Carolina Rodrigues da Costa Doria, an ichthyologist at the Federal University of Rondônia in Porto Velho, Brazil. The threat may not be limited to fish: Freshwater dolphins and river otters may also migrate along Amazonian rivers, and how dams affect their behavior is unknown, says Paul Van Damme, director of the Institute for Applied Research on Water Resources, a research center in Cochabamba, Bolivia.
During the rainy season—from November to May—water levels rise in the Amazon Basin, flooding large tracts of forest. Various species of fish swim into the forest, where they feed on fruits—and later disperse seeds. By blocking migration routes or changing water levels, dams change seed dispersal patterns, says Sandra Bibiana Correa, a freshwater ecologist at Mississippi State University in Starkville. The fish-forest pas de deux "is a really delicate interaction," she says. "It has been going on for tens of millions of years. We can disrupt that very easily."
Some species are taking advantage of the disorder. Another kind of giant catfish known as the manitoa or piramutaba once rarely ventured upstream of rapids that predated the Madeira River dams. Unlike its cousin, this species (B. vaillantii) can make it through both dam bypasses and into the upper reservoir, and from there swims another 1000 kilometers or so upstream to Peru's Madre de Dios watershed. Whether the newcomer will fill the dorado's ecological role or prey on different fish and thus skew species assemblages is unclear, says Carlos Cañas, a river ecologist at the Wildlife Conservation Society here, who plans to monitor the migration of large catfish in the watershed.
Two other threats—climate change and the deforestation that accompanies road building during dam construction—could amplify the severity of ecological deterioration, Anderson says. At stake, she says, are the livelihoods of indigenous peoples who depend on fishing. Other projects, such as Peru's plans to dredge rivers to improve navigability, could exacerbate the ecological impact by changing flows, disturbing spawning sites, and disrupting the river-forest connection, says Fabrice Duponchelle of the French Institute of Research for Development in Marseille.
Amazon nations should work together to craft a basin-wide management plan for migratory fish, says Thomas Lovejoy, a tropical ecologist at George Mason University in Fairfax, Virginia. "You have to manage the Amazon as a system," he says. It's not too late to preserve much of the ecology of the western Amazon Basin, where "we still have a lot of free-flowing rivers," Anderson adds. "There is a huge opportunity to protect at least a subset of them by thinking at a regional scale."