New megadams threaten world’s biggest fish

Phnom Penh, CambodiaThere was a time when scores of some of the world’s largest freshwater fishes swam up the Mekong River, past the Laotian town of Luang Prabang, now a UNESCO World Heritage site.

Giant catfish the size of grizzly bears, seven-striped barb, giant pangasius, and other huge fish once made their way past the city’s historic Buddhist temples, French colonial villas, and traditional wooden buildings on their way north to their spawning grounds. Following decades of overfishing in the Mekong River, it’s a rarity to see such river titans today. (Read how the giant freshwater stingray is likely the world’s biggest freshwater fish.)

Still, many scientists held out hope for their recovery: As long as the Mekong River south of China remained undammed, smaller scale fishing and other conservation efforts could eventually lead to the recovery of these critically endangered species.

But now that hope appears to be dimming, with Laos planning to construct at least 10 new dams on the Mekong’s main stem in the next decade. Among the first projects is Nam Sang, a massive hydropower plant to be constructed just upstream of Luang Prabang. 

Laos’ Communist regime, which promotes hydropower as part of its bid to become the battery of Southeast Asia, has built dozens of dams on Mekong tributaries. The government hopes new dams, such as Nam Sang—to be completed by 2027—will generate government revenue by selling electricity to neighboring countries, such as Thailand.

“If these dam projects go ahead, the stretch of the Mekong that was once core habitat and spawning grounds for several giant fish species is going to be chopped into ever-smaller pieces,” says National Geographic Explorer Zeb Hogan, a fish biologist at the University of Nevada, Reno, who has studied Mekong megafishes for more than two decades. “For fish that need free-flowing rivers to survive, this may be the death blow.”

Nam Sang’s Vietnamese developer, PetroVietnam, did not respond to National Geographic’s requests for comment about the project and its potential effects on wildlife. Satellite images show housing has been set up for work crews, but construction of the dam has not begun. (Learn more about how dams are constructed.)

Beyond the Mekong, large fish, dolphins, crocodiles, and other big freshwater animals are also threatened by new dams. Around the world, more than 3,400 major hydropower projects are either planned or under construction, and a big share of those are in biodiverse rivers in tropical regions, according to a recent study in the journal Biological Conservation led by Fengzhi He, a freshwater ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Germany. It finds that the dams there will disproportionately affect the largest river animals.

If they’re built, says He, the dams “will add great uncertainties to the survival” of many freshwater species.

Adds Hogan, a study co-author, “it’s a pattern we’re seeing in other tropical rivers as well, like the Amazon and the Congo, but the Mekong is the poster child for this problem, with accelerating hydropower development driving extinction risk of some of the world’s most iconic animals.”

Mekong ghosts

Freshwater megafauna, loosely defined as species that weigh over 66 pounds on average, are among the most endangered animals on Earth. Global populations have declined by almost 90 percent since 1970—twice as much as the loss of vertebrate populations on land or in the oceans, according to a 2019 study in Global Change Biology.

Large fish, such as sturgeons, salmons, and giant catfishes, particularly in the Northern Hemisphere, have experienced even higher declines, because of overfishing, pollution, and dams.

The Mekong River, which runs through six countries, has historically been home to more species of extremely large fish than any other river system in the world. But of the biggest found here, almost all are now critically endangered. Hogan, for example, hasn’t seen a Mekong giant catfish since 2015.

While China operates a cascade of dams in the Mekong River’s upper reaches—beyond the spawning grounds for many of the region’s megafishes—entangled politics in the Lower Mekong Basin and pressure from conservationists have helped stave off plans for mainstream developments to the south. (Read more about the controversies surrounding dams on the Mekong.)

Conservationists hoped new dams would follow the less-destructive model of two megadams that are already up and running on the Mekong’s main stem.

One, Don Sahong, was built where the Mekong branches into different channels, giving fish other passageways around the dam. At the other, the Thai-financed and constructed Xayaburi Dam, more than $300 million has been spent on efforts to help fish bypass it, including the installment of sophisticated fish ladders. 

Conservationists had hoped “the investment at Xayaburi would be the gold standard, and all future dams would either meet, or exceed, the investment in facilities,” says Lee Baumgartner, a freshwater fish ecologist at Charles Sturt University in Sydney, who studies dam impacts in the Mekong. “But this is not the case. Not even close.”

Interrupted migration

Perhaps the best example of how dams can harm large fish is in China, where researchers declared in 2019 that the Chinese paddlefish, an ancient species capable of growing up to 23 feet long, had gone extinct. While overfishing of the paddlefish had been a problem, scientists concluded it was the Gezhouba Dam, built in the 1980s on the Yangtze River, that ultimately had caused its demise, by cutting the fish off from their only spawning grounds upstream.

“Typical impact assessments still focus on analyzing a small buffer around the construction site itself,” says Günther Grill, a geographer at Canada’s McGill University and a co-author on the recent study. What’s missing, he says, is bigger efforts by governments to find a location for a dam that won’t harm megafauna on a regional scale.

In addition to blocking fish movement, dams also alter the hydrological conditions from which migratory fish take their cues to feed and spawn. The Mekong River system is governed by a giant flood pulse that in the wet season can raise the river by as much as 40 feet. In recent years, however, that flood pulse has been disrupted by regional drought exacerbated by climate change and by China withholding water from its dams in the upper watershed, according to satellite data from the Stimson Center. 

That data also show that in the past three years, water levels throughout the Mekong River system have reached historic lows. (Learn why Southeast Asia is building dams so quickly.)

“For fishes that have evolved to migrate at the onset of the flood pulse, this altered flow regime can create a timing mismatch between when fish actually migrate and the ideal environmental conditions for their offspring,” says National Geographic Explorer Aaron Koning, a University of Nevada, Reno, conservation ecologist who works with Hogan on a USAID project called Wonders of the Mekong to boost biodiversity and ecosystem health in the Lower Mekong.

“In this way,” he says, “dams not only affect the Mekong fish of today but affect the future of fish in the Mekong.”

Dolphin dilemma

If all the 3,400 proposed dams are built, more than 600 rivers longer than 60 miles would no longer be considered free-flowing, according to the study. Many large freshwater fish are highly migratory, and their ability to move along free-flowing rivers is essential to their survival. One of them is the Irrawaddy River in Myanmar, which is home to the endangered Irrawaddy dolphin and the critically endangered Ganges shark.

In the Amazon Basin, which is the most species-rich river in the world, there are more than 400 hydropower dams planned in various tributaries. Of particular concern are two river dolphin species, the tucuxi dolphin and the larger pink river dolphin, which were recently listed as endangered. (Read about Hogan’s search for the world’s megafishes.)

Although river dolphins don’t migrate the way many large fish do, they rely on migratory fish for food. Dams also threaten to box the dolphins into smaller populations, which can lead to inbreeding and a dilution of genetic diversity.

If all these dams are built, “we could be watching the same fate here as in Asia,” says Mariana Paschoalini, an ecologist with the Aqualie Institute in Juiz de Fora, Brazil, referring to the functional extinction of the baiji river dolphin in China’s Yangtze River in 2006.

For his part, Hogan says he hopes he won’t have to bear witness to the demise of the Mekong giant catfish and other river giants. There are some glimmers of hope. For instance, Cambodia recently announced a 10-year moratorium on the building of new dams on the main stem of the Mekong.

But, Hogan warns, “this is how extinctions play out. And if more dams are built in high biodiversity regions in the future, it’s likely to get worse.”