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Why damming the world’s most productive river could have serious negative consequences

No one knows more about the giant freshwater fish of the Mekong and the world’s other big rivers than Zen Hogan, an aquatic ecologist and National Geographic Emerging Explorer. Hogan has been featured in dozens of National Geographic stories on the web and on television as a champion of megafishes. In this blog post Hogan comments on...

No one knows more about the giant freshwater fish of the Mekong and the world’s other big rivers than Zen Hogan, an aquatic ecologist and National Geographic Emerging Explorer. Hogan has been featured in dozens of National Geographic stories on the web and on television as a champion of megafishes. In this blog post Hogan comments on a report issued last week by the international conservation charity WWF, which warned warns that wild populations of giant catfish will be driven to extinction if hydropower dams planned for the Mekong go ahead: Dog-eating catfish, other river giants threatened by Mekong dam plan.

By Zeb Hogan

It’s unclear why so many species of giant fish occur in the Mekong River–certainly part of the answer is the river’s size: Large rivers have more space and more food to accommodate larger fish.

Part of the answer many also lie in the productivity of Mekong ecosystem, the floodplains and flooded forest in particular provide an abundant source of food for many species of fish during the rainy season.

The Mekong River is also–depending on who you ask–either the second or third most biodiverse river on Earth (in terms of freshwater fish) and it’s logical that a river with so many species of fish would also support several species of giants.

All that said, the Mekong is not only amazing in its diversity of large fish, but also these fishes’ persistence given the number of people and level of fisheries exploitation on the river. It just goes to show that fish populations can be remarkably resilient: It’s not typically overfishing that drives species to extinction but rather habitat degradation or invasive species.

In this sense, the Mekong River is still a relatively healthy, natural, free-flowing river–a river that, in large part due to that most habitats and connections between habitats are still intact–is still capable of producing 2,500,000 million tons of fish a year (making it the most productive river in the world).

National Geographic Mekong giant catfish video courtesy of Zeb Hogan

Given that the Mekong does produce so much fish, it’s not unreasonable to question whether or not the benefits of the proposed dams will outweigh the costs. It’s a question that needs to be answered (and will require more study) before construction of the dams moves forward.

Sayabouly dam.jpg

Map courtesy of WWF

WWF is absolutely correct to suspect that mainstream Mekong dams will have deleterious effects on the giant fish of the Mekong. Almost all of the information that we have about these species (e.g. the Mekong giant catfish is highly migratory, endemic to the Mekong, seems to need specific cues to spawn, cannot reproduce in reservoirs, and probably spawns in northern Thailand/Laos) suggests that the Sayabory dam and other Mekong dams will have serious negative impacts.

The same is true of other species of Mekong giants: We know very little about the ecology of these species and what we do know suggests that they need healthy, free-flowing rivers to survive.


A boy poses with a giant barb on the Tonle Sap River near Phnom Penh, Cambodia. The fish, landed as bycatch by a local fishing operation, was tagged and released as part of a study of large freshwater species in the Mekong River Basin. There is evidence that giant barb once reached sizes of 10 feet (3 meters) long and 660 pounds (300 kilograms), but today specimens even half that size are extremely rare.

Photograph courtesy Zeb Hogan

Without further study, it’s highly likely that mainstream dams will drive at least one, if not all, of these species to extinction. We’ve seen something similar happen on the Yangtze where the two largest species in that river are in grave danger (one, the Chinese paddlefish, may already be extinct).

Mainstream Mekong dams are the number one threat to the survival of these species. Because we do not fully understand the impacts of the dams, based on what we do know the dams are highly likely to have serious negative impacts, and these impacts will be very difficult to mitigate.

Other threats include over-harvest (which has already brought populations of giant Mekong species to very low levels), habitat degradation (like dredging and blasting upstream of the only known spawning ground of Mekong giant catfish), and invasive species.


One of the largest fish in the world, Mekong giant catfish, such as this one on a fisherman’s boat, can reach 10 feet (3 meters) long and weigh up to 650 pounds (300 kilograms). This critically endangered species has suffered from overfishing, dam building, and habitat destruction.

Photograph courtesy Zeb Hogan

There are several actions that would help ensure the survival of the giant fish species of the Mekong, including:

  • Maintenance of connectivity between rearing grounds and spawning habitat: Many species of Mekong fish have complex life cycles that involve long distance migrations. Maintenance of migratory pathways is crucial.
  • Management of the river for environmental flows: Both the fish and the fisherfolk of the Mekong rely on the natural dry season/rainy season cycle. Flows often cue fish to migrate or spawn and the high flows of the rainy season open up vast habitats for feeding fish. Likewise, local people have invented all manner of ingenious ways of catching fish and most of these methods are adapted to a specific site, flow, and time of year.
  • Regulation and monitoring of harvest: Over-harvest is a serious threat to the Mekong’s largest, longest-lived, and most vulnerable species. In areas with heavy fishing pressure (and that includes virtually the entire Mekong Basin), catch of the largest fish must be regulated to ensure their survival. Lessons from other parts of the world indicate that relatively slow growing large-bodied fish cannot sustain heavy fishing pressure indefinilty.
  • Research and decision-making based on research: This may seem like standard scientist-speak–i.e. “more research is needed”–but in the Mekong River Basin research on the ecology and conservation status of giant fish is urgently needed. The “dog-eating” catfish Pangasius sanitwongsei is an excellent case in point: We know almost nothing about its ecology or conservation status and yet it is undoubtedly one of the largest, rarest, and most vulnerable fish in all of Southeast Asia.

The risk of losing these fish before we understand them (and the threats they face) cannot be overstated.

70-80% of Mekong giant fish are at risk of extinction.

Several large-bodied catfish of the Mekong are migratory.

Mekong giant catfish, “dog-eating” catfish, and giant barb (Catlocarpio siamensis) are extremely rare with only 5-10 adult fish caught per year.

Two other comments:

  • It’s important to emphasize how little research is currently being done on Mekong fish. I don’t think it’s an exaggeration to say that there is at least a hundred times more research being done on salmon in the Pacific Northwest U.S. than on fish in the Mekong, but the consequences [of losing the fish] are a hundred times more significant (in terms of biodiversity and potential impact to livelihoods) on the Mekong.
  • The two dams that are the closest to being approved (Sayabouri and Sahong) are two of the potentially more damaging: Sayabouri is a suspected spawning location for many species of fish and Sahong channel is the most important migratory pathway in Southern Laos.

The Mekong is truly a special place, and it has been gratifying to see recognition of the existence and uniqueness of the Mekong’s imperiled giants, but we now need to take concrete steps to protect these species for future generations.


Photograph of Zeb Hogan by Brant Allen

Zeb Hogan earned an undergraduate degree in ecology and evolutionary biology from the University of Arizona. He later became a visiting Fulbright student at the Environmental Risk Assessment Program at Thailand’s Chiang Mai University. Returning to the United States, Hogan completed a National Science Foundation-sponsored Ph.D. in ecology at the University of California, Davis. He is currently a fellow at the University of Wisconsin and a World Wildlife Fund fellow. Hogan also leads Megafishes, a National Geographic Society project to identify and protect the world’s largest freshwater fishes.

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