By Zeb Hogan
Assistant Research Professor, University of Nevada
It's unclear why so many species of giant fish occur in the Mekong River, the 2,700-mile (4,350-kilometer) river that runs from southern China to the delta south of Ho Chi Minh City, Vietnam. Certainly part of the answer is the river's size: Large rivers have more space and more food to accommodate larger fish.
Another part of the answer may lie in the productivity of the Mekong River Basin ecosystem, including the floodplains and flooded forests that provide an abundant source of food for many species of fish during the rainy season.
The Mekong River is also—depending on whom 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.
Not only is the diversity of large fishes found in the Mekong amazing, so is these fishes' persistence, given the number of people who live on the river and the level of fisheries’ exploitation. It just goes to show that fish populations can be remarkably resilient: It's not typically overfishing that drives species to extinction. Usually, it’s 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 the fact that most habitats and connections between habitats are still intact—is still capable of producing 2,500,000 million tons of fish a year. That makes it the most productive river in the world.
Given that the Mekong does produce so much fish, it's not unreasonable to question whether the benefits of proposed dam projects will outweigh the environmental costs. It's a question that needs to be answered (and will require more study) before construction of the dams moves forward.
The hydropower dam planned on the Mekong River in Sayabouly Province, northern Laos, is a threat to the survival of the wild population of Mekong giant catfish. Under threat are the suspected spawning locations for many species of fish. The Sayabouly dam is the first lower Mekong River mainstream dam to enter a critical stage of assessment before construction is approved by the Mekong River Commission, which includes representatives from Cambodia, Laos, Thailand, and Vietnam, according to a recent World Wildlife Fund (WWF) report on the ecological implications of new dam projects.
The other dam closest to being approved is the Sahong. The Sahong channel is the most important migratory pathway in Southern Laos.
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 and in 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.
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 now in grave danger after dam construction (one, the Chinese paddlefish, may already be extinct).
Beyond dams, the other threats to the Mekong’s megafish include over-harvest (which has already brought populations of giant Mekong species to very low levels), habitat degradation (such as 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, the Mekong giant catfish can reach 10 feet (3 meters) long and weigh up to 650 pounds (300 kilograms). This critically endangered species has suffered from all of the above—overfishing, dam building, and habitat destruction.
The risk of losing these fish before we understand them—and the threats they face—cannot be overstated.
Up to 80 percent 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.
Mitigation
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 indefinitely.
• Research and decision-making based on research: This may seem like standard scientist-speak, but research on the ecology and conservation status of giant fish is urgently needed in the Mekong River Basin. The "dog-eating" catfish (Pangasius sanitwongsei) is a case in point. We know almost nothing about its ecology or conservation status and yet it is undoubtedly one of the largest, most rare, and most vulnerable fish in all of Southeast Asia. It’s likely that at least a hundred times more research is being done on salmon in the Pacific Northwest of the United States than on fish in the Mekong, but the consequences of losing the Mekong’s fish are a hundred times more significant in terms of biodiversity and potential impact to livelihoods
