for National Geographic News
The mighty Mekong River in Southeast Asia was recently the starting point for a scientific adventure to find the world's biggest freshwater fish. Researchers will continue to cast their nets far and wide, from the Amazon to the rivers of the Mongolian steppes.
Their goal: to assess the conservation status of giant catfish, stingrays, gars, carp, salmon, sturgeon, and other freshwater fish species that grow to at least 6.5 feet (2 meters) in length or 220 pounds (100 kilograms) in weight.
Many of these aquatic monsters are thought to be seriously threatened by overfishing and habitat destruction. But because such fish often inhabit deep, murky waters in remote regions, relatively little is known about them.
Researchers also hope to identify the planet's largest freshwater fish. The current record holder is the Mekong giant catfish, according to Guinness World Records. Among possible challengers for the title are a massive river stingray from Cambodia and the Chinese paddlefish of the Yangtze River.
Jointly funded by the World Wildlife Fund (WWF) and the National Geographic Society, the project will involve a network of more than a hundred scientists in 17 countries. WWF freshwater conservation biologist Robin Abell says the study should assist in establishing urgent conservation priorities and raising awareness about these poorly understood species.
"These giants are the freshwater equivalents of elephants and rhinos. And if they were visible to us on land, the world wouldn't stand by while they disappeared," she said. "This study will give us new insight into how these species live and what threatens their survival. In the end we'll know better how to manage fishing and protect habitats to save the species for the future."
The lead scientist is biologist Zeb Hogan, a WWF conservation science fellow. "This is the first study to examine all of the world's giant freshwater fish. Like previous studies of coral reefs and the deep sea, the study will serve as a window to an amazing underwater world that people seldom see," he said.
Hogan is currently collecting data in Cambodia's Mekong River Basin, home to an impressive range of freshwater leviathans. These include the Mekong giant catfish, officially the world's biggest river fish, which can tip the scales at more than 660 pounds (300 kilograms). Other notable Mekong heavyweights are the giant carp, Pangasius catfish, and giant stingray.
Next month Hogan and other researchers will travel the length of the Cambodian Mekong in search of clues about past and present distributions of giant fish. The scientists will interview local fishers to locate areas where such fish may still occur.
The researchers will use population counts, distribution trends, and other biological indicators to determine the conservation status of each species.
The Mekong giant catfish (Pangasianodon gigas) is considered the most endangered fish in Southeast Asia. Last year the World Conservation Union classified the species as critically endangered.
Hogan says the fish is threatened with extinction because of overfishing, dam construction, and the controversial Upper Mekong Navigation Project. "This project involves dredging and blasting in areas where the giant catfish spawns," he noted.
The biologist says species like the giant carp (Catlocarpio siamensis), which can weigh up to 660 pounds (300 kilograms), are vulnerable to the negative effects of fishing, because it takes the fish a long time to become sexually mature.
"The large fish hold enormous reproductive potential. One good spawning year can make an enormous difference to the population," Hogan said. "But kill too many adult fish and, in all likelihood, the population will crash."
The giant stingray (Himantura chaophraya) is thought to be threatened with extinction in rivers such as the Mekong. Some reports suggest this species could reach 1,100 pounds (500 kilograms) in weight and 16.4 feet (5 meters) in length. If confirmed, this would make the stingray larger than the Mekong giant catfish, the current world record holder.
"Based on initial reports from fishermen, we believe very large stingray may still occur in the deep pools of northern Cambodia," Hogan said. "Until now no one has gone looking specifically to find this species."
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