Search Is on for World's Biggest Freshwater Fish

James Owen
for National Geographic News
December 14, 2004
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.

Mekong Catfish

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."

Depleted Stocks

The disappearance of such species is a potentially serious problem for rural peoples that depend on fish for protein, according to the World Resources Institute in Washington, D.C. The environmental nonprofit recently reported that depleted freshwater fish stocks in Cambodia have led to violent conflict between fishers.

Elsewhere, the WWF- and National Geographic Society-sponsored big-fish project will focus on species such as the Amazon's arapaima (Arapaima gigas), the largest fish in South America, and the Yangtze River's Chinese paddlefish (Psephurus gladius), a critically endangered species plagued by overfishing and the construction of dams that block migrations and isolate populations.

Dambuilding has similarly impacted on the Tigris salmon (Barbus esocinus) of Syria, Iraq, and Iran, according to Brian Coad, an ichthyologist (fish biologist) at the Canadian Museum of Nature in Ottawa.

The fish is actually a type of huge carp, Coad said. He noted that populations of Tigris salmon are also severely threatened by water extraction, pollution, and fishing with explosives and poisons.

In Mongolia the taimen (Hucho taimen), a true member of the salmon family, is under pressure from illegal poaching and degraded water quality caused by logging and mining operations. (See related story.)

Jake Vander Zanden, a freshwater ecologist at the University of Wisconsin, Madison, said the taimen's "range includes the former Soviet Union and Mongolia, though it is now quite rare or [exterminated] throughout much of its historical range."

Zeb Hogan says time is now running out for this and other charismatic species. "Due to the precarious state of populations of large freshwater fish, this new project is a race against the clock," he said. "We must identify and protect these aquatic giants before they are gone forever."

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