The Mekong Catfish

The world’s largest scaleless freshwater fish lives a tenuous existence in the murky brown waters of Southeast Asia’s Mekong River. Capable of reaching an almost mythical 10 feet (3 meters) in length and 650 pounds (295 kilograms), Mekong giant catfish live mainly in the lower half of the Mekong River system, in Cambodia, Laos, Thailand, and Vietnam.

Once plentiful throughout the Mekong basin, population numbers have dropped by some 95 percent over the past century, and this critically endangered behemoth now teeters on the brink of extinction. Overfishing is the primary culprit in the giant catfish’s decline, but damming of Mekong tributaries, destruction of spawning and breeding grounds, and siltation have taken a huge toll. Some experts think there may only be a few hundred adults left.

Mekong giant catfish have very low-set eyes and are silvery to dark gray on top and whitish to yellow on the bottom. They are toothless herbivores who live off the plants and algae in the river. Juveniles wear the characteristic catfish “whiskers,” called barbels, but these features shrink as they age.

Highly migratory creatures, giant catfish require large stretches of river for their seasonal journeys and specific environmental conditions in their spawning and breeding areas. They are thought to rear primarily in Cambodia’s Tonle Sap lake and migrate hundreds of miles north to spawning grounds in Thailand. Dams and human encroachment, however, have severely disrupted their lifecycle.

International efforts are under way to save the species. It is now illegal in Thailand, Laos, and Cambodia to harvest giant catfish. And recently in Thailand, a group of fishers pledged to stop catching giant catfish to honor the king’s 60th year on the throne. However, enforcement of fishing restrictions in many isolated villages along the Mekong is nearly impossible, and illicit and bycatch takings continue.

The waters of the Mekong River are very murky and make it difficult to track the movements of the fish. In order to determine where the Mekong catfish have been, scientists examined chemical markers called isotopes in bone and muscle tissue of catfish from the Khone Falls region of the Mekong. The team found evidence that the freshwater-dwelling fish had migrated recently from a marine habitat. Although this method of fish tracking is a highly regarded method, it appears that this is the first time it has been used to track the migration routes of river catfish species from a marine environment.

The recent discovery that Mekong catfish are anadromous, (moving from coastal waters into fresh waters to spawn) has surprised even scientist's long-held notions of freshwater species. It may be that many other species of catfish also have similar migration habits, and that other species of freshwater fish may be found living part of their lives at sea. It certainly gives new meaning to the concept of "freshwater fish", if they spend part of their lives living in the salty waters of a marine environment.

As big as the Giant Mekong catfish can get, there are rumored to be other species of freshwater fish whose dimensions rival those of the Mekong catfish. Among them are the Arapaima and huge freshwater stingrays, both found living in the Amazon River. The giant Chinese paddlefish is also a serious contender for world's biggest freshwater fish, but very little is known about this species that seems to be rapidly disappearing.