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The river catfish may be overshadowed by its famed cousin, the Mekong giant catfish, which shares the same river system in Southeast Asia. But this less celebrated species is a remarkable fish in its own right and, at 5 feet (1.5 meters) and 99 pounds (45 kilograms), it has no apologies to make in the size department.

River catfish are known to inhabit Tonle Sap Lake in Cambodia. Every year they leave the lake and enter the Mekong River as part of their annual spawning cycle. Breeding begins at the onset of the region's rainy season (May-July).

These unique fish have the nickname "iridescent shark catfish," a seemingly ill-suited comparison for a toothless, freshwater fish. The nickname arises from its tendency to swim close to the surface, raising its dorsal fin above the water. As for iridescence, river catfish have no scales, but their delicate skin is covered with a protective layer of slime that gives them a shiny glow.

These fish are threatened with habitat fragmentation, industrial pollution, and overfishing. Many millions of people depend on the Mekong River for sustenance, and river catfish have long been targeted for their succulent meat.

They are extensively farmed along some sections of the river, such as in Thailand, where wild specimens have become quite rare.

National Geographic Emerging Explorer Zeb Hogan is attempting to learn more about these catfish by conducting a large-scale tagging and monitoring study. Since 2004, his team has tagged more than 3,000 river catfish in hopes of documenting their behavior and helping authorities to provide better protections.

Sometimes called the "king of fish," the giant barb has a storied history in its Southeast Asia range. In Vietnam, this distinctive, large-headed species is called cá ho. In Cambodia, the barb appears in ancient temple carvings at Angkor and has been named the national fish.

The giant barb is a river fish that often frequents deep pools but may move seasonally into canals or river floodplains. Juveniles are often seen in swamps or smaller river tributaries.

Though they've been known to reach 660 pounds (300 kilograms), specimens above 220 pounds (100 kilograms) have become exceptionally rare in recent years. These massive fish sustain their bulk on tiny plants such as phytoplankton as well as algae, seaweed, and, during periods of high water, the fruits of submerged terrestrial plants.

Scientists fear that cá ho populations have declined to the point where few survive to reach the age of sexual reproduction. This fish has become severely threatened along its native range, which stretches from Cambodia to the Mekong Delta, because of water pollution, river traffic, and especially overfishing pressures.

The flesh of the giant barb has long been considered a delicacy among residents of the Mekong River Basin, and it is a popular eating and pickling option for the tens of millions of people who depend on the Mekong's aquatic fauna for food.

Government programs have focused on captive breeding in an attempt to save this regional icon. Young giant barb can become acclimated to pond life and may be suitable for farming.