The Mekong and Tonle Sap

The Mekong flows about 4,900 km from the Tibetan plateau through China, Myanmar, Laos, Thailand, Cambodia and Vietnam. At Phnom Penh it meets the Tonle Sap river, which links it to the great Tonle Sap lake. The Lower Mekong basin is the largest inland fishery in the world, landing around two million tonnes of wild fish a year, and holds some of the largest freshwater fish known.

The flood pulse

Each monsoon, the Mekong rises so high that the Tonle Sap river reverses and flows upstream into the lake, which swells to several times its dry-season size and floods the forests around it. Fish move into the flooded forest to feed and spawn, and as the water drains in the dry season they return to the river, where fishers catch them in huge numbers. Cambodians eat fish at nearly every meal, often as prahok, a fermented fish paste.

Giants under threat

The Mekong giant catfish can weigh around 300 kg, and in 2022 a giant freshwater stingray of about 300 kg was caught and released in Cambodia, reported as the largest freshwater fish on record. Both are endangered or critically endangered. Dozens of hydropower dams on the main river and its tributaries in China and Laos block fish migration and trap sediment, and scientists and fishing communities warn they threaten the fishery. Governments argue the dams are needed for power and income.

Still to be written: Mekong giant catfish, Giant freshwater stingray, Giant barb, Iridescent shark, Giant pangasius.