Fishing communities are facing declining catches and revenue due to the contamination of the Mekong River. The toxic runoff from upstream rare earth mines is threatening millions who rely on these waters for sustenance.

Perched on the bow of his long-tail fishing boat, 75-year-old Sukjai Yana untangled a handful of small fish from his net, disappointed by his catch and fretting over whether he can sell them. Some days Sukjai earns nothing: demand for fish is falling due to worries over contamination of the Mekong River and its tributaries by toxic run-off from rare earth mines upstream that is threatening millions who rely on those waters for farms and fisheries. Chiang Saen, a fishing hub in northern Thailand,...