Chiang Saen, THAILAND (AP) — 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 Yana earns nothing: demand for fish is falling due to worries over contamination of the Mekong River and its tributaries by toxic runoff 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, has been Yana’s family’s home for decades. “I don’t know where else I’d go,” he said. Yana is one of 70 million people in mainland Southeast Asia who depend on the nearly 5,000-kilometer (3,100-mile) Mekong River. Rising demand for rare earth materials is driving an unregulated mining boom centered in war-torn Myanmar, to the west, that is spreading to Laos, in the east.…