The nets yield almost no fish today, the same as yesterday and the day before that. For generations, Bun Neang's family has depended on the bounty of Cambodia's Tonle Sap, a vast lake fed by one of the world's greatest rivers, the Mekong. Two decades ago, his father could rely on a daily catch totaling about 65 lbs. (30 kg). When the water gods were feeling particularly charitable, he would land a Mekong catfish, a massive bottom-feeder that can weigh as much as a tiger. But today, when Bun Neang dips his net into the caramel-hued waters near Chong...