Beneath the surface of the planet’s rivers and lakes, the historically heaving migrations of freshwater fish are thinning out. The blubbery-lipped Siamese giant carp of Asia’s Mekong River, the mottled brown goonch of India’s Ganges and the ancient-in-appearance beluga sturgeon of Europe’s Danube River are declining. Facing existential threats along their migratory paths, an ecological collapse is...