Dark morning clouds hang over Phnom Penh, the Cambodian capital, as Zeb Hogan readies to head out on the muddy Mekong on a different kind of mission.

Usually when the University of Nevada, Reno fish biologist—and host of the Nat Geo Wild TV show “Monster Fish”—goes out on the river, he’s in search of aquatic giants. This time he has his sights set on much smaller quarry: the billions—or more likely trillions—of microscopic fish that are swept down the Mekong River during the monsoon season. Some of them are in steep decline, and Hogan wants to help scientists find out why.

What could be one of the most mysterious and least-studied mass movement of river fish in the world begins hundreds of miles upstream, in the deep pools and rugged river stretches of Laos and northern Cambodia, where soon after the rains set in, all kinds of different fish begin to spawn. As the Mekong transforms into a rushing, sediment-filled monster, it sends massive numbers of larvae and baby fish with it downstream.

Moving unseen in the dark and mud, the tiny fish disperse with the floodwaters that spread over large parts of Cambodia and Vietnam. Over time, a fraction of them will survive to grow into full-size edible fish, powering what is the largest inland fishery in the world with a quarter of global freshwater catches.