As the Colorado River fades, biologists are racing to save ancient fish from predators — including "public enemy number one"

Barrett Friesen steers a motorboat toward the shore of Lake Powell, with the Glen Canyon Dam towering overhead. Pale "bathtub rings" line the canyon's rocky face, starkly illustrating how water levels have slumped in the second-largest U.S. reservoir amid rising demand and a multi-year drought.

The Utah State University graduate student and colleagues are on a mission to save the humpback chub, an ancient fish under assault from nonnative predators in the Colorado River. The reservoir's decline may soon make things worse, enabling these introduced fish to get past the dam to where the biggest groups of chub remain, farther downstream in the Grand Canyon.  

On the brink of extinction decades ago, the chub has come back in modest numbers thanks to fish biologists and other scientists and engineers. But an emerging threat becomes evident in early June as Friesen hauls up minnow traps and gillnets packed with carp, gizzard shad, green sunfish and, ominously, three smallmouth bass.