Denver: For National Park Service fisheries biologist Jeff Arnold, it was a moment he’d been dreading. Bare-legged in sandals, he was pulling in a net in a shallow backwater of the lower Colorado River last week, when he spotted three young fish that didn’t belong there. “Give me a call when you get this!” he messaged a colleague, snapping photos.
Minutes later, the park service confirmed their worst fear: smallmouth bass had in fact been found and were likely reproducing in the Colorado River below Glen Canyon Dam.
They may be a beloved sport fish, but smallmouth bass feast on humpback chub, an ancient, threatened fish that’s native to the river, and that biologists like Arnold have been working hard to recover. The predators wreaked havoc in the upper river but were held at bay in Lake Powell where Glen Canyon Dam has served as a barrier for years — until now. The reservoir’s recent sharp decline is enabling these introduced fish to get past the dam and closer to where the biggest groups of chub remain, farther downstream in the Grand Canyon.
If bass and other predator fish continue to get sucked into the penstocks, survive and reproduce below the dam, they will have an open lane to attack chub and other natives, potentially unraveling years of restoration work and upending the Grand Canyon aquatic ecosystem — the only stretch of the river still dominated by native species.
On the brink of extinction decades ago, the chub has come back in modest numbers thanks to fish biologists and other scientists and engineers. Agencies spend millions of dollars annually to keep intruders in check in the upper portion of the river.