GRAND CANYON NATIONAL PARK, Ariz.— Federal researchers reported Wednesday that despite last fall’s eradication efforts the number of invasive smallmouth bass more than doubled in the Colorado River below Glen Canyon Dam since last year, imperiling the already threatened native humpback chub.

“I’m afraid this bass population boom portends an entirely avoidable extinction event in the Grand Canyon,” said Taylor McKinnon, Southwest director for the Center for Biological Diversity. “Losing the humpback chub’s core population puts the entire species at risk. The U.S. Bureau of Reclamation is utterly failing to respond to this emergency with the urgency it requires.”

Smallmouth bass are a nonnative invasive fish whose predation eliminated an entire population of humpback chub from the Yampa River in the Colorado River’s upper basin. Most remaining humpback chub now live at the confluence of the Colorado and Little Colorado rivers in the Grand Canyon. Failure to prevent bass from passing through Glen Canyon Dam and reproducing downstream threatens this largest and last remaining source population of the chub.

Since 2016 the Bureau has said it would “pursue means of preventing nonnative fish from passing through Glen Canyon Dam” but has failed to do so. A planning process to manage river flows to prevent bass reproduction has been delayed, and officials now say it will be five years until dam screens are installed to block nonnative fish from the Grand Canyon. The Bureau has proposed modifying the area below the dam where most smallmouth bass and other nonnative fish are detected, but has not announced timelines for the project.

Climate-driven declines in Colorado River flows and historically low Lake Powell levels have caused warm water and smallmouth bass, green sunfish, walleye and other nonnative fish to pass through Glen Canyon Dam into the lower basin of the Colorado River. Researchers first detected smallmouth bass reproduction below the dam in 2022. Contrary to forecasts and despite higher Lake Powell levels this year, the river below the dam has warmed enough in recent months to support bass spawning in the lower basin.

The current humpback chub emergency comes two years after the U.S. Fish and Wildlife Service, relying on the health of its now-threatened Little Colorado River population, downlisted the fish’s status to threatened from endangered. The decision was sharply criticized for ignoring the effects of climate change, including Colorado River declines.

“It’s deeply concerning that after four decades of endangered fish recovery the humpback chub is still on the precipice of extinction,” said McKinnon. “To give native fish any chance for survival, agencies must not just plan but aggressively implement measures to prevent invasive species in the Grand Canyon. The climate crisis means the status quo of playing catch-up is a recipe for extinction.”