Invasive Smallmouth Bass Threaten Colorado River's Native Humpback Chub Population

By author of The Realblog with Stephanie Mallory

Efforts are underway to contain the predatory fish

Colorado National Park Service fisheries biologists have discovered that smallmouth bass, a non-native predatory fish, are reproducing in the lower Colorado River.

Although smallmouth are a much-loved sport fish, their presence can be devastating to the humpback chub, an ancient, threatened fish that’s native to the river.

According to, smallmouth diminished the chub population in the upper river but were contained in Lake Powell where, until now, Glen Canyon Dam has acted as a barrier for years. Because of the reservoir’s recent sharp decline, smallmouth can now get past the dam and closer to where the biggest groups of chub can be found downstream in the Grand Canyon.

As water levels drop, nonnative fish that live in warm surface waters in Lake Powell are moving closer to the dam and are getting sucked in to the submerged steel tubes that carry water to turbines, where it generates hydroelectric power and is released on the other side.

Bass and other predatory fish that make it out on the other side can reproduce and feed on chub and other native species below the dam. They risk undoing years of work spent on restoring the chub population and upsetting the Grand Canyon aquatic ecosystem, which is sadly the only stretch of the river still dominated by native species.

“It’s pretty devastating to see all the hard work and effort you’ve put into removing other invasive species and translocating populations around to protect the fish – and to see all that effort overturned really quickly,” Brian Healy, founder of the Native Fish Ecology and Conservation Program, said.

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The chub almost went extinct 10 years ago, but its numbers have risen somewhat thanks to fish biologists and other scientists and engineers. Agencies spend millions of dollars each year to mitigate the impact of invasive species in the upper river section.

In August, federal, state and tribal leaders are scheduled to release a plan to policymakers with solutions for responding to the threats brought by smallmouth bass and other predators below the dam.

Many of the solutions will likely require significant changes to infrastructure.

Currently, the National Park Service, U.S. Geological Survey and Arizona Game and Fish are trying to contain the issue. They plan to increase their monitoring efforts in other shallow areas and block off the entire backwater to keep the bass from swimming out into the river.

“Unfortunately, the only block nets we have are pretty large mesh, so it will not stop these smaller fish from going through. But it will keep the adults from going back out,” Arnold said.

Experts recommend leaving more water in Lake Powell to ensure cool water can be released through the dam, but water levels are already under a lot of stress due to lack of precipitation and increased usage.

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