Chemical treatment to be deployed against invasive fish in Colorado River

The National Park Service will continue efforts to rid an area of ​​the Colorado River in northern Arizona of invasive fish by killing them with chemical treatment, the agency said Friday.

A substance deadly to fish but approved by federal environmental agencies called Rotenone will be circulated starting August 26. It’s the latest tactic in an ongoing battle to keep alien smallmouth bass and moonfish at bay below Glen Canyon Dam and to protect an endangered native fish, the humpback chub.

Treatment will require a weekend closure of the Colorado River Slough, a cobblestone area surrounding the backwater where the smallmouth bass was found, as well as a short stretch upstream and downstream. Chemical substances were also used last year.

According to the National Park Service, the effort will be “carefully planned and executed to minimize exposure” to humans and “desirable fish species.” An “impermeable barrier of cloth” is erected at the mouth of the swamp to prevent water from crossing with the river.

Once the treatment is complete, another chemical will be released to dilute the rotenone, the park service said.

In the past, smallmouth bass in Lake Powell were held back behind the Glen Canyon Dam, which had served as a barrier for them for years. But last summer they were found in the river below the dam.

Due to climate change and drought, Lake Powell, a key reservoir on the Colorado River, fell to historically low levels last year, making it less of a barrier to smallmouth bass. The predatory fish managed to get close to the Grand Canyon, where the largest groups of ancient and rare humpback chub still live.

Environmentalists accuse the federal government of not having acted quickly. The Center for Biological Diversity cited National Park Service data released Wednesday that showed the smallmouth bass population more than doubled in the past year. The group also said there was no timeline for changing the area below the dam yet.

“I’m afraid this boom in bass populations points to an entirely avoidable extinction event in the Grand Canyon,” said Taylor McKinnon, the center’s southwest director. “The loss of the humpback whale’s core population puts the entire species at risk.”

Conservation groups also continue to criticize the 2021 decision to downgrade the humpback chub from endangered to endangered. At the time, federal authorities said the fish, which gets its name from a fleshy bump behind its head, was saved from extinction after decades of protection.