PHOENIX — The National Park Service will use an EPA-approved fish piscicide in attempts to remove invasive species from the Colorado River below the Glen Canyon Dam in northern Arizona starting on Aug. 26.
NPS will deploy the chemical rotenone to kill nonnative smallmouth bass and green sunfish that were found breeding in areas below the dam.
The number of smallmouth bass in the Colorado River more than doubled since last year, according to the Center for Biological Diversity, threatening the existence of the local humpback chub. Smallmouth bass are predators that feed on native species.
NPS said in a press release failure to control the smallmouth bass population in the Colorado River above the Grand Canyon could “likely lead to the demise of the humpback chub.”
Smallmouth bass and green sunfish thrive in warmer waters, which is how drought and climate change have impacted this environmental issue.
Warmer water is passing through the dam after Lake Powell’s levels dropped to historic lows last year, increasing temperatures in the Colorado River. That allows the invasive fish to pass through the dam and survive, spawn and reproduce.
Lake Powell’s lower water levels also increased rates of fish passage through dams, according to the U.S. Geological Survey, considering smallmouth bass live on the surface which is now closer to the dam’s turbines.
Juvenile smallmouth bass were found in the Colorado River below Glen Canyon Dam this month.
“It’s deeply concerning that after four decades of endangered fish recovery the humpback chub is still on the precipice of extinction,” Taylor McKinnon, Southwest director for the Center for Biological Diversity, said in a press release. “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.”
NPS said tests will be conducted to learn the minimum effective concentration of rotenone for use before treatment begins. The chemical is produced from roots and stems of tropical and sub tropical plant species and is highly toxic to fish. It is also used as an insecticide.
Once treatment begins, the cobble bar surrounding the backwater slough in between Glen Canyon Dam and Lees Ferry where invasive fish were found will be closed to the public. The river mainstem will not be closed, and signs will mark where the restrictions are.
NPS plans to use a fabric barrier at the mouth of the slough to minimize the exchange of water with the river. Potassium permanganate will be added to neutralize the rotenone. A similar strategy was used last September.
The Bureau of Reclamation will hold Glen Canyon Dam releases at approximately 10,500 cubic feet per second for four days to facilitate the treatment, NPS said.
The humpback chub was downlisted from endangered to threatened in October 2021 after nearly four decades. It was first listed under the Endangered Species Act in 1973, but the U.S. Fish and Wildlife Service said in 2021 that improved habitat and river flow conditions led to its reclassification.
It was a decision that is still criticized by conservation groups.
“I’m afraid this bass population boom portends an entirely avoidable extinction event in the Grand Canyon,” McKinnon said. “Losing the humpback chub’s core population puts the entire species at risk.”
Smallmouth bass were introduced to Lake Powell in the 1980s and have been stocked in the West for recreational fishing, according to the U.S. Geological Survey.