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LAKE POWELL — Federal wildlife biologists are preparing to take their first steps in an effort to remove a pair of invasive and predatory fish species that have entered the lower Colorado River ecosystem as a result of record-low Lake Powell water levels.
The National Park Service says it plans to conduct a round of rotenone treatment this weekend. Rotenone is a substance found in the roots of a tropical bean that works as a piscicide to kill off the unwanted smallmouth bass and green sunfish.
The decision comes after the two fish species, smallmouth bass and green sunfish, were found in the Colorado River below the Glen Canyon Dam in Glen Canyon National Recreation Area earlier this year.
"These nonnative predatory fish were recently discovered breeding in areas where they have not previously been found in large numbers, threatening the recovery of humpback chub, which is listed as threatened under the Endangered Species Act," the agency wrote in a statement on Sept. 9.
Park service officials say they will work with other government agencies Saturday and Sunday to use rotenone at levels that align with Environmental Protection Agency regulations. The substance is lethal to fish but isn't considered a threat to humans or animals as long as it is used in low enough quantities, according to wildlife officials.
A second round of treatment in the area is planned within the next two months in an effort to handle any invasive species that either hatched after the treatment or eluded the first attempt.
The "cobble bar area" surrounding the backwater slough at river mile 12 and a "short distance up and downstream" will be closed for the duration of the treatments, according to park service officials. All closed areas will be marked. They add that a fabric barrier will be installed at the mouth of the slough for the duration of the treatments to "minimize the exchange of water with the river."
"Potassium permanganate, a chemical used to purify drinking water, will be added to the slough, and into the river just above the fabric barrier to neutralize the rotenone," the agency wrote. "Should any rotenone enter the main channel, it will immediately be diluted to concentrations that are insignificant to wildlife or humans, due to the volume of flow in the Colorado River."
Smallmouth bass and green sunfish were discovered by biologists over the summer. Experts believe the reservoir's record-low levels in March and April, caused by the region's ongoing drought and other factors, resulted in Colorado River temperatures being warm enough to help the species cross through the dam.
Both species thrive on warmer water temperatures closer to the lake's surface, park officials explain. They believe the warmer water reached the dam's intakes, allowing the nonnative fish species a better chance of survival as each passed through the dam.
Smallmouth bass is a popular sportfish; however, the species are a predator of the humpback chub, a threatened fish species native to the river's ecosystem. The humpback chub had been downlisted from endangered to threatened just last year.
"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, the founder of the Native Fish Ecology and Conservation Program, told the Associated Press at the time the species were discovered.
Lake Powell is currently listed as being at 24% of capacity as it again nears record-low levels, according to the Utah Department of Natural Resources.
