Invasive Fish Preying on Threatened Species in Lake Powell to be Eradicated as Water Levels Fall

Federal wildlife biologists are preparing to take their first steps to eradicate invasive fish species that have entered the lower Colorado River ecosystem as a result of record-low Lake Powell water levels.

This weekend, the National Park Service says it will carry out another round of rotenone treatment. Rotenone, a compound present in the roots of some tropical beans, functions as a piscicide to eliminate unwanted green sunfish and smallmouth bass.

The choice was made in response to the discovery of the two fish species, green sunfish and smallmouth bass, in the Colorado River in Glen Canyon National Recreation Area earlier this year.

On September 9, the agency wrote that the Endangered Species Act lists the humpback chub as threatened, and the recent discovery of breeding nonnative predatory fish in areas where they had not previously been abundantly found puts that species' recovery in jeopardy.

Rotenone

To use rotenone at levels compliant with Environmental Protection Agency guidelines, park service officials say they will collaborate with other governmental organizations on Saturday and Sunday. Wildlife officials claim that although the substance is fatal to fish when used in small enough doses, it poses no threat to people or other animals.

Within the next two months, the second round of treatment will be conducted in the area to deal with any invasive species that either emerged after the first treatment or evaded it.

According to park service representatives, the cobble bar area around the backwater slough at river mile 12 and near upstream and downstream will be closed for the duration of the treatments. All restricted areas will be flagged. For the duration of the treatments, they add, a fabric barrier will be put in place at the slough's mouth to reduce the water exchange with the river.

The agency explained that to neutralize the rotenone, potassium permanganate, a substance used to purify water, will be added to the slough and into the river just above the fabric barrier. Given the volume of flow in the Colorado River, if any rotenone were to enter the main channel, it would immediately be diluted to concentration levels that would not be harmful to humans or animals.

Read also: Fish Leather: Sustainable Leather Made from Invasive Species May Become the Next Big Thing 

Invasive Fish: Smallmouth Bass and Green Sunfish

Over the summer, biologists made the discoveries of green sunfish and smallmouth bass. Due to the region's persistent drought and other factors, experts think the reservoir's record-low levels in March through April allowed the Colorado River to warm up enough for the species to pass through the dam.

According to park officials, both species prefer water that is warmer and closer to the lake's surface. They contend that because of the warmer water that entered the intakes of the dam, the non-native fish species had a better chance of surviving as they passed through the structure.

The humpback chub, a listed threatened species, and native of the river ecosystem is preyed upon by smallmouth bass, a common sportfish. Just last year, the status of the humpback chub was changed from endangered to threatened.

Brian Healy, the Native Fish Ecology and Conservation Program founder, said that It is pretty upsetting to see how quickly all the effort that was put into eradicating other invasive species and moving populations to protect the fish was undone.

According to the Utah Department of Natural Resources, Lake Powell is currently listed as being at 24 percent of its maximum capacity as it once again approaches record-low levels, KSL reports.

Related article: The Moment Biologists Dread Has Come, Invasive Fish Comes into Colorado River