News Release

National Park Service Taking Action to Remove Invasive Predatory Fish from the Colorado River Slough at Glen Canyon

Swimming green fish with dark scales, with yellow fins and tail, and a red eye.
Smallmouth Bass

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News Release Date: August 18, 2023

Contact: Mary Plumb, 928-608-6200

Starting next week, the National Park Service (NPS) will begin active efforts to remove invasive smallmouth bass and green sunfish from the Colorado River Slough below the Glen Canyon Dam in Glen Canyon National Recreation Area.

These non-native 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 humpback chub was downlisted from endangered to threatened in October 2021, in part based upon the success of the species in the Colorado River through Grand Canyon. Because smallmouth bass are aggressive predators, failure to quickly control their population in the Colorado River above the Grand Canyon could likely lead to the demise of the humpback chub.

Beginning Saturday, August 26, 2023, NPS staff, will deploy the EPA-approved fish piscicide rotenone (Prenfish) to kill these invasive, predatory fish. The cobble bar area surrounding the backwater slough at river mile -12 where the invasive fish were found, the slough itself, and a short distance up and downstream will remain closed to the public for the duration of treatments that weekend (see map). The closure will be marked by signs. The river mainstem itself will not be closed.

The treatment will be carefully planned and conducted to minimize exposure to rotenone and protect the health and safety of humans, the environment, desirable fish species, and livestock. Prior to the treatment, tests will be conducted to determine the minimum effective concentration of rotenone for use during the treatment. Rotenone is a natural substance derived from plant roots. It has been effective at eliminating localized populations of smallmouth bass, including in the Colorado River Basin. An impermeable 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, just above the fabric barrier to neutralize the rotenone. 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. To facilitate the treatment, the Bureau of Reclamation will hold Glen Canyon Dam releases at approximately 10,500 cubic feet per second for four days (72 hours, noon August 25 through noon August 28).

Lake Powell water elevations dropped to historically low levels in April 2023 and have had indirect adverse impacts on the federally listed and native populations of fish below Glen Canyon Dam. Smallmouth bass and green sunfish live in the warmer levels of the lake’s waters closer to the surface. As warmer water reaches the dam’s water intakes, the nonnative predatory fish have a greater chance of passing through the dam alive. This increases the threats to the native fish in the Grand Canyon and Glen Canyon’s rainbow trout fishery downstream of Glen Canyon Dam.

Threats to native fish are increasing due to the warmer temperatures of water passing through the dam and related increased river temperatures below the dam, which increase non-native fish spawning and reproduction, and allow for predation on native fish populations downriver. Juvenile smallmouth bass were found in the Colorado River below Glen Canyon Dam on June 30 in 2022 and during July and August in 2023, underscoring the urgency of this emergent issue.

For more information please visit: fish-threats.htm

Map of Colorado River Slough Closure

Last updated: March 28, 2024

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Contact Info

Mailing Address:

PO Box 1507
Page, AZ 86040


928 608-6200
Receptionist available at Glen Canyon Headquarters from 7 am to 4 pm MST, Monday through Friday. The phone is not monitored when the building is closed. If you are having an emergency, call 911 or hail National Park Service on Marine Band 16.

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