Starting Saturday, August 26, the National Park Service (NPS) will begin efforts to remove invasive smallmouth bass and green sunfish from the Colorado River Slough below Glen Canyon Dam.
The recent discovery of smallmouth bass and green sunfish breeding in new stretches of the Colorado River threatens the recovery of the humpback chub, listed as threatened under the Endangered Species Act.
The humpback chub was downlisted from endangered to threatened status in October 2021 thanks to successes in the Grand Canyon area. However, wildlife officials warn the aggressive predatory bass could decimate the native chub population if left unchecked. Prompt action is needed to control the invasive fish above the canyon and prevent the demise of the recovering humpback chub.
NPS staff will deploy the EPA-approved fish toxin rotenone to eradicate the non-native fish. The cobble bar and backwater slough area where the fish were found will be closed to the public during treatments through August 28.
Signs will mark the closure. The river mainstem will remain open. This stretch of river is located within Glen Canyon National Recreation Area (see map below).
Rotenone is a natural substance made from plant roots that has successfully removed invasive bass populations before. NPS says the treatment will be carefully conducted to minimize environmental and human impacts. An impermeable barrier will be installed at the slough mouth to contain the rotenone. Potassium permanganate will also be added above the barrier to neutralize any toxin that escapes.
The Bureau of Reclamation will hold dam releases steady at about 10,500 cubic feet per second for four days (noon August 25 through noon August 28) to facilitate the treatment. Lake Powell elevations dropped to historic lows this year, allowing warmer water harboring non-native fish to reach the dam intakes. This increases threats to native species downstream.
Juvenile bass were found below the dam in 2022 and 2023, underscoring the urgent need for action. NPS says smallmouth bass are aggressive predators that could lead to the demise of the humpback chub if left unchecked. The treatment aims to prevent this and protect the native fish population’s recovery.
What is the Humpback Chub and Why is it Important?
The humpback chub is a federally protected fish species found only in the Colorado River basin. This large minnow gets its name from the pronounced hump behind its head, an adaptation that helps it navigate through deep, fast-moving canyon waters.
Once abundant, the humpback chub saw major declines due to habitat loss and competition from non-native predators. By the 1960s, its population dropped dramatically, leading to its listing as endangered in 1967. Thanks to conservation efforts, its status was downgraded to threatened in 2021.
Today, six self-sustaining wild populations of humpback chub remain, mostly isolated within remote, canyon-bound stretches of the Colorado River and its tributaries. The largest population, numbering around 12,000 adults, is found in the Little Colorado River’s lower 13 miles within Grand Canyon National Park.
The humpback chub plays an important role in its ecosystem. Its diet of insects and other invertebrates makes it a key part of the aquatic food web. Protecting this unique fish helps maintain ecological balance and native biodiversity. Wildlife officials hope continued efforts can secure the species’ long-term survival.
Cover photo courtesy of: Sharon Mollerus