As part of its 5-year effort to protect native fish populations in the Muddy River, including the Moapa dace, the Nevada Department of Wildlife (NDOW) recently completed a nonnative fish eradication project using the chemical rotenone in the lower section of the river.
“The goal of this project was to eradicate nonnative blue tilapia (Oreochromis aurea) and other nonnative fishes in the Muddy River in an effort to conserve and recover imperiled, native fishes. This treatment was the last of three main-stem Muddy River treatments planned under our 5-year eradication project,” said Brandon Senger, supervising fisheries biologist for NDOW.
The blue tilapia is a predatory species that originates in North Africa and preys upon the endangered Moapa Dace (Moapa coriacea) and other native fishes. Tilapia were first reported in the Muddy River in 1991, but the invasive species quickly expanded its range throughout the drainage and had a significant impact on native fish populations.
Ultimately, this project will contribute to removing the dace from the Federal Endangered Species list and help prevent future ESA listings of other Muddy River native fishes.
“Removal of tilapia is an essential step toward removing threats to Moapa Dace and other Muddy River native fishes,” said Amos Rehm, the Muddy River fisheries biologist for NDOW. “Mechanical removal and other physical methods are not adequate to control blue tilapia, so we used rotenone.”
Rotenone is a naturally occurring substance derived from the roots of a South American plant and has been used historically by indigenous tribes as a method to capture fish, Rehm explained. “Since 1934, it has been safely used as a fisheries management tool, and has a long history of successful applications.”
Due to the large scale nature of the eradication effort, biologists utilized a systematic approach, focusing their eradication efforts on relatively small sections of the Muddy River at a time. Their focus for this phase of the project was that portion of the river that begins near Hidden Valley Road and ends at the Wells Siding Diversion.
“Follow up surveys have shown that native fish populations in the previously treated portions of the river are doing well and increasing in number,” Senger said.
Biologists used potassium permanganate, a commonly used water purifier, to neutralize the rotenone at the terminal end of the project. During the project, the river immediately downstream turned purple in color, the natural result of using potassium permanganate. The color change is temporary.
At concentrations used in fisheries management applications, rotenone is lethal only to gill breathing organisms like fish or early-stage amphibians and has no lethal or harmful effects on non-aquatic wildlife. To lessen the impact of the project on native fishes, biologists spent several months prior to the rotenone treatment salvaging native fish out of the treatment area and moving them to previously treated portions of the river.