Preparing For Restoration

Cutthroat trout spawning in a creek
Cutthroat trout spawning

NPS/Neal Herbert


Nonnative fish cause harm to native fish in Yellowstone National Park by preying upon them, competing for food resources, or interbreeding (hybridizing) with them. Biologists use “selective removal” techniques, such as electrofishing, to reduce the abundance of nonnatives and minimize these impacts. Anglers can assist with these conservation efforts by harvesting the nonnative fish they catch. Angling regulations include no creel limits and (in some areas) a mandatory kill requirement for nonnatives, which have proven effective for selectively removing them.

In some instances selective removal methods cannot effectively suppress the invaders and the native species have been largely lost. In those cases, in-stream barriers that isolate headwater refuge areas followed by chemical treatments to completely remove the nonnatives are considered. Some natural waterfall barriers already exist; sometimes modifications to the natural waterfalls can form complete barriers to upstream movement by fish. In a few places, the barriers must be completely fabricated from concrete or other materials. Once the headwater refuge is protected from invasion by nonnative fish located downstream, a chemical (rotenone) treatment is used to eliminate nonnative species from the restoration area.

Piscicides are toxins which remove fish from habitats where nets, electrofishing, angling, traps, or other mechanical methods are impractical or ineffective. Fish removal projects in the United States use piscicide containing the natural compound rotenone. Biologists in Yellowstone National Park have used rotenone in formulations approved by the Environmental Protection Agency to remove nonnative fish species from High Lake and East Fork Specimen Creek (2006-2009), Goose Lake (2011), Elk Creek (2012-2014), Grayling Creek (2013-2014), Soda Butte Creek (2015-2016), and the upper Gibbon River including Wolf, Grebe, and Ice lakes (2017-2019).

Rotenone occurs in the roots, stems, and leaves of tropical plants in the pea family (Fabaceae). Ingestion has a relatively minor effect on land animals because the enzymes and acids of the digestive system break it down. Rotenone must be absorbed into the bloodstream, usually across the gill membrane of fish, to be harmful. It kills by inhibiting the biochemical reaction some cells use to turn nutrients into energy. Essentially, rotenone starves the cells, causing death.

To treat a section of stream, rotenone is dripped in at a rate determined by the volume, speed, and temperature of the water. At the downstream end of the restoration area, potassium permanganate is added to the water to neutralize the rotenone. Rotenone is quickly broken down in the environment by sunlight and readily binds to sediments or organic matter in the water. The rapid degradation and dissipation mean that managers have a short window of time to successfully remove nonnative fish.

Unfortunately, piscicides may impact other gill-breathing aquatic organisms, including non-target fish species (i.e., native fishes if present), larval amphibians, and macroinvertebrates. To reduce potential impacts on non-target organisms, specialists use a minimum dosage of rotenone for short periods of time. Biologists limit treatment areas and leave recovery intervals between treatments. All treatments in Yellowstone National Park have been conducted during late summer or fall to avoid impacts to amphibians in their early (larval, gill-breathing) developmental stages. Research conducted during these treatments provides strong evidence benthic macroinvertebrates and amphibians in Yellowstone have not been significantly impacted in the long-term. Following removal of the harmful species, stocking of the native fish restores their populations.

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1 minute, 32 seconds

To maintain the natural biodiversity of the Yellowstone ecosystem, sometimes you have to start small. Fish biologist Todd Koel discusses efforts to restore native fish in Grayling Creek, a cup of eggs at a time.

Last updated: October 29, 2019

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