"This work provides strong evidence that rotenone treatments have not significantly impacted benthic macroinvertebrates or amphibians in YNP in the longterm." from Skaar et al, 2017
by Donald R. Skaar, Jeffrey L. Arnold, Todd M. Koel, Michael E. Ruhl, Joseph A. Skorupski, & Hilary B. Treanor
Piscicides are fish toxins approved by the Environmental Protection Agency and used by managers to eradicate non-native fishes. With the exception of sea lamprey control in the Great Lakes, all fish removal projects in the United States utilize piscicide formulations containing either rotenone or antimycin as the active ingredient. Both of these natural compounds have been used extensively in fisheries management since the 1930s to control invasive species, recover native species, or restore sport fish (e.g., removing suckers to make habitat available for a sport fish). Over the past decade, biologists in Yellowstone National Park (YNP) have used rotenone in High Lake and East Fork Specimen Creek (2006-2009), Goose Lake (2011), Elk Creek (2012-2014), Grayling Creek (2013-2014), and Soda Butte Creek (2015-2016) to remove non-native fish species. Piscicides are effective at removing fish from habitats where nets, electrofishing, angling, traps, or other mechanical methods are impractical or ineffective. However, piscicides are also non-specific, meaning they can impact all gill-breathing organisms, including larval amphibians and macroinvertebrates. Therefore, when using this powerful tool, natural resource managers need to consider these potential impacts and seek ways to lessen them.
What is Rotenone, and How Does it Work?
Rotenone occurs in the roots, stems, and leaves of tropical plants in the pea family (Fabaceae), including the jewel vine (Derris involuta), lancepod or cube plant (Lonchocarpus utilis), and Tephrosia genus found in southeast Asia, South America, and east Africa, respectively. Rotenone and other related compounds are produced by these plants for a variety of functions, including defense against the growth of microorganisms (Dixon and Passinetti 2010). Indigenous peoples discovered that the roots of these plants are toxic to fish and developed a variety of ways to apply these roots to water to kill fish for consumption (Cannon et al. 2004).
Rotenone’s toxicity results from the inhibition of a biochemical reaction called oxidative phosphorylation, which occurs in the energy-producing mitochondria within the cells of animals. The resulting loss of usable energy for cellular function results in death. To reach most tissues in an animal, rotenone must first be absorbed into the bloodstream. Ingestion of rotenone has a relatively minor effect on land animals because the enzymes and acids of the digestive system break it down, thus limiting absorption through the lining of the intestinal tract. On the other hand, the absorption of rotenone in water across the gill membrane by fish or other aquatic organisms (amphibians, immature insects) is a direct route into the blood.
The physical and chemical properties of rotenone combine with environmental conditions to determine its fate and toxicity in the environment. It is a large compound quickly broken down in the environment by sunlight and other factors. The degradation time in lakes ranges from one day in warm water to several weeks in cold water. Rotenone is also somewhat hydrophobic, meaning once applied in the environment, it will readily bind to sediments or organic matter in the water. These factors result in a relatively quick dissipation and degradation of rotenone from the environment, which poses a challenge for biologists attempting to kill fish before rotenone concentrations decrease to nontoxic levels. This challenge is greatest in rapidly moving waters where rotenone, once applied, will either degrade or bind to streambed sediments within 1-5 hours of travel time, thus requiring reapplication to maintain toxic concentrations (figure 1).
Impacts of Rotenone on Organisms Other than Fish
The use of rotenone to kill fish can affect non-target organisms. In YNP, this includes most gill-breathing, immature forms of aquatic macroinvertebrates, specifically the insect taxa Ephemeroptera (mayflies), Plecoptera (stoneflies), and Tricoptera (caddisflies; Magnum and Madrigal 1999, Hamilton et al. 2009). Factors such as age and size contribute to sensitivity, as younger insects have thinner cuticles and smaller animals have higher surface area to volume ratios, both leading to greater absorption of rotenone. Life history characteristics are also a factor, since animals living on the water-sediment interface of lakes or streams are more likely to be exposed to rotenone than those living in the spaces between gravel/cobble or burrowed into mud (Minckley and Mihilack 1981, Whelan 2002). Additionally, insects with high oxygen requirements will typically succumb more quickly to rotenone because it inhibits the oxygen-mediated production of energy molecules in the body (Engstrom-Heg et al. 1978). Finally, aquatic invertebrates with tracheal gills for respiration generally are more sensitive to rotenone than those that acquire oxygen through the skin, air, or respiratory pigments (Vinson et al. 2010).
Immature gill-breathing forms of amphibians may also be inadvertently impacted by rotenone treatments. In YNP, this potentially includes the boreal toad (Anaxyrus boreas), a species of concern in YNP and the Rocky Mountain West, blotched tiger salamander (Ambystoma tigrinum melanostictum), boreal chorus frog (Pseudacris maculata), and Columbia spotted frog (Rana luteiventris). Rotenone generally has a greater impact on larval forms of both frogs and salamanders than on adult forms (Farringer 1972, Burress 1982, Fontenot et al. 1994, Grisak et al. 2007). However, during the larval stage, frogs undergo lung development as they approach metamorphosis and rely very little on gill respiration; whereas, toads remain gill-breathers during the entire larval period (McDiarmid and Altig 1999). Frog tadpoles, therefore, may be less susceptible to the negative effects of rotenone as they grow older.
Research Conducted During East Fork Specimen Creek Rotenone Treatments
Rotenone treatments were used in the East Fork of Specimen Creek (EFSC) drainage during 2006-2009 to remove non-native trout. The first treatment occurred in 2006 at the upper end of the drainage, which included High Lake and its outlet stream, ending at a waterfall barrier (Koel et al. 2008). This was followed by treatment of Specimen Creek in 2008 and 2009 from the waterfall to a man-made barrier near the confluence with the North Fork Specimen Creek (figure 2), approximately 27 km (16.6 mi.) downstream. In all treatments, a liquid formulation of rotenone called CFT-Legumine (5% active rotenone) was applied at a concentration of 1 part per million (ppm). For the stream treatments, the primary means of applying rotenone was through the use of “drip stations” consisting of a 5-gallon container filled with a CFT-Legumine/water mixture, metered out at a constant rate to maintain a 1 ppm concentration in the stream. Because the rotenone degraded and bound with streambed materials after it was applied, there was a decrease in the concentration of rotenone in the water with increasing distance from each drip station. Therefore, it was necessary to utilize additional drip stations spaced evenly throughout the treated section. On Specimen Creek, each drip station applied rotenone for 8 hours; stations were spaced at a distance equivalent to 2-3 hours travel time in the stream water.
The CFT-Legumine rotenone formulation used at High Lake and the EFSC was a relatively new product whose impacts to non-target organisms had not been previously evaluated in the field. In spite of this information gap, this formulation was chosen over traditional products because it contained oil-based solvents, which contain fewer and less persistent contaminants, and reduced odor, rather than petroleum-based solvents. Biologists used this opportunity to investigate how CFT-Legumine impacts benthic macroinvertebrates (EFSC) and amphibians (High Lake and outlet EFSC) in an effort to identify ways to mitigate effects during future treatments.
Don Skaar is Special Projects Bureau Chief for the Fisheries Division of Montana Fish, Wildlife & Parks in Helena. His experience applying piscicides spans more than 30 years, and he has been an instructor of the American Fisheries Society course “Planning and Executing Successful Rotenone and Antimycin Projects” since 2007.
Last updated: July 26, 2017