There have been many attempts to remove non-native fish using what is referred to as mechanical removal: that is capturing and removing fish using nets, electrofishing and any method of capture available. This is not only extremely time, labor, and resource intensive, but it is almost never successful at removing all the non-native fish. For instance, there has been an effort to mechanically remove Brook Trout in Soda Butte Creek in Yellowstone National Park that has been unsuccessful after a decade of mechanical removal work.

The most powerful and arguably useful tool for the removal of non-native fish is the use of a piscicide- a chemical that kills fish. Rotenone is a piscicide that is present in plants from the Fabaceae family that is effective at killing fish in very low concentrations measured in parts per billion. Rotenone interferes with uptake of oxygen in a fishes gills, and results in suffocation.  Rotenone is classified as a pesticide, and the Environmental Protection Agency has put forth strict regulations for safe application that protects both the ecosystem and people from any potential harm. When rotenone is used in accordance with these regulations, it poses no danger to humans or the environment. In addition, rotenone is not a persistent chemical and is quickly broken down into harmless organic compounds by oxygen and sunlight. Rotenone can also be chemically deactivated using Potassium permanganate, a commonly used chemical in municipal water treatment plants. This enables the use of rotenone in only a specified section of a stream, after which it can be deactivated so it doesn’t effect fish downstream of that point.

I was lucky to be able to attend a rotenone application training class as part of my internship, which was hosted by the American Fisheries Society. This week-long training covered the safe and effective use of rotenone for non-native fish removal. This included the use of personal protective equipment for protecting the people handling the chemical in its most concentrated form, how to use the chemical in accordance with regulations, how to calculate the effective dose for a particular stream, river or pond, and how to conduct a bio-assay, using water from the treatment location to test the amount of rotenone needed to effectively treat a particular waterbody based on the water chemistry (Pictured).

While it may seem counterintuitive for fisheries biologists to be learning how to kill fish, the reality is that human introduction of non-native fish is one of the main causes of native fish species becoming threatened or endangered. Three of the 14 subspecies of Cutthroat Trout native to the Western US are currently listed as threatened with extinction by the US Fish and Wildlife Service, and one subspecies has been declared extinct. Within Montana, genetically pure Westslope Cutthroat Trout that are not hybridized with non-native Rainbow Trout currently only occupy an estimated 2-4% of their historic range, and typically exist in small, isolated populations that are highly susceptible to extinction. As a result, the long-term survival of Westslope Cutthroat trout is dependent upon some help to expand their habitat, increase their population numbers, and remove non-native fish. Rotenone is a specialized tool that fisheries biologists can use to help meet those goals to reintroduce native fish, and help ensure that the native Westslope Cutthroat can persist in this ecosystem in perpetuity.