Efficacy of one-time rotenone application for Westslope Cutthroat Trout restoration in Boulder Creek, Montana

ObjectiveAlthough piscicides are an important tool for native fish management, our understanding of native fish population dynamics posttreatment (i.e., in putatively vacant habitat) is limited.MethodsHere, we describe long-term trends in size and abundance of Cutthroat Trout Oncorhynchus clarkii in Lower Boulder Lake and Boulder Creek (Montana).ResultNo significant differences were found before and after treatment with rotenone in mean length and population density in Boulder Creek. Posttreatment genetic samples from Boulder Lake and upper Boulder Creek consisted entirely of the Westslope Cutthroat Trout Oncorhynchus clarkii lewisi used to refound the population. Middle and lower Boulder Creek genetic samples contained 99.4% and 98.4% Westslope Cutthroat Trout, indicating that some nonnative (Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri hybrid) fish survived the treatment. Population demographics showed robust recovery posttreatment; however, genetic results revealed the limitations of a one-time treatment-that is, an incomplete fish kill.ConclusionWe demonstrate that Cutthroat Trout populations achieve recovery after rotenone treatment in the long term, but one-time rotenone treatments on large drainages often fail to totally eradicate populations. Therefore, managers should plan on multiple treatments and utilize environmental DNA to help achieve total removal of target species. The Westslope Cutthroat Trout is an iconic native species of the northern Rocky Mountains, but hybridization with introduced species, including Rainbow Trout and Yellowstone Cutthroat Trout, has greatly reduced the distribution and abundance of genetically pure populations. This case study reviews an attempt to remove a hybridized population using a single rotenone treatment before restocking with a hatchery-reared, genetically pure population. These results can aid fisheries managers as they plan and implement fish removal projects.Impact statement