Response of Endangered Species to Nonnative Eradication through Piscicide Application

Authors: Skyler Hedden, Julie Carter, Matthew Rinker, Scott Rogers, Kevin Guadalupe

Year: 2022

Abstract

Nonnative fish are believed to be a major imperilment to native fish persistence and recovery and thus are a main focus of native fish conservation plans. But nonnative species management is complicated due to the difficulty of eradicating undesirable species while simultaneously having minimal effects on rare and endangered species that occupy similar areas and habitats. The most successful nonnative eradication tool in large riverine systems is piscicide application, but native species that occupy reaches in which problematic nonnative fish populations exists will also perish during treatment. We examined the response of native species following treatment of a 17-mile reach of the Virgin River, in Utah and Arizona to assess short term (less than 2 years) recolonization rates and if immediate benefits could be observed in the native fish community. In June 2021, over 200 gallons of rotenone solution was applied to the Virgin River to eradicate nonnative Red Shiners (Cyprinella lutrensis), resulting in complete mortality of all fish species, including some endangered species. Two months post treatment revealed large numbers of Virgin River Chub (Gila seminuda), an endangered species, had recolonized the upper reaches of the treatment area, while nonnative Red Shiners were still absent. Follow up sampling 14 months after the treatment showed a further increase in Virgin River Chub densities throughout the treated area with densities typically more than twice as high than downstream control reaches that have abundant Red Shiner populations. Our results highlight that nonnative eradication is possible with the use of piscicides even when endangered species are present, and that native fish populations have the ability to quickly recolonize treated reaches and appear be on a trajectory where robust native fish populations will exist.