Assessing the Success of Conservation Efforts for a North American Topminnow at Risk of Extinction from Spatially Variable Mosquitofish Invasions

Invasive species can cause population declines and extinctions of native species through several mechanisms including predation and competition. One such species, western mosquitofish (Gambusia affinis), is an invasive, aggressive predator and competitor of numerous freshwater species around the world that has caused precipitous declines of native species. The Barrens topminnow (Fundulus julisia) is a spring-endemic fish of the south-eastern U.S.A., and the western mosquitofish limits its recruitment via predation of smaller individuals but there is also potential for competition (i.e. interference competition) between the species in laboratory trials. All of these negative interactions probably contribute to the Barrens topminnow's precipitous decline since the 1980s and the listing of the species as endangered by the U.S. Fish and Wildlife Service in late 2019. Ongoing conservation measures for Barrens topminnow include long-term monitoring of populations and reintroductions through stocking of hatchery-reared individuals into numerous sites. Here, we investigate trends in Barrens topminnow and western mosquitofish occupancy and abundance over an 8-year period (three or four generations for Barrens topminnow) and assess the effectiveness of a stocking programme. We also included habitat covariates including distance to the nearest downstream confluence, stream order, and habitat size. Our findings revealed significant range-wide Barrens topminnow declines in both occupancy and abundance were occurring over the 8-year period. These declines were associated with the presence of western mosquitofish. Overall, the stocking programme of hatchery-reared individuals did not significantly enhance or restore populations of Barrens topminnow but had a negative effect on topminnow abundance when western mosquitofish were present. The lack of a positive effect of stocking hatchery-reared individuals on Barrens topminnow populations and the long-term population declines of Barrens topminnow suggest that propagation and stocking alone are not effective at creating self-sustaining populations of the Barrens topminnow. The presence of western mosquitofish does not currently appear to be compatible with long-term persistence of topminnow populations. Moving forward, conservation strategies will be needed to create and maintain mosquitofish-free habitats, which may include eradication of mosquitofish, construction of barriers, or other actions.