Great Basin Naturalist


Efforts between 1982 and 1990 have failed to recover and secure three natural populations of endangered Sonoran topminnow (Poeciliopsis o. occidentalis) at Bylas Springs, Arizona. Flooding in the Gila River in 1977–78 allowed ingress by predatory mosquitofish (Gambusia affinis), and topminnows began to decline. Since that time (1) one stock has been replaced twice and is again nearly gone because of depredations by mosquitofish that resisted two eradication attempts; (2) topminnows at a second spring were extirpated through vegetation encroachment after fencing to protect the habitat from livestock; and (3) a third population was lost to mosquitofish, restocked after the nonnative was removed, and the restocked population is again in jeopardy, or extirpated, since mosquitofish reinvaded. Recommendations for a more intensive program of recovery are based on reassessments of past efforts and new suggestions for eradication and exclusion of mosquitofish.