Response of endangered Poeciliopsis occidentalis sonoriensis in the Río Yaqui drainage,Arizona, to introduced Gambusia affinis

Synopsis Potential coexistence of the native Yaqui topminnow, Poeciliopsis occidentalis sonoriensis, with introduced mosquitofish, Gambusia affinis, was examined in spring pools and streams in San Bernardino National Wildlife Refuge, southeastern Arizona. Poeciliopsis never exceeded 12% of total poeciliid numbers in a spring pool where Gambusia was present. Body size, fecundity and reproductive effort of Poeciliopsis were significantly higher in this pool than in similar spring pools where Gambusia was absent. Where Poeciliopsis and Gambusia were syntopic in a stream, numbers of Poeciliopsis declined over 1.7 years, until none were collected. Poeciliopsis then increased to > 60% of total poeciliid numbers following flash flooding. Persistence of Poeciliopsis with Gambusia in the spring pool appeared to be a result of compensatory increase in reproductive output, while in the stream it was associated with recurrent flash flooding and a uniform temperature springhead which provided refuge.