Gila Topminnow (Poeciliopsis occidentalis occidentalis) and P. monacha-occidentalis

Authors: Matthew Mayer, Peter Reinthal

Year: 2025

Abstract

Gila Topminnow, Poeciliopsis occidentalis, a USFWS Listed Endangered Species, was rediscovered in the main stem of the Santa Cruz River in 2015. Genetic analyses (Mussman et al. 2023) revealed the presence of an exotic Poeciliopsis species, the hybridogenic Poeciliopsis monacha–occidentalis, also in the Santa Cruz main stem. We examined demographics among P. occidentalis, P. monacha-occidentalis and Mosquitofish (Gambisua affinis) in the river across eight geographic localities in the United States from annual collections made from 2016 to 2024. Sites are located from Nogales, Arizona where the Santa Cruz River reenters the United States to Marana north of Tucson. Subsamples were originally collected during an annual fall fish survey in the Santa Cruz River to determine Mosquitofish and Topminnow populations given difficulty in field identification of these species. These collections are part of the Sonoran Institute annual Living River Reports. They provide an invaluable opportunity to determine geographic and temporal changes in the distributions of the three species during this period of changing ichthyofauna composition in the Santa Cruz River. Currently we have completed an analysis regarding abundance and sex ratios of both Topminnow and Mosquitofish across the sites and years. The faunal composition shows a decreasing number of Mosquitofish, an increasing abundance of female Poeciliopsis species across sites and time. We demonstrate that P. occidentalis and P. monacha-occidentalis can be distinguished by examining dentition. The P. monacha-occidentalis is present in samples 2016 and becomes the prevalent Poeciliopsis over time. This suggests that the recovery of Poeciliopsis in the Santa Cruz River is not solely due to the upgrade of the water treatment facilities prior to the rediscovery. We suggest that the hybridogenetic species played an important role in the return of P. occidentalis to the Santa Cruz by out-competing G. affinis but requires P. occidentalis males for reproductive success.