Gila Topminnow (Poeciliopsis occidentalis) Demographics (2016-2023) in the Santa Cruz River, Arizona

Authors: Matthew Mayer, Peter Reinthal

Year: 2024

Abstract

Gila Topminnow Poeciliopsis occidentalis, an USFWS listed endangered species, was rediscovered in the main stem of the Santa Cruz River in the mid-2010s. Genetic analyses in 2024 revealed the presence of an exotic Poeciliopsis species, the hybridogenic Poeciliopsis monacha–occidentalis. We examined demographics among P. occidentalis, P. monacha occidentalis and Mosquitofish (Gambusia affinis) in the river across eight geographic localities in the United States from 2016-2023. Sites are located from where the Santa Cruz River reenters the United States at Nogales, Arizona to Marana, north of Tucson. Subsamples were collected during fall fish survey efforts in the Santa Cruz River to determine Mosquitofish and Topminnow populations given the difficulty in field identification of these species. These collections are part of the Sonoran Institute annual Living River surveys. 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 a preliminary analysis regarding abundance and sex ratios of both Topminnow and Mosquitofish across sites and years. The faunal composition shows a decreasing number of Mosquitofish, an increasing abundance of female Poeciliopsis species, and a shift in fish sizes across sites and time.