Featured Story – Gila Topminnow in Action Against Mosquitos

Believed to have been the most abundant native fish in Arizona waters, the range and number of Gila Topminnow (Poeciliopsis occidentalis) within the state has declined dramatically due to habitat degradation or loss, and predatory nonnative fishes. The nonnative species particularly identified with its decline is the Mosquitofish (Gambusi affinis), which has been commonly and widely spread around to control mosquitoes outside of its natural range. Ironically, Gila Topminnow are recognized as just as effective and efficient at controlling mosquito larvae, as are many other fish species of a similar size.

In July of 2017, a new option was added to the toolbox for mosquito control for Pima County Health Department (PCHD) in the form of the Gila Topminnow. In a cooperative effort with the Arizona Game and Fish Department, U.S. Fish and Wildlife Service, and the Phoenix Zoo, PCHD began using Gila Topminnow as a mechanism for mosquito control within certain waters in Pima County. Listed as Endangered under the Endangered Species Act in 1967, this is a new role for the little fish in a wider effort to eliminate threats and reestablish it across southern Arizona. And while certainly not an end-all solution for mosquito control, it provides PCHD with another tool in its arsenal of treatments available for mosquito control and the spread of certain mosquito-borne diseases.

To date, PCHD has had the opportunity to use topminnow as a vector control tool on two properties in the Tucson Metropolitan area. After previous efforts were exhausted in both instances, a warrant was obtained allowing PCHD employees onto the property to treat the abandoned waters. Gila Topminnow were then stocked to help reduce mosquito-linked health risks at both locations.

Conservation efforts for the Gila Topminnow over the past several decades have focused on establishing refuge populations free of nonnative species and reestablishing wild populations across its former range; these efforts have ultimately improved the status of topminnow. This most-recent action will use topminnow to control mosquitoes in abandoned pools, fountains or ponds, rather than using chemical or other biological means. Though limited in scope as to where they can be used, this is widely believed to be an effective first step towards reducing or eliminating the spread of Mosquitofish and the continuing threat they pose to topminnow, while providing PCHD with an effective option for controlling mosquitoes at a number of sites.

Our hope is that in the future, other counties within the Gila River Basin will also follow suit, using this or other native fishes as an means of mosquito control, rather than a detrimental nonnative species.


Topminnows are transferred to the abandoned water, where they will aggressively prey on mosquito larvae (Photo courtesy of Pima County Communications Department).


An abandoned urban water that generates a health risk due to an infestation of mosquitos. This water was recently stocked with topminnow for mosquito control (Photo courtesy of Pima County Communications Department).



Gila Topminnow, an endangered species, is now being introduced for vector control to replace the use of Mosquitofish, a nonnative species.