After going undetected for ten years, an endangered fish species has once again been found in the Santa Cruz River in southern Arizona. Last month the Gila topminnow, a native Arizona species listed under the Endangered Species Act in 1967, was found in the Santa Cruz River near the U.S.-Mexico border during the annual fish survey conducted by the Sonoran Institute, Arizona Game and Fish Department, Friends of the Santa Cruz River, National Park Service and other partners.
Surface flows along most of the Santa Cruz River originate from effluent (cleaned wastewater), and have historically been so polluted that no fish of any kind were found for several years and odor alone was a deterrent to recreation. Scientists believe that cleaner water is what led to the fish’s return.
“It’s rare to have a native fish species re-appear and occupy its former habitat, so this rediscovery is significant for our efforts to recover imperiled fish species throughout Arizona,” says Ross Timmons, topminnow recovery coordinator for the Arizona Game and Fish Department. “Without the ongoing monitoring of this partnership, it may have been some time before we realized they were back in the Santa Cruz River.”
Many southwestern rivers and streams depend on effluent for continued flows. As water becomes ever scarcer in the desert southwest, the value of wastewater inputs will only increase.
Massive upgrades to the Nogales International Wastewater Treatment Plant have resulted in the elimination of odor, reduced levels of toxicity for fish, and a breakdown of the so-called “Schmutzdecke” (dirty blanket)—a clogging layer of algae and microorganisms that kept water from infiltrating into the groundwater table. University of Arizona scientists found that this clogging layer was largely responsible for an eight mile die-off of trees along the Santa Cruz River near Rio Rico in 2005.