The long-endangered Gila topminnow seems to be thriving in the Santa Cruz River near Tumacacori National Historical Park/Arizona Game and Fish Department

An endangered minnow that was returned to the Santa Cruz River in southern Arizona a decade ago via a recovery project now appears to be thriving in the river that flows by Tumacacori National Historical Park.

The endangered Gila topminnow, a native Arizona species, was listed under the Endangered Species Act in 1967. Last year, it was found in the Santa Cruz River near Nogales, Arizona, for the first time since 2005. Annual surveys conducted in November confirm that Gila topminnow remain in the river and have likely increased in number.

“At one site, I saw a small pool with over 200 topminnow. We are thrilled to be finding them this numerous since this is a good indicator that their return last year was not a brief blip on the radar,” said Doug Duncan, a fish biologist with the U.S. Fish and Wildlife Service.

Surface flows along most of the Santa Cruz River originate from effluent (highly treated wastewater) and have in recent decades been so polluted that no fish of any kind were found for several years. Massive upgrades to the Nogales International Wastewater Treatment Plant beginning in 2009 resulted in the elimination of odor, reduced levels of toxicity for fish, and a breakdown of 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.

After the treatment plant upgrades were completed, scientists and local residents eagerly awaited the fish's return as the water quality in the river began improving. Scientists believe that cleaner water led to the fish’s return. Survey methods do not estimate population numbers, but the ease with which the Gila topminnow were found this year suggests that they are doing very well, a release from the park said.

The implications of the endangered topminnow discovery extend far beyond Santa Cruz County and even Arizona. Many Southwestern rivers and streams depend on effluent for continued flows. As water becomes scarcer in the desert Southwest, the value of returning wastewater to the ecosystem will only increase.

“Often communities discharge effluent into rivers out of convenience and not with intent to benefit the environment,” said Claire Zugmeyer, an ecologist at the Tucson-based nonprofit Sonoran Institute, and longtime coordinator of the annual fish survey. “We are now seeing that highly treated wastewater is a vital component to maintaining this region’s living river. With the release of effluent into the Santa Cruz and other rivers, we can create rich oases for both people and wildlife while simultaneously benefiting from functions provided by a healthy river, such as flood control, recharge, and cooling riparian vegetation.”

“We are ecstatic to know the Gila topminnow appear to be thriving again,” said Sherry Sass of the Friends of the Santa Cruz River, an all-volunteer organization at the forefront of river health advocacy. “We have been tracking water quality and river conditions since the early-1990s. The return of this sensitive species speaks volumes about the river’s recovery.”

This year’s survey was conducted by Sonoran Institute, U.S. Fish and Wildlife Service, University of Arizona, National Park Service, and other partners. The groups have been conducting the annual survey since 2008 as a means to track the overall health of the Santa Cruz River. Additional community partners who have participated in this annual survey effort include the Arizona Game and Fish Department, Friends of the Santa Cruz River, National Park Service Sonoran Desert Network, United States Geological Survey, and Global Community Communications Schools at Avalon Gardens.