Endangered Gila topminnows have once again been found in the Santa Cruz River of Southern Arizona after a 10-year absence, and scientists say that improved water quality in the river might have led to the reappearance of the fish there.

Several of the inch-long topminnows, a native Arizona species, were found in the river near the U.S.-Mexico border during an annual fish survey conducted by the Arizona Game and Fish Department along with other agencies and organizations, said Tony Robinson, a native fish biologist with the department.

“It’s excellent news. We’re glad to have that population back,” Robinson said.

He said there are only eight natural populations of Gila topminnows remaining in Arizona, but he estimated that there are “thousands” of the fish in the state — “some in a wild setting and some in ponds.”

Spokespersons for other agencies and groups said it appears that cleanup of the river led to the return of topminnows.

“We have seen such an improvement in water quality from a 2009 upgrade to the international wastewater treatment plant that we knew Gila topminnow would come back to the river,” said Doug Duncan, fish biologist with the U.S. Fish and Wildlife Service.

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“The endangered topminnow has been living in Sonoita Creek, upstream of the Santa Cruz River, and (in) the Santa Cruz River in Mexico, so most likely floods washed them down,” Duncan said. “Now we need to ensure they survive and thrive long term.”

Claire Zugmeyer, ecologist at the nonprofit Sonoran Institute and a long-time coordinator of the annual fish survey, said, “Historically, communities have discharged effluent into river channels out of sheer convenience. Upgrades to wastewater treatment facilities pay substantial community dividends. As rivers bounce back to health, they provide rich oases for both people and wildlife.”

Robinson, of Game and Fish, said the reappearance of topminnows in the Santa Cruz River is “a very positive thing” for an ongoing effort to improve populations to the point where they are no longer endangered.

“The Game and Fish Department and the Fish and Wildlife Service are working to establish the population” of Gila topminnows, Robinson said. “We’re hoping to get them down-listed from endangered to threatened in the next 10 years.”

Contact reporter Doug Kreutz at dkreutz@tucson.com or at 573-4192. On Twitter: @DouglasKreutz