That compared to eight captured a year earlier along the river.

On the same day the latest samples were taken, federal fish biologist Doug Duncan saw more than 200 topminnow in a small pool near Santa Gertrudis, in an anecdotal observation that didn’t involve sampling.

“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 Duncan, of the U.S. Fish and Wildlife Service.

The topminnow, a thin fish with tan- to olive-colored bodies, was first collected in the Santa Cruz near Tucson in 1851. Its numbers dropped dramatically after World War II due to non-native competition, groundwater pumping and other forces that have damaged many of Arizona’s native fish populations. Until last year, it hadn’t been seen in the Upper Santa Cruz since 2005.

The main credit for the topminnow’s improved presence here goes to the federal government’s $59 million project to improve the Nogales International Wastewater Treatment Plant, said Reinthal and Claire Zugmeyer, an ecologist for the Sonoran Institute, which is involved in the Santa Cruz fish monitoring.

Before the upgrade was finished in 2009, the plant was polluting the Upper Santa Cruz with discharges high in ammonia and phosphorus.