The discovery of endangered Gila topminnows in the Santa Cruz River near Tucson puts the tiny native fish a step closer to recovery, a federal biologist said Friday.
“Dozens” of the imperiled minnows were captured as part of a fish survey on the Lower Santa Cruz in early November, said Doug Duncan, a fish biologist for the U.S. Fish and Wildlife Service. It was the first time the endangered minnow has been seen in the Santa Cruz near Tucson since 1943.
Finding these fish in this stretch of river is probably the most significant conservation discovery for the topminnow since it was listed in 1967 as endangered, Duncan said. The fish were found Nov. 9 downstream of the Aqua Nueva sewage treatment plant near Sweetwater Drive. It was one of four locations where fish surveys were conducted.
The fish’s return to the Santa Cruz comes two years after the topminnows were found for the first time in decades in the Upper Santa Cruz in the Tubac area south of Tucson.
These discoveries are generally credited to the cleanup of long-polluting sewage-treatment plants in Nogales and Tucson. Sewage effluent has been the only year-round water source in the river since groundwater pumping dried up fresh water in the river back in the 1940s.
The topminnows were found in the latest of the annual “Living River” surveys sponsored by the Sonoran Institute conservation group and joined by a host of other state and federal agencies and other entities.
“Any fish in a river is a really good sign of a river’s health,” said Claire Zugmeyer, a Sonoran Institute ecologist. “Finding Gila topminnow underscores how critical effluent is for maintaining our native fish and wildlife.”
The latest discovery of topminnows is particularly significant because the Santa Cruz is a large river system where topminnows used to occur commonly, Duncan said. It’s also happening in a city where multiple native fish species used to dwell in the Santa Cruz but now only live in tributaries such as Sabino Canyon and Cienega Creek, northeast and southeast of Tucson respectively.
It also means that:
- The Santa Cruz could be host to the largest of 11 remaining wild populations of topminnow known to exist, Duncan said. Until now, Cienega Creek southeast of Tucson had the largest known population, he said.
But with this finding, the combined populations in the Santa Cruz may be larger. Cienega Creek has the fish in nine miles, whereas dozens of miles in the Santa Cruz have the endangered topminnows, Duncan said.
- Combined with other conservation actions helping the topminnow over the past five years, “I think we’re getting close to downlisting. ... We’re definitely on our way to downlisting” the fish from an endangered to a threatened species, Duncan said.
Removing the topminnow entirely from the endangered species list will take longer, he said. The criteria and timetable for downlisting are unknown, because the service is in the middle of revising its topminnow recovery plan, he said.
- If the U.S. Bureau of Reclamation continues to seek to remove treated sewage effluent from the Santa Cruz for groundwater recharge outside it, it will likely require a formal review by the wildlife service to see how the loss of effluent would affect the topminnow species, Duncan said.
- The wildlife service has begun a similar review of the ongoing $148 million rebuilding of the Ina Road-Interstate 10 interchange and the building of a new Ina Road bridge over the river.
Duncan said he won’t rule out the possibility that some of the 1,000-plus fish killed in the river below Ina as part of that construction project in late October and early November were topminnows. But authorities don’t believe any were there at that time, since surveys downstream didn’t find any. They could occur there in the future.
In any case, “We believe that bridge construction can continue and minimize potential impacts to the topminnow. We would expect certain losses, but as long as the rest of the population’s fine, for management purposes we want a viable population,” Duncan said.
In the Upper Santa Cruz River in the Tubac area, surveyors found eight Gila topminnows in 2015 and 135 last year.
This year, surveyors found “hundreds” of topminnows near the river’s Tubac bridge and at nearby Santa Gertrudis Lane, said Peter Reinthal, curator of fishes at the University of Arizona, who participated in the Upper Santa Cruz survey.
The topminnow used to be abundant across the Gila River Basin, which includes the Santa Cruz. In 1941, it was considered one of the most common fish in the southern Colorado River Basin, the wildlife service said. But its numbers dropped dramatically after World War II due to non-native fish competition, groundwater pumping and other forces that have damaged many of Arizona’s native fish populations. It occupies a fraction of its former range.
The Nogales International Wastewater Treatment plant was given a $59 million overhaul that finished in 2009. Pima County spent more than $600 million cleaning up discharges, by 2013, from the Aqua Nueva and Tres Rios sewage plant at Ina along the Lower Santa Cruz.
For decades, both sections were so polluted that few fish of any kind were found during annual surveys. The Sonoran Institute’s annual Living River report says fish now thrive in healthier living conditions, including reduced toxin levels and increased dissolved oxygen.
Since 2013, the number of non-native fish found in the Lower Santa Cruz has increased, but until now no native fish were seen there.
Zugmeyer said she’s hopeful that in the future, native longfin dace will be found in the Lower Santa Cruz. They’ve already been found in recent years in the Upper Santa Cruz.
Exactly how the topminnows reached the Lower Santa Cruz is uncertain, but officials will be conducting genetic testing to try to find out. A possible or likely source is the Upper Santa Cruz, particularly since the two rivers were connected with floodwaters during July when nearly 7 inches of rain fell.
