The diminutive Gila topminnow is a big indicator of change. This endangered fish, no bigger than your thumb, is native to the region but disappeared from the Santa Cruz River near Tucson decades ago as the water dried up. Now there is a glimmer of hope for its return to a newly flowing stretch of the river near downtown Tucson.
Three years ago, the topminnow reappeared in northwest Tucson thanks to Pima County’s investment in major upgrades to wastewater treatment facilities. When Tucson Water launched the Santa Cruz River Heritage Project last year, some of this high-quality recycled water started flowing in the long-dry riverbed near downtown. The hope was that the Gila topminnow would eventually follow, once the ecosystem had time to mature. But no one expected the fish to be there a mere 10 months later.
The hope was that the Gila topminnow would eventually follow, once the ecosystem had time to mature. But no one expected the fish to be there a mere 10 months later.
Aquatic ecologist and University of Arizona professor, Michael Bogan, has been going out to the river every two weeks since the water started to flow in June 2019. He is studying the reappearance of aquatic wildlife, such as the 40+ dragonfly species that were found flitting over the new flows. During a recent routine field visit, Dr. Bogan found Gila topminnow swimming in a small pool, which has since dried up.
The source of the Gila topminnow in this small pool is unknown, but it is unlikely that they naturally populated the pool from the Nogales area or downstream reaches of the river. “The river in the Heritage Project reach has dried completely several times when the water was shut off for project maintenance—so even if topminnow naturally colonized the reach last monsoon season, they would have disappeared during those dewatering events,” explains Dr. Bogan. “Furthermore, all other subsequent flood events have been very small and originated completely as urban runoff.” Dr. Bogan thinks the more likely explanation is that the fish were placed there by human hands.
Although this likely introduction may have been done with good intention, moving any aquatic species, native or non-native is problematic because there are likely to be unintended consequences for the wildlife and the ecosystem—especially in a highly managed system like this reach of the Santa Cruz River.
Although this likely introduction may have been done with good intention, moving any aquatic species, native or non-native is problematic…
The U.S. Fish and Wildlife Service and the Arizona Game and Fish Department, who are leading the recovery of this endangered species, were unable to test the genetic make-up of this specimen, which is important to manage healthy populations of Gila topminnow. “Unfortunately, the COVID-19 pandemic has resulted in the closure of the genetics lab we partner with at the University of Arizona. We won’t be able to investigate the potential origin of these fish for months,” said Doug Duncan of U.S. Fish and Wildlife Service.
“Moving animals around is also illegal—this misdemeanor can come with a big fine, up to $8,000 for endangered species,” said Nate Berg of the Arizona Game and Fish Department. These laws are meant to protect the safety of both people and wildlife. No one should try to introduce animals to the river.
The most immediate problem for the fish in the Heritage Project is here—our spring weather has turned into summer heat. Warmer temperatures naturally cause the length of a desert river’s flow to shrink as evaporation increases and plants along the banks use more water. Just two weeks after Gila topminnow was found downtown, their small pool was isolated, and flows no longer traveled very far. As temperatures soared to 100 degrees, the small pool completely dried.
However the fish returned, it was too soon. The pool was in a reach that is undergoing critical maintenance work this month to remove sediment and vegetation to reduce the risk of flooding to residences and businesses in the area. Given this urban setting and flood risk, the ecosystem created by the Heritage Project flows will always have to be highly managed, with careful consideration about whether, when, and how to introduce fish or other wildlife.
Given this urban setting and flood risk, the ecosystem created by the Heritage Project flows will always have to be highly managed, with careful consideration about whether, when, and how to introduce fish or other wildlife.
The weeks of dredging work ahead will change the immediate look and feel of this reach of the river for the short term. However, the story is ultimately a positive one. In its preliminary year, the Heritage Project experiment has helped to recharge the aquifer as intended, has become a valued amenity along the Loop in the downtown area, and has supported wildlife flocking to a new desert oasis. Although it will be temporarily shut off for much-needed maintenance, many benefits are expected to quickly return when it begins flowing long-term in the newly improved channel.
Furthermore, Tucson Water, Arizona Game and Fish Department, and the U.S. Fish and Wildlife Service are now discussing how the Gila topminnow might be incorporated responsibly into the water flowing downtown at a future date. This interagency cooperation will be critical to achieve a successful return of the Gila topminnow near downtown Tucson after nearly a century of absence.
Dr. Bogan is hopeful about the project’s future. “It’s truly exciting to be part of a process where the scientific data we collect is informing discussions that may bring the fish back to its historic home.”
“It’s truly exciting to be part of a process where the scientific data we collect is informing discussions that may bring the fish back to its historic home.”
–Michael Bogan, aquatic ecologist and assistant professor at the University of Arizona
Blog Post By: Claire Zugmeyer, Ecologist