TUCSON, Ariz.— A newly built segment of border wall and paved road across Arizona’s California Gulch is blocking streamflow critical to the survival of one of only two U.S. populations of Sonora chub. The rare desert fish is protected as threatened under the Endangered Species Act.

“The new wall and road will push these imperiled fish to the brink of extinction,” said Krista Kemppinen, Ph.D., a senior scientist at the Center for Biological Diversity. “The Sonora chub’s survival depends on being able to access scarce desert water on both sides of the border, exchange genetic material with nearby populations in Mexico, and bolster its populations with upstream migrations of fish from Sonora after droughts. The new construction makes all that impossible.”

Critical habitat designated for the Sonora chub in 1986 includes the entire U.S. range of the species known at the time. The nearby California Gulch population was discovered later.

In 2023 the Center filed a formal petition to designate approximately 4 miles of California Gulch as critical habitat, starting at the international border. In July the U.S. Fish and Wildlife Service announced that expanded critical habitat designations for the species may be warranted. But the agency also decided to delay any action until at least 2027.

The petition identified addition of the newly constructed wall segment as a primary threat to the population’s survival. Other major threats to Sonora chub in California Gulch are climate change — which is fueling the region’s megadrought and drying up Sonora chub habitat — mining and grazing by livestock. Damage from livestock, which trample the fish and pollute and degrade their habitat, is regularly documented in the area.

“With new border infrastructure cutting this population off from its lifeline in Sonora, designating California Gulch as critical habitat is more urgent than ever to minimize other threats, such as by keeping cows out of the Sonora chub’s pools,” said Kemppinen. “It’s also imperative that carefully designed culverts be added to the new border infrastructure to allow at least some semblance of a natural streamflow and migration. If federal officials are serious about saving this fish, they need to act now.”

The Sonora chub is a small, moderately chubby fish that feasts on a variety of native food sources and has a unique and distinctly red coloration on the underside when in breeding condition.