BLM conducts important fish surveys in the San Pedro Riparian National Conservation Area

Story by Fisheries Biologist Christina Perez

The BLM manages the approximately 57,000-acre San Pedro Riparian National Conservation Area (SPRNCA), which includes about 40 miles of the upper San Pedro River.  The primary purpose for the area’s 1988 Congressional designation is to protect and enhance the desert riparian ecosystem, a rare remnant of what was once an extensive network of similar riparian systems throughout the Southwest.

The San Pedro River historically supported 13 native fish species known to the Gila River System, however only two natives – longfin dace Agosia chrysogaster and the desert sucker Catostomus clarkii – remain in the San Pedro River. Common non-native fish species, which pose a threat to native species in the San Pedro River, are yellow bullhead, common carp, green sunfish, mosquito fish, fathead minnows, crayfish, spiny soft-shelled turtles, and bull frogs. Other threats to native aquatic species include climate change, groundwater pumping, and changes to the natural flow regime of the river.

The BLM has a 25-year data set that began with data collected by two volunteers tracking fish populations on the San Pedro River. Each year, a team of BLM interdisciplinary resource specialists survey the river in late spring.  The historical data set focuses on five fixed sites in the San Pedro River. However, since the selection of aquatic sites over 20 years ago, one site is completely dry (Babocomari site) and two are now intermittent (near Fairbank and near Hereford). A new perennial site was selected three years ago to replace the intermittent sites. Backpack electrofishing is used to sample fish species at each of the three river sites. The captured fish are identified, measured (only desert sucker) and counted and habitat data is recorded for each site. The data is used to track how the native fish are responding to conditions of the river over time.

This year, the BLM team and a volunteer completed annual fish surveys April 26-28. The team observed native desert sucker and longfin dace and non-native bullfrog, green sunfish, western mosquito fish, fathead minnow, crayfish, and common carp.  Data collected during the annual surveys reveal that in many instances, habitat that was available to fish and aquatic species 20 years ago is no longer available. Of the two native fish observed, desert sucker persists in the San Pedro River; however, this species is restricted to limited habitats that are few and far between in the upper San Pedro River.  In two of the sites, where staff used to catch larger numbers of desert sucker, numbers have decreased. There are a significant number of non-native species still in the river system, however, overall catch of non-natives has decreased. This important work each year helps to track fish populations in the river, identify the native fish species that still exist in the system, and inventory change in habitat. The resulting data will be analyzed and included with other monitoring data to develop future management actions to improve both fish habitat and populations.