Influence of low flows on abundance of fish in the upper San Pedro River, Arizona

The relationship between stream discharge and abundance of fish in the upper San Pedro River, Arizona, was examined. Backpack electrofishing equipment was used to capture fish at four sites in a 50-kilometer reach between the International Boundary and Fairbank. Surveys were done annually in the spring, 1990 to 1997. Catch per unit of effort (CFUE: number of individuals per 15 minutes shocking) varied between years (mean = 89.1, range = 32.7 to 167.8). Native desert sucker, Catostomus clarki, and longfin dace, Agosia chrysogaster, comprised 57 to 85% of the annual catch. In addition to the two native species, black bullhead, Ameiurus melas, and fathead minnow, Pimephales promelas, were captured in all eight efforts. Seven surveys included western mosquitofish, Gambusia affinis, green sunfish, Lepomis cyanellus, were in the last six efforts, largemouth bass, Micropterus salmoides, were in two, and common carp, Cyprinus carpio, was taken once. Fish abundance appeared related to extreme low flow during the previous year, but not peak or mean flows, or total runoff. Lowest daily mean discharges (mean = 25 l/sec, range = 1 to 48) explained 82% of the variation in catch for all species and all sites combined, and 78% and 70% of the variation for native and normative species, respectively.