Effects of Flooding on Abundance of Native and Nonnative Fishes Downstream from a Small Impoundment

—Flooding can benefit native fishes in southwestern streams by disproportionately displacing nonnative fishes. We examined how the presence of an upstream impoundment affected this relationship in lower Sonoita Creek, Arizona. Nonnative species not found in the reservoir decreased in abundance in lower Sonoita Creek after flooding. The catch and relative abundance of some nonnative species found in both the reservoir and the creek increased in lower Sonoita Creek after flooding. Movement of nonnative fishes out of the reservoir via the spillway during periods of high water probably contributes to the persistence and abundance of these species downstream. Both preventing nonnative fishes from escaping reservoirs and the release of flushing flows would aid conservation of native southwestern fishes downstream. Many species of fish native to the American Southwest have decreased to the extent that they are now federally listed as threatened or endangered (Minckley and Deacon 1991). Loss and modification of aquatic habitats (Miller 1961; Hendrickson and Minckley 1984; Rinne 1992; Rinne et al. 1998) and negative interactions with introduced species (Meffe 1985; Marsh and Brooks 1989; Douglas et al. 1994; Marsh and Douglas 1997; Dudley and Matter 2000) are primarily responsible for these declines. In the Southwest, flooding has been shown to benefit native fishes by disproportionately displacing nonnative fishes (Meffe 1984; Minckley and Meffe 1987; Rinne and Stefferud 1997; Rinne et al. 1998). This is not surprising because fishes native to the Southwest have evolved in arid systems where precipitation is often torrential and localized, and runoff is often abrupt through channels that may be narrow, of high gradient, canyonlike, and lacking dense vegetative cover. Discharge may increase by three or more orders of magnitude in seconds. In comparison, many of the * Corresponding author: [email protected] 1 Present address: School of Renewable Natural Resources, University of Arizona, 325 Biosciences East, Tucson, Arizona 85721, USA. Received July 30, 2001; accepted August 14, 2002 fishes introduced to the Southwest evolved in mesic systems, where channels meander, having low gradients and broad floodplains, and floods build over periods of hours to days (Minckley and Meffe 1987). The disproportionate displacement of nonnative fishes by flooding is theorized as a factor that allows long-term coexistence of some native and nonnative fishes (Meffe 1983, 1984; Minckley and Meffe 1987). Reservoirs often alter downstream discharge regimes and habitat conditions, provide habitat for nonnative fishes and an impetus for stocking them, and subsequently affect fish assemblages downstream (Vanicek et al. 1970; Edwards 1978; Ward and Stanford 1979; Clarkson and Childs 2000). The impact of regulation may be enhanced in dryland streams because the regulated regime often is drastically different from natural flow conditions (Davies et al. 1994). Our objectives were to quantify the effects of flooding on the fish assemblage of a southwestern stream below an impoundment and identify management strategies that would favor native fishes in these types of systems.