Long-Term Dynamics of Native and Nonnative Fishes in the San Juan River, New Mexico and Utah, under a Partially Managed Flow Regime

Nonnative fishes and flow alteration are primary threats to native fish persistence in lotic systems. We used several flow regime attributes and fish sampling data obtained from the San Juan River, New Mexico and Utah, during 1993–2010 to evaluate the potential use of flow manipulations to increase recruitment of native fishes that must cope with nonnative species. During this period, discharge in the river was partially manipulated by reservoir releases that augmented naturally high spring flows in this snowmelt-driven system. An information theoretic approach was used to rank candidate models that predicted species densities based upon selected combinations of flow attributes and abundances of nonnative species. Autumn density of age-0 fishes in secondary channels was the main response variable. The main predictor variables included flow attributes associated with interannual variation in daily discharge and water temperature; densities of nonnative competitors; and catch rates of a numerically dominant nonnative predator (the channel catfish Ictalurus punctatus). Top-ranked models for native species included positive associations with small-bodied nonnative fishes and negative associations with the abundance of channel catfish adults. Densities of native speckled dace Rhinichthys osculus and flannelmouth suckers Catostomus latipinnis increased with mean spring discharge, but the density of native bluehead suckers C. discobolus did not. With the exception of juvenile channel catfish, the top candidate models predicting densities of nonnative fishes all included the duration of low summer flows. These results confirmed findings from a previous study that demonstrated different responses of native and nonnative fishes to seasonal flows; the present study also revealed that densities of all fishes were generally lower in years with greater abundance of adult channel catfish. Regression analysis indicated that seasonal flow manipulations and suppression of nonnative predator populations could be effective management tools to restore and maintain the native fish community. Manipulating flows in regulated rivers is at least conceptually an important conservation tool (Poff et al. 1997), but implementation of effective flow management requires a datadriven understanding of how stream organisms respond to manageable attributes of flow regimes. The most comprehensive framework for prescribing environmental flows is to mimic a natural flow regime or attributes of that flow regime that restore natural stream ecosystem processes (Poff et al. 1997, 2010; *Corresponding author: [email protected] Received March 2, 2011; accepted November 20, 2011 Bunn and Arthington 2002). Specifically, the timing, frequency, duration, magnitude, and rate of change are thought to influence stream organisms because of evolutionary, behavioral, and morphological adaptations structured by these flow attributes (Lytle and Poff 2004). Although there are excellent examples of how fishes respond to flow manipulations (Murchie et al. 2008), understanding how entire communities respond to specific attributes of flow regimes (either natural or manipulated) over 645 D ow nl oa de d by [ K an sa s St at e U ni ve rs ity L ib ra ri es ] at 1 0: 53 1 4 M ay 2 01 2