Responses of Native and Nonnative Fishes to Natural Flow Regime Mimicry in the San Juan River

Abstract

AbstractThe maintenance or restoration of natural flow regimes has been proposed as one means of conserving native fishes. Native fish conservation is enhanced either through the restoration of natural fluvial geomorphic processes (and thus the maintenance of essential habitats) or by the suppression of nonnative fishes. The San Juan River of Colorado, New Mexico, and Utah was dammed in 1962 and its natural flow regime was lost. Beginning in 1993, the river was regulated to mimic a natural flow regime by increasing reservoir releases to mimic timing, but only partially to mimic amplitude, volume, and duration of spring snowmelt discharge. We evaluated the responses of native and nonnative fishes to this natural flow regime mimicry by comparing their autumn densities (number/m2) in San Juan River secondary channels to those during spring runoff and summer base flow over a 9‐year period. Densities of native speckled dace Rhinichthys osculus, bluehead sucker Catostomus discobolus, and flannelmouth sucker C. latipinnis increased with elevated spring discharge. Total native fish density was 10 times greater in 1993 (the year of highest spring discharge) than in 2000 (the year of lowest spring discharge). Collectively, nonnative fish density was negatively related to spring discharge, but western mosquitofish Gambusia affinis was the only commonly collected nonnative that had a significant relationship. Mean daily summer discharge did not affect the density of native or nonnative fishes. Nonnative fishes, however, responded positively to sustained low summer flows (days discharge was less than 14 m3/s). Densities of red shiner Cyprinella lutrensis, common carp Cyprinus carpio, and western mosquitofish were four or more times greater in 2000 (a year of sustained low summer discharge) than in years with comparatively high summer discharge. Speckled dace was the only native species negatively affected by extended low summer discharge. Our results suggest that manipulating spring discharge to mimic a natural flow regime enhances native fish recruitment but might have limited effect in suppressing nonnative fishes, particularly fecund, rapidly growing, small‐bodied species.