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

Abstract

The 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.

Historical Fish Assemblage Flux in the Cheyenne River below Angostura Dam

“…Studies elsewhere have found that red shiner abundance may dramatically increase during low-flow years and in river reaches directly below dains (Anderson et a]. 1983, Bonner and Wilde 2000, Propst and Gido 2004, similar to our observations. In contrast, drought and stable flows may negatively impact riverine species such as plains minnow and flathead chub (Cross andMoss 1987, Bonner andWilde 2000), which may explain their absence from our 2004 collections.…”

Endangered river fish: factors hindering conservation and restoration

“…Although mimicking the natural flow regime may enhance native fish recruitment, it does not necessarily reduce non-native fish recruitment, particularly for highly fecund small-bodied nonnative fishes (Propst & Gido 2004). Therefore, identifying the mechanisms and specific aspects of flow that help maintain native river fishes may be needed to maintain these native populations.…”

Preliminary synthesis and assessment of environmental flows in the middle Verde River watershed, Arizona

“…Large floods have been shown to be beneficial for the succession of fish species native to the Southwest (Rinne 2005;Propst and Gido (2004), and biota that have evolved under conditions of such variable discharge will return to a proper functioning condition following a flood. The magnitude and timing of a large flood are important to know because they affect the characteristics of the succession period that follows.…”