A Dynamic Flow Regime Supports an Intact Great Plains Stream Fish Assemblage

Intact native fish assemblages-those with all or most historical taxa present and a strong numerical dominance of native species-are rare in streams and other aquatic systems worldwide. Our sampling during four intervals from 1983 to 2007 showed that the Purgatoire River, a Great Plains stream in eastern Colorado, supported an assemblage that was 99.6% native taxa, and all but one rare species persisted through time. An additional rare Arkansas River native fish colonized the Purgatoire River and was widespread in 2006-2007, and only three nonnative taxa were detected. The perennial Purgatoire River supported all native fishes in the basin, while tributaries supported mainly native fishes that were tolerant of intermittent, harsh habitat. In the Purgatoire River, frequency and abundance of dominant species varied spatially during 2006-2007. Persistence of native fishes was unchanged over time in the Pinon Canyon reach of the river even though abundance varied substantially. Despite the presence of upstream Trinidad Reservoir, which reduced flood peak magnitude and frequency and increased base flows, the flow regime of the river remained flashy, with high, turbid, and unpredictable flood flows. Peak flows resulted from thunderstorms in tributaries of the Purgatoire River downstream from the reservoir or from snowmelt runoff and usually occurred in August or July but also were observed in May and June. We attribute persistence of the native fish assemblage to sustained base flows and, more importantly, to the continued (albeit lower) magnitude and frequency of peak flows and otherwise harsh habitat, which limit invasion by nonnative fishes from upstream and downstream reservoirs. Persistence of the intact native fish assemblage in the Purgatoire River requires maintenance of the dynamic flow regime and prevention of invasions by nonnative fishes, including riverine-adapted centrarchids like Smallmouth Bass Micropterus dolomieu. Conservation of the Purgatoire River basin should be a priority for ecologists and managers alike given its value as a study system and as habitat for rare fishes.