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00158~ La Plata River Native Fishes Information and Mitigation Recommendations for Proposed Long Hollow Reservoir Colorado Division of Wildlife February 4,2004 I. Fish species distribution and relative abundance within the area of interest. The area of interest includes the lower La Plata River drainage extending from Hesperus, Colorado south to the state line. Native fishes present in the lower La Plata River include: roundtail chub (Species of Special Concern in Colorado), flannelmouth sucker, bluehead sucker, mottled sculpin and speckled dace. From Hesperus to its confluence with Cherry Creek, La Plata River flows are regularly dewatered by existing irrigation diversions and no permanent fishery exists. Flows occasionally dry up in the La Plata River from Cherry Creek to Long Hollow Creek, especially during the severe drought in recent years. From its confluence with Cherry Creek to the state line, base flows in the La Plata River are heavily dependent on tributary inflow (including irrigation return flows) from Cherry Creek and Long Hollow Creek. In the Final Report, Animas-La Plata Project, Native Fish Studies William J. Miller (March, 1995) reported 98% of fish captured on the La Plata River were native species, which is unusual for western rivers that usually contain numerous non-native fishes. Miller (1995) reported seasonal movement offlannelmouth sucker, bluehead sucker, and roundtail chub in the La Plata River between the Thomas Ditch diversion (0.5 mile south of state line) to the Cherry Creek confluence. The Thomas Ditch diversion forms a fish rnigration barrier that prevents upstream movement of fish in the La Plata River. The lower La Plata River native fish community has been genetically and geographically isolated from the rest of the San Juan basin for many years by this irrigation diversion structure. As a result, competing non-native fishes like green sunfish, channel catfish, and common carp have not invaded from the San Juan River. Geographic isolation has allowed the native fish community in the La Plata River from Cherry Creek to the state line to persist for many years, despite marginal flow conditions. Historically the native fish community of the lower La Plata River has occupied three distinct (but interconnected) reaches, shown in Figure 1. Extreme drought conditions in 2002 and 2003 coupled with existing irrigation practices greatly reduced the abundance and distribution of native fishes in the La Plata River. If stream flows return to average conditions found in the last 100 years, native fishes are expected to re-colonize suitable habitat in the La Plata River from the state line to the Cherry Creek confluence (~l 0 river miles). Relative abundance of native fishes within each of these stream reaches is listed in declining order: Reach 1: La Plata River from the state line to Long Hollow confluence (~5 river miles): flannelmouth sucker, speckled dace, bluehead sucker, roundtail chub, and mottled sculpin. (Miller, 1995). This is the core area for native fishes in the lower La Plata River. Fish habitat within this reach contains deep pools with abundant woody debris and overhanging vegetation that is utilized by roundtail chubs. The Biological Assessment for the Proposed Long Hollow Reservoir (BA) identifies potential significant negative impacts to native fishes in this reach caused by reduced winter base flows resulting from storage of water in Long Hollow Reservoir.