An occupancy-based quantification of the highly imperiled status of desert fishes of the southwestern United States

Desert fishes are some of the most imperiled vertebrates worldwide due to their low economic worth and because they compete with humans for water. An ecological complex of fishes, 2 suckers (Catostomus latipinnis, Catostomus discobolus) and a chub (Gila robusta) (collectively managed as the so-called three species) are endemic to the U.S. Colorado River Basin, are affected by multiple stressors, and have allegedly declined dramatically. We built a series of occupancy models to determine relationships between trends in occupancy, local extinction, and local colonization rates, identify potential limiting factors, and evaluate the suitability of managing the 3 species collectively. For a historical period (1889-2011), top performing models (AICc) included a positive time trend in local extinction probability and a negative trend in local colonization probability. As flood frequency decreased post-development local extinction probability increased. By the end of the time series, 47% (95% CI 34-61) and 15% (95% CI 6-33) of sites remained occupied by the suckers and the chub, respectively, and models with the 2 species of sucker as one group and the chub as the other performed best. For a contemporary period (2001-2011), top performing (based on AICc ) models included peak annual discharge. As peak discharge increased, local extinction probability decreased and local colonization probability increased. For the contemporary period, results of models that split all 3 species into separate groups were similar to results of models that combined the 2 suckers but not the chub. Collectively, these results confirmed that declines in these fishes were strongly associated with water development and that relative to their historic distribution all 3 species have declined dramatically. Further, the chub was distinct in that it declined the most dramatically and therefore may need to be managed separately. Our modeling approach may be useful in other situations in which targeted data are sparse and conservation status and best management approach for multiple species are uncertain

Frequent Usage of Tributaries by the Endangered Fishes of the Upper Colorado River Basin: Observations from the San Rafael River, Utah

The importance of main-stem rivers and major tributaries to endangered Colorado River fishes is well documented, but the use and significance of small tributary streams remains poorly understood. Historically, these fishes probably used smaller tributaries for spawning, rearing, feeding, and refuge. Currently, the proliferation of nonnative species and altered flows may have affected tributary use by endangered fishes. In February 2008 and 2009, we installed a PIT-tag passive interrogation array (PIA) in the San Rafael River, Utah, approximately 2 km upstream from the confluence with the Green River, and another PIA approximately 60 km upstream from the Green River confluence. Using passive detections and active captures in the San Rafael River from 2008 to 2010, we detected 15 Colorado Pikeminnow Ptychocheilus lucius, 16 Bonytails Gila elegans, 20 Razorback Suckers Xyrauchen texanus, and five “undocumented” fish. Several endangered fishes were detected on multiple occasions and across years, often moving into and out of the San Rafael River from distances up to 360 km away (range, 6–360 km). Our findings demonstrate the use and the potential importance of small tributaries and their fragile habitats to endangered fishes