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.

Una Cuantificación Basada en la Ocupación del Estado Altamente en Peligro de los Peces del Desierto del Suroeste de los Estados Unidos Resumen Los peces del desierto son unos de los vertebrados en mayor peligro a nivel mundial debido a su bajo valor económico y a que compiten por el agua con los humanos. Un complejo ecológico de peces, dos miembros de la familia Catostomidae (Catostomus latipinnis, C. discobolus) y un bagre (Gila robusta) (todos manejados colectivamente como las llamadas tres especies), endémico a la cuenca estadunidense del Río Colorado, está afectado por múltiples factores estresantes y supuestamente ha declinado dramáticamente. Construimos una serie de modelos de ocupación para determinar las relaciones entre las tendencias de ocupación, la extinción local y las tasas de colonización, para identificar los factores limitantes potenciales y para evaluar la idoneidad del manejo colectivo de las tres especies. Durante un periodo histórico (1889 - 2011), los modelos con el mejor desempeño (AICc) incluyeron una tendencia positiva de tiempo en la probabilidad de extinción local y una tendencia negativa en la probabilidad de colonización local. La probabilidad de extinción local incrementó conforme disminuyó la frecuencia de inundaciones pos-desarrollo. Al final de la serie de tiempo, el 47% (95% IC 34 - 61) y el 15% (95% IC 6 - 33) de los sitios permanecieron ocupados por los catostómidos y el bagre, respectivamente, y los modelos con las dos especies de catostómidos como un grupo y el bagre como otro tuvieron el mejor desempeño. Para un periodo contemporáneo (2001 - 2011), los modelos con el mejor desempeño (basados en AICc) incluyeron a la mayor descarga anual. La probabilidad de extinción local disminuyó y la probabilidad de colonización local incrementó conforme incrementó la mayor descarga. Para el periodo contemporáneo, los resultados de los modelos que dividieron a las tres especies en grupos separados fueron similares a los resultados de los modelos que combinaron a los dos catostómidos pero no al bagre. Colectivamente, estos resultados confirmaron que las declinaciones de estos peces estuvieron fuertemente asociadas con el desarrollo del agua y que, en relación a su distribución histórica, las tres especies han declinado dramáticamente. Además, el bagre se distinguió por declinar más dramáticamente y por lo tanto tal vez requiera de manejo por separado. Nuestra estrategia de modelado puede ser útil en otras situaciones en las que los datos de importancia son escasos y el estado de conservación y la mejor estrategia de manejo para múltiples especies son inciertos.

Unintended consequences of restoration: loss of riffles and gravel substrates following weir installation.

We used pre- and post-restoration channel surveys of the Donner und Blitzen River, Oregon, to evaluate the effects of grade-control structures on channel morphology and baseflow habitat conditions for native redband trout and other aquatic biota. Six years after installation, we found that the channel had a smaller proportion of riffles and pools and less gravel substrate, combined with an increase in the proportion of flat waters and consolidated clay on the bed surface. Both local scour downstream from weirs and backwater effects upstream from weirs appear to have caused the general flattening and fining of the channel. A direct-step backwater calculation indicates that backwaters extended to the upstream weir at both low and high flows, creating long sections of flat water separated by short, steep drops. Despite backwater effects, a comparison of longitudinal profiles before and six years after weir installation showed bed erosion downstream of nearly all weirs, likely a consequence of the cohesive clay material that dominates the channel bed and banks. A deep inner channel reflects the cohesive nature of the clay and the mechanisms of abrasion, and indicates that sediment load is low relative to the transport capacity of the flow. Unfortunately, weirs were problematic in this system because of the cohesive clay substrate, limited sediment supply, and low channel gradient. Although deeper flows due to backwaters might be more favorable for resident trout, less gravel and fewer riffles are likely to negatively impact trout spawning habitat, macroinvertebrate communities, and biofilm productivity. Our results demonstrate the potential limitations of a single-feature approach to restoration that may be ineffective for a given geomorphic context and may overlook other aspects of the ecosystem. We highlight the need to incorporate geomorphic characteristics of a system into project design and predictions of system response.