Open-File Report 2010-1236
Warming during the 20th century drove a series of environmental trends that have profound implications for many aspects of salmonid habitat including disturbance regimes, such as wildfire, and unfavorable changes to thermal and hydrologic properties of aquatic systems. As dramatic and extensive as climatic and environmental trends are for salmonid habitats, global climate models (GCMs) project that many of these trends will continue and even accelerate until at least the middle of the 21st century. Clearly, managers of native salmonids in the western United States should consider adjusting management strategies to accommodate a warmer and possibly drier future. Tools are needed to forecast where important changes may occur and how conservation efforts should be prioritized. Our study examined the influence of changing climate on the persistence of native trout and grayling within 11 western States. The study area ranged from the crests of the Cascades and Sierra Nevada eastward through the Rocky Mountains, Great Basin, and Southwest Deserts within the western United States. The focal taxa were westslope cutthroat trout (Oncorhynchus clarkii lewisi), Yellowstone cutthroat trout (O. clarkii bouvieri), Lahontan cutthroat trout (O. clarkii henshawi), Bonneville cutthroat trout (O. clarkii utah), Colorado River cutthroat trout (O. clarkii pleuriticus), greenback cutthroat trout (O. clarkii stomias), Rio Grande cutthroat trout (O. clarkii virginalis), Apache trout (O. gilae apache), Gila trout (O. gilae gilae), and Montana Arctic grayling (Thymallus arcticus). All of the species and subspecies that were examined in this study have a high risk in some portion of their range for one or more of the factors analyzed. Drought is the most pervasive threat with 40 percent or more of the historic range for seven taxa at a high risk. Bonneville cutthroat trout are the most vulnerable to drought (>70 percent of the historic range at a high risk). Westslope cutthroat trout are the least likely to be affected by drought, and only 16 percent of its historically occupied subwatersheds is at a high risk. However, much of the area at a high risk lies in the Missouri River Basin where the distribution of westslope cutthroat trout has already contracted significantly and 85 percent of the remaining populations do not meet persistence criteria. A large number of populations within the taxa examined did not meet persistence criteria, particularly in the central and southern extent of the study. Current management and restoration strategies have often focused on the occupied habitat on Federal lands or within conservation easements on private lands. Many of the target populations are small (<500 individuals) and located at higher elevations above barriers to upstream movement; therefore, there is limited opportunity for large-scale movements within habitat patches or for dispersal among patches. Where conditions are limiting, populations may have little opportunity for long-term persistence. Climate change stressors commonly are synergistic with existing ecosystem stressors. That is, the existing integrity of stream channels, riparian habitats, and floodplains may moderate or exacerbate negative effects of climate. If habitats are degraded by existing stressors, negative consequences of climate change may be underestimated by our analysis. On the other hand, more intact watersheds where riparian areas are properly functioning and rivers have access to natural floodplains will be more resilient to disturbances associated with climate change. In spite of these limitations, our results do provide a valuable overview of regional effects and risks to coldwater fish as a result of climate change. The broad-scale perspective can help guide managers and interested stakeholders in developing a strategic conservation framework that will increase resilience in native trout populations and improve resistance to the environmental changes brought on by a warming planet.
First posted October 1, 2010
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Haak, A.L., Williams, J.E., Isaak, D., Todd, A., Muhlfeld, C., Kershner, J.L., Gresswell, R., Hostetler, S., and Neville, H.M., 2010, The potential influence of changing climate on the persistence of salmonids of the inland west: U.S. Geological Survey Open-File Report 2010–1236, 74 p.