Anticipated Climate Warming Effects on Bull Trout Habitats and Populations Across the Interior Columbia River Basin

—A warming climate could profoundly affect the distribution and abundance of many fishes. Bull trout Salvelinus confluentus may be especially vulnerable to climate change given that spawning and early rearing are constrained by cold water temperatures creating a patchwork of natal headwater habitats across river networks. Because the size and connectivity of patches also appear to influence the persistence of local populations, climate warming could lead to increasing fragmentation of remaining habitats and accelerated decline of this species. We modeled the relationships between (1) the lower elevation limits of small bull trout and mean annual air temperature and (2) latitude and longitude across the species’ potential range within the interior Columbia River basin of the USA. We used our results to explore the implications of the climate warming expected in the next 50 or more years. We found a strong association between the lower elevation limits of bull trout distributions and longitude and latitude; this association was consistent with the patterns in mean annual air temperature. We concluded that climate does strongly influence regional and local bull trout distributions, and we estimated bull trout habitat response to a range of predicted climate warming effects. Warming over the range predicted could result in losses of 18–92% of thermally suitable natal habitat area and 27–99% of large (.10,000-ha) habitat patches, which suggests that population impacts may be disproportionate to the simple loss of habitat area. The predicted changes were not uniform across the species’ range, and some populations appear to face higher risks than others. These results could provide a foundation for regional prioritization in conservation management, although more detailed models are needed to prioritize actions at local scales. Distribution shifts in many species (Parmesan and Yohe 2003; Root et al. 2003) and environmental trends consistent with broad-scale warming (Mote et al. 2005a; Stewart et al. 2005; Westerling et al. 2006; Hamlet and Lettenmaier 2007) show that climate change is no longer an abstraction. Official statistics compiled by the Intergovernmental Panel on Climate Change (IPCC) suggest these trends were associated with a 0.68C warming during the 20th century (IPCC 2007). Predictions of future global climates suggest larger and faster changes, and current models project a minimum warming of 18C in mean annual or seasonal air temperatures over the next 50 years and possibly a 68C increase in 100 years (Boer et al. 1992; Kerr 1997; IPCC 2007). Similar scenarios hold for predictions downscaled to the Columbia River basin, where models project warming of 1–2.58C or more by 2050 (Leung et al. 2004; Mote et al. 2005b). A warming climate can have important effects on the regional distribution and local extent of habitats available to salmonids (Meisner 1990; Keleher and Rahel 1996; Nakano et al. 1996; Rahel et al. 1996) and other fishes (Shuter and Meisner 1992; Eaton and Scheller 1996) because local climates influence surface water (Stephan and Preud’homme 1993; Stoneman and Jones 1996; Mohseni and Stefan 1999) and groundwater temperatures (Meisner 1990; Shuter and Meisner 1992). For coldwater fishes near the southern margins * Corresponding author: [email protected] 1 Present address: Post Office Box 1541, Seeley Lake, Montana 59868, USA. Received February 8, 2007; accepted July 19, 2007 Published online November 5, 2007 1552 Transactions of the American Fisheries Society 136:1552–1565, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T07-028.1 [Article]