Fragmentation and thermal risks from climate change interact to affect persistence of native trout in the Colorado River basin JAMES J. ROBERTS* † , KURT D. FAUSCH* † , DOUGLAS P. PETERSON ‡ and MEVIN B. HOOTEN* † § *Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, CO 80523, USA, †Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO 80523, USA, ‡Abernathy Fish Technology Center, US Fish and Wildlife Service, Longview, WA 98632, USA, §U.S. Geological Survey-Colorado Cooperative Fish and Wildlife Research Unit, Colorado State University, Fort Collins, CO 80523, USA Abstract Impending changes in climate will interact with other stressors to threaten aquatic ecosystems and their biota. Native Colorado River cutthroat trout (CRCT; Oncorhynchus clarkii pleuriticus) are now relegated to 309 isolated high- elevation (>1700 m) headwater stream fragments in the Upper Colorado River Basin, owing to past nonnative trout invasions and habitat loss. Predicted changes in climate (i.e., temperature and precipitation) and resulting changes in stochastic physical disturbances (i.e., wildfire, debris flow, and channel drying and freezing) could further threaten the remaining CRCT populations. We developed an empirical model to predict stream temperatures at the fragment scale from downscaled climate projections along with geomorphic and landscape variables. We coupled these spa- tially explicit predictions of stream temperature with a Bayesian Network (BN) model that integrates stochastic risks from fragmentation to project persistence of CRCT populations across the upper Colorado River basin to 2040 and 2080. Overall, none of the populations are at risk from acute mortality resulting from high temperatures during the warmest summer period. In contrast, only 37% of populations have a  90% chance of persistence for 70 years (simi- lar to the typical benchmark for conservation), primarily owing to fragmentation. Populations in short stream frag- ments <7 km long, and those at the lowest elevations, are at the highest risk of extirpation. Therefore, interactions of stochastic disturbances with fragmentation are projected to be greater threats than warming for CRCT populations. The reason for this paradox is that past nonnative trout invasions and habitat loss have restricted most CRCT popula- tions to high-elevation stream fragments that are buffered from the potential consequences of warming, but at risk of extirpation from stochastic events. The greatest conservation need is for management to increase fragment lengths to forestall these risks. Keywords: climate change, cutthroat trout, fragmentation, multiple stressors, native fish, stream temperature model, stream warming Received 25 June 2012 and accepted 28 November 2012 Introduction The changing climate worldwide has already influ- enced the distribution and persistence of biota, and this change is predicted to continue (Parmesan & Yohe, 2003; Root et al., 2003; Rosenzweig et al., 2008). Biota in aquatic ecosystems have been particularly vulnerable, because fishes and aquatic invertebrates are ectothermic and have species-specific temperature requirements that make them sensitive to changes in thermal regimes (Ficke et al., 2007; P€ ortner & Farrell, 2008). For example, substantial changes in tempera- tures owing to climate change have been reported for rivers and streams (Webb & Nobilis, 2007; Isaak et al., 2012a), and changes to these and other abiotic charac- teristics have already had strong consequences for many aquatic biota (Burgmer et al., 2007; Daufresne & Boet, 2007; Buisson & Grenouillet, 2009). Moreover, fish often will be unable to avoid rising temperatures because their movements are restricted to river net- works. Although temperature is a critical factor affecting aquatic biota in lotic ecosystems, other abiotic variables also alter habitat and shape life histories. Modeling and long-term measurements from western North America have revealed changes in precipitation patterns (Pag- ano & Garen, 2005) and, more importantly for fishes, the timing and magnitude of stream flows (Luce & Holden, 2009; Clow, 2010; Leppi et al., 2012). The Correspondence: James J. Roberts, tel. +970-226-9362, fax +970-226-9230, e-mail: jroberts@usgs.gov Present address: USGS-Fort Collins Science Center, 2150 Centre Ave. Bldg. C., Fort Collins, CO 80526-8118. © 2013 Blackwell Publishing Ltd 1 Global Change Biology (2013), doi: 10.1111/gcb.12136