ARTICLE Are brown trout replacing or displacing bull trout populations in a changing climate? Robert Al-Chokhachy, David Schmetterling, Chris Clancy, Pat Saffel, Ryan Kovach, Leslie Nyce, Brad Liermann, Wade Fredenberg, and Ron Pierce Abstract: Understanding how climate change may facilitate species turnover is an important step in identifying potential conservation strategies. We used data from 33 sites in western Montana to quantify climate associations with native bull trout (Salvelinus confluentus) and non-native brown trout (Salmo trutta) abundance and population growth rates (). We estimated  using exponential growth state-space models and delineated study sites based on bull trout use for either spawning and rearing (SR) or foraging, migrating, and overwintering (FMO) habitat. Bull trout abundance was negatively associated with mean August stream temperatures within SR habitat (r = −0.75). Brown trout abundance was generally highest at temperatures between 12 and 14 °C. We found bull trout  were generally stable at sites with mean August temperature below 10 °C but significantly decreasing, rare, or extirpated at 58% of the sites with temperatures exceeding 10 °C. Brown trout  were highest in SR and sites with temperatures exceeding 12 °C. Declining bull trout  at sites where brown trout were absent suggest brown trout are likely replacing bull trout in a warming climate. Résumé : Il importe de comprendre comment le climat pourrait faciliter le renouvellement des espèces pour cerner des stratégies de conservation potentielles. Nous avons utilisé des données de 33 sites de l’ouest du Montana pour quantifier les associations climatiques avec l’abondance et les taux de croissance de populations () d’ombles a ` tête plate (Salvelinus confluentus) indigènes et de truites brunes (Salmo trutta) non indigènes. Nous avons estimé  en utilisant des modèles d’espaces d’états de croissance exponentielle et délimité les sites étudiés selon leur utilisation par l’omble a ` tête plate soit comme habitat de frai et d’alevinage (SR) ou d’approvisionnement, de migration et d’hivernage (FMO). L’abondance des ombles a ` tête plate était néga- tivement associée aux températures moyennes des cours d’eau en août dans les habitats SR (r = −0,75). L’abondance de la truite brune était généralement maximum a ` des températures entre 12 et 14 °C. Nous avons constaté que les  des ombles a ` tête plate étaient généralement stables aux sites présentant une température moyenne en août inférieure a ` 10 °C, mais qu’il diminuait significativement, l’espèce y étant rare ou disparue, dans 58 % des sites où cette température dépasse 10 °C. Les  des truites brunes étaient maximums dans les habitats SR et les sites caractérisés par des températures supérieures a ` 12 °C. Des  en baisse des ombles a ` tête plate dans des sites exempts de truites brunes donnent a ` penser que ces dernières remplacent probablement les ombles a ` tête plate dans un climat en réchauffement. [Traduit par la Rédaction] Introduction Climate change is likely to substantially alter stream ecosystems with pronounced effects for cold-water fishes such as salmonids (Jonsson and Jonsson 2009; Williams et al. 2009; Elliott and Elliott 2010). Salmonid life histories, vital rates, and demographics are strongly tied to factors influenced by climate, including thermal and hydrologic regimes (Elliott 1994; Lobon-Cervia 2004; Crozier et al. 2008; Warren et al. 2012). With different thermal tolerances and life-history expressions, the effects of changing climatic con- ditions are likely to differ across species. Understanding how cli- mate, among other factors, may be influencing populations is critical for identifying the potential for effective management and restoration scenarios. Non-native salmonids are an additional concern for the conser- vation of native salmonids across North America (Dunham et al. 2002). Widespread introductions for recreation have resulted in naturally producing populations of non-native salmonids in many streams. The mechanistic threats of non-native species to native salmonids, particularly in the context of climate change, are often not well understood (e.g., Rahel and Olden 2008; Lawrence et al. 2014). This uncertainty stems partly from the paucity of situations where changing climatic conditions have been empirically linked with salmonid population and demographic data (Kovach et al. 2016). Likewise, delineating between non-native displacement (i.e., declines in native salmonids due to negative interactions with non-natives) or replacement (i.e., declines in native salmonids due to factors unrelated to non-natives) is an inherent challenge in species turnover studies (Dunham et al. 2002). Refining our understanding of the influences of climate change and non-native species on extant populations of bull trout (Salvelinus confluentus) is essential in designing conservation strategies to en- hance long-term persistence. Bull trout are currently listed as “Threat- ened” in the United States under the Endangered Species Act and ranked “Of Special Concern” or “Threatened” for three of four geo- graphic populations by the Committee on the Status of Endangered Wildlife in Canada. Bull trout are extremely temperature-sensitive Received 10 June 2015. Accepted 31 December 2015. R. Al-Chokhachy. US Geological Survey, Northern Rocky Mountain Science Center, 2327 University Way, Suite 2, Bozeman, MT 59715, USA. D. Schmetterling, P. Saffel, B. Liermann, and R. Pierce. Montana Fish, Wildlife and Parks, 3201 Spurgin Road, Missoula, MO 59804, USA. C. Clancy and L. Nyce. Montana Fish, Wildlife and Parks, 1801 North 1st Street, Hamilton, MO 59840, USA. R. Kovach. US Geological Survey, Northern Rocky Mountain Science Center, Glacier Field Station, West Glacier, MT 59936, USA. W. Fredenberg. US Fish and Wildlife Service, Creston Fish & Wildlife Center, Kalispell, MT 59901, USA. Corresponding author: Robert Al-Chokhachy (email: ral-chokhachy@usgs.gov). Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. Pagination not final (cite DOI) / Pagination provisoire (citer le DOI) 1 Can. J. Fish. Aquat. Sci. 73: 1–10 (2016) dx.doi.org/10.1139/cjfas-2015-0293 Published at www.nrcresearchpress.com/cjfas on 23 February 2016. Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by USGS LIBRARY on 06/03/16 For personal use only.