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Are Brown Trout Replacing Or Displacing Bull Trout Populations In Pagination not final (cite DOI) / Pagination provisoire (citer le DOI) 1 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 For personal use only. les ombles a` tête plate dans un climat en réchauffement. [Traduit par la Rédaction] Introduction salmonids, particularly in the context of climate change, are often Climate change is likely to substantially alter stream ecosystems not well understood (e.g., Rahel and Olden 2008; Lawrence et al. with pronounced effects for cold-water fishes such as salmonids 2014). This uncertainty stems partly from the paucity of situations (Jonsson and Jonsson 2009; Williams et al. 2009; Elliott and Elliott where changing climatic conditions have been empirically linked 2010). Salmonid life histories, vital rates, and demographics are with salmonid population and demographic data (Kovach et al. strongly tied to factors influenced by climate, including thermal 2016). Likewise, delineating between non-native displacement and hydrologic regimes (Elliott 1994; Lobon-Cervia 2004; Crozier (i.e., declines in native salmonids due to negative interactions with et al. 2008; Warren et al. 2012). With different thermal tolerances non-natives) or replacement (i.e., declines in native salmonids due and life-history expressions, the effects of changing climatic con- to factors unrelated to non-natives) is an inherent challenge in ditions are likely to differ across species. Understanding how cli- species turnover studies (Dunham et al. 2002). mate, among other factors, may be influencing populations is Refining our understanding of the influences of climate change critical for identifying the potential for effective management and non-native species on extant populations of bull trout (Salvelinus and restoration scenarios. confluentus) is essential in designing conservation strategies to en- hance long-term persistence. Bull trout are currently listed as “Threat- Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by USGS LIBRARY on 06/03/16 Non-native salmonids are an additional concern for the conser- vation of native salmonids across North America (Dunham et al. ened” in the United States under the Endangered Species Act and 2002). Widespread introductions for recreation have resulted in ranked “Of Special Concern” or “Threatened” for three of four geo- naturally producing populations of non-native salmonids in many graphic populations by the Committee on the Status of Endangered streams. The mechanistic threats of non-native species to native 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: [email protected]). Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. 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. Pagination not final (cite DOI) / Pagination provisoire (citer le DOI) 2 Can. J. Fish. Aquat. Sci. Vol. 73, 2016 Fig. 1. Major rivers, locations of the long-term monitoring sites (grey circles with numbers distinguishing sample sites; see Table 1), and stream gauges (triangles) in western Montana, USA. Inset denotes location of the study area relative to the western portion of the USA. For personal use only. relative to other salmonids (Selong et al. 2001), and distributions changing climatic conditions are associated with population trends during summer months are strongly tied to ambient stream temper- and abundance; and (iii) the putative threat brown trout may repre- atures (Dunham et al. 2003; Wenger et al. 2011). However, bull trout sent to extant bull trout populations. exhibit complex life histories, and adults and subadults can season- ally demonstrate large upstream and downstream movements to Materials and methods access foraging and overwintering habitat (Swanberg 1997; Howell et al. 2010; Starcevich et al. 2012). Study area Brook trout (Salvelinus fontinalis) and lake trout (Salvelinus namaycush) Our study area included 33 long-term sampling sites in the Clark have been identified as considerable threats to bull trout populations Fork, Bitterroot, and Blackfoot river drainages in western Mon- through predation (Martinez et al. 2009; Hansen et al. 2010; tana (Fig. 1). Lands within the study area include a mixture of Fredenberg 2014), competition (Guy et al. 2011; Warnock and mostly private lands in the valley bottoms and river corridors and Rasmussen 2014), and nonintrogressive hybridization resulting in state and federal ownership in the tributaries and headwaters. gametic wastage (Leary et al. 1993; DeHaan et al. 2010). Recently, Native vegetation within riparian zones varies elevationally across however, there has been growing concern regarding the effects of sites, but reflects montane forests commonly found in the region. introduced brown trout (Salmo trutta) on bull trout populations in Riparian communities commonly includes a mixture of grasses and Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by USGS LIBRARY on 06/03/16 western Montana, USA, particularly in light of recent evidence sug- sedges (Carex spp.), shrub-dominated taxa (e.g., Salix spp.), mixed gesting expansion of brown trout into historic bull trout habitat conifer (Pinaceae; e.g., Abies spp.), and poplars (Populus spp.). Cli- (USFWS 2015). Similar to bull trout, brown trout are a fall-spawning mate within the study area is characterized by relatively cold, wet species and considered a top-level predator within streams; however, winter and spring months and relatively warm, dry summers. bull trout have substantially lower thermal tolerances than brown Streamflows are typical for snowmelt-influenced streams in the trout (Elliott 2009) and, as a result, often occupy headwater streams upstream of the distribution of brown trout. northern Rocky Mountains, with high spring flow events during Recent observations of expansion in brown trout abundance May and June and declining flow throughout
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