Ice age fish in a warming world: minimal variation in thermal acclimation capacity among lake trout (Salvelinus namaycush) populations

Nicholas I. Kelly 2 Gary Burness 1 Jenni L. McDermid 0 Chris C. Wilson 3 Steven Cooke 0 Wildlife Conservation Society Canada, Trent University , Peterborough, ON , Canada K9J 7B8 1 Department of Biology, Trent University , 2140 East Bank Drive, Peterborough, ON , Canada K9J 7B8 2 Environmental and Life Sciences Graduate Program, Trent University , Peterborough, ON , Canada K9J 5G7 3 Ontario Ministry of Natural Resources, Trent University , Peterborough, ON , Canada K9J 8M5 In the face of climate change, the persistence of cold-adapted species will depend on their adaptive capacity for physiological traits within and among populations. The lake trout (Salvelinus namaycush) is a cold-adapted salmonid and a relict from the last ice age that is well suited as a model species for studying the predicted effects of climate change on coldwater fishes. We investigated the thermal acclimation capacity of upper temperature resistance and metabolism of lake trout from four populations across four acclimation temperatures. Individuals were reared from egg fertilization onward in a common environment and, at 2 years of age, were acclimated to 8, 11, 15 or 19C. Although one population had a slightly higher maximal metabolic rate (MMR), higher metabolic scope for activity and faster metabolic recovery across all temperatures, there was no interpopulation variation for critical thermal maximum (CTM) or routine metabolic rate (RMR) or for the thermal acclimation capacity of CTM, RMR, MMR or metabolic scope. Across the four acclimation temperatures, there was a 3C maximal increase in CTM and 3-fold increase in RMR for all populations. Above 15C, a decline in MMR and increase in RMR resulted in sharply reduced metabolic scope for all populations acclimated at 19C. Together, these data suggest there is limited variation among lake trout populations in thermal physiology or capacity for thermal acclimatization, and that climate change may impact lake trout populations in a similar manner across a wide geographical range. Understanding the effect of elevated temperatures on the thermal physiology of this economically and ecologically important cold-adapted species will help inform management and conservation strategies for the long-term sustainability of lake trout populations. Introduction Global climate change is predicted to impact ecosystems significantly over the next century (Magnuson et al., 1997; Schindler, 1997; Prtner, 2002; Brander, 2010), with expected implications for species and populations (Walther et al., 2002; Parmesan, 2006; Eliason et al., 2011; Pauls et al., 2013). Climate models project a global increase in average atmospheric temperature by 3.54.2C over the next 50 years (IPCC, 2007). A change in both the average temperature and The Author 2014. Published by Oxford University Press and the Society for Experimental Biology. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), whichpermits unrestricted distribution and reproduction in any medium, provided the original work is properly cited. the thermal heterogeneity of terrestrial and aquatic environments will expose many populations to suboptimal conditions. Predicted effects include changes in the geographical distribution of species (Perry et al., 2005; Pinsky et al., 2013), alterations to phenological processes (Bradshaw and Holzapfel, 2006; Shuter et al., 2012) and species interactions (Tylianakis et al., 2008). Overall, this may potentially result in the extinction or extirpation of many terrestrial, marine and aquatic species over the next century (Thomas et al., 2004; Xenopoulos et al., 2005; Somero, 2010). Freshwater ecosystems are considered to be particularly vulnerable to climate change (Magnuson et al., 1997; Schindler, 1997; Ficke et al., 2007; McCullough et al., 2009). A chronic increase in atmospheric temperature is predicted to impact the thermal properties of freshwater lakes and their resident biota, with elevated epilimnetic temperatures and increased magnitude and duration of thermal stratification reducing the availability of suitable thermal habitats for cold-adapted species (De Stasio et al., 1996; Stefan et al., 1998; Ficke et al., 2007). Many cold-adapted populations will be exposed to temperatures above their thermal optimum, which will increase energetic demands and drive selection on physiological traits to maximize performance at the new environmental temperatures (Stockwell et al., 2003; Somero, 2010; Hoffmann and Sgro, 2011). Consequently, coldwater species may become extirpated from much of their present range (Casselman, 2002; Chu et al., 2005). The persistence of cold-adapted species and populations may therefore be determined by their capacity to cope with or adapt to elevated temperatures, which may be constrained by limited genetic resources originating from finite ancestral populations in glacial refugia and post-colonization restrictions on local population sizes (Bernatchez and Wilson, 1998; Willi et al., 2006). In a changing environment, the ability to maintain performance over a range of environmental conditions determines the persistence of populations and species (Stillman, 2003; Hoffmann and Sgro, 2011). Phenotypic plasticity (e.g. acclimatization) allows individuals to adjust physiological performance over a range of environmental conditions, which can enhance fitness in an unstable environment (Wilson and Franklin, 2002; Somero, 2010). For many species, the capacity of local populations to buffer the negative effects of temperature change through thermal acclimatization will determine their persistence over the longer time periods required for evolutionary adaptation to changing climatic conditions (Stillman, 2003; Calosi et al., 2008; Somero, 2010, 2011; Seebacher et al., 2012). Relatively few studies have assessed variation in thermal acclimatization capacity among intraspecific populations, but the limited evidence suggests that thermal acclimatization for physiological traits may vary among conspecific populations (Lucassen et al., 2006; Sylvestre et al., 2007; Seebacher et al., 2012). Understanding the degree of variation within and among populations for physiological traits, as well as the capacity of these traits for thermal acclimatization, is an important knowledge gap for identifying the potential impacts of climate change on cold-adapted species. Previous investigations of the degree of interpopulation variation in the thermal physiology and acclimatization capacity of salmonid species have yielded conflicting results. Intraspecific variation in thermal physiology has been reported among populations of sockeye salmon (O. nerka; Lee et al., 2003; Eliason et al., 2011), cutthroat trout (Oncorhynchus clarkia pleuriticus; Underwood et al., 2012) and brook trout (Salvelinus fontinalis; McDermid et al., 2012). In contrast, multiple studies suggest that the th (...truncated)