Global Warming of Salmon and Trout Rivers in the Northwestern U.S.: Road to Ruin or Path Through Purgatory?

Large rivers constitute small portions of drainage networks but provide important migratory habitats and fisheries for salmon and trout when and where temperatures are sufficiently cold. Management and conservation of coldwater fishes in the current era of rapid climate change require knowing how riverine thermal environments are evolving and the potential for detrimental biological impacts. Robust estimates of warming rates, however, are lacking due to limited long-term temperature monitoring, so we compiled the best available multidecadal records and estimated trends at 391 sites in the 56,500-km river network of the northwestern USA. Warming trends were prevalent during summer and early fall months in recent 20and 40-year periods (0.18–0.35°C per decade during 1996–2015 and 0.14–0.27°C per decade during 1976–2015), paralleled air temperature trends, and were mediated by discharge trends at regional and local levels. To illustrate the biological consequences of warming later in this century, trend estimates were used to inform selection of river temperature scenarios and assess changes in thermal exposure of adult Sockeye Salmon Oncorhynchus nerka migrating to four population areas as well as thermal habitat shifts for resident Brown Trout Salmo trutta and Rainbow Trout O. mykiss populations throughout the region. Future warming of 1–3°C would increase Sockeye Salmon exposure by 5–16% (3–143 degree-days) and reduce thermally suitable riverine trout habitats by 8–31% while causing their upstream shift. Effects of those changes on population persistence and fisheries are likely to be context dependent, and strategic habitat restoration or adaptation strategies could ameliorate some biological impairments, but effectiveness will be tempered by the size of rivers, high costs, and pervasiveness of thermal effects. Most salmon and trout rivers will continue to provide suitable habitats for the foreseeable future, but it also appears inevitable that some river reaches will gradually become too warm to provide traditional habitats. Salmon and trout have been cultural icons and important food resources for millennia, are the subjects of multi-billion-dollar global commercial and recreational fisheries, and are the focus of similar investments for conservation and habitat restoration efforts where populations have declined (Mills 1989; Montgomery 2003; Rieman et al. 2015). Given their requirement for cold water (Jonsson and Jonsson 2009; Isaak et al. 2017a), the specter of global warming has spawned concerns about the vulnerability of some populations, beginning with the earliest work on climate change and aquatic environments (Meisner et al. 1988; Eaton and Scheller 1996; Keleher and Rahel 1996). Those fears are not unfounded, as climaterelated environmental trends are linked to shifting distributions and range contractions (Comte and Grenouillet 2013; Eby et al. 2014; Al-Chokhachy et al. 2016), adjustments in phenology (Wedekind and Kung 2010; Martins et al. 2012), and evolutionary change (Crozier et al. 2011; Kovach et al. 2012). Documented extirpations of local populations, however, remain exceedingly rare despite the frequency of this prediction in the literature and several decades of rapid, accumulated climate change. In mountain headwater environments, populations are buffered by slow climate velocities and the presence of cold refuge *Corresponding author: disaak@fs.fed. us Received December 4, 2017; accepted March 14, 2018 Transactions of the American Fisheries Society 147:566–587, 2018 Published 2018. This article is a U.S. Government work and is in the public domain in the USA ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1002/tafs.10059