Abstract:Fishes of the family Salmonidae, including trout (Oncorhynchus), charr (Salvelinus), salmon (Oncorhynchus and Salmo), grayling (Thymallus) and whitefish (Prosopium), are expected to be particularly vulnerable to climate change because of their dependence on cold, clean water. Salmonids are among the most sought after fish by recreational anglers. In North America, their native range includes much of the continent from the Arctic Plains, along Pacific and Atlantic coasts, and throughout most mountainous regions (Behnke 2002). In Mexico, trout naturally occur in the mountainous regions of Baja California, and throughout the Sierra Madre Occidental as far south as the Rio Presidio and Rio Baluarte basins (Hendrickson et al. 2002). Brown trout (Salmo trutta) are native to Europe but have been broadly introduced in North America. Additionally, rainbow trout (O. mykiss) and brook trout (Salvelinus fontinalis) and other salmonids that are native to North America have been widely introduced into lakes, reservoirs, and river systems outside of their native ranges to increase angling opportunities.Climate change is likely to continue affecting salmonids throughout their ranges. Increasing air temperatures have been warming stream and lake temperatures (Schneider and Hook 2010; Asaak et al. 2012) with impacts ranging from growing stress and metabolic rates to loss of lower elevation habitats as waters warm (Eby et al. 2014; Keefer and Caudill 2015). Warmer conditions will also impact salmonids through changes in winter precipitation and altered flow regimes (Haak et al. 2010). Disturbance events, such as wildfires, floods, and drought, are likely to increase as well (Westerling et al. 2006) with resulting stream sedimentation (Goode et al. 2012). Many existing stressors for salmonids are likely to be made worse by climate change (Williams et al. 2015). For instance, non-native fishes, which now prey on and compete with native salmonids, are likely to increase in numbers and distributions as climate changes (Rahel and Olden 2008; Lawrence et al. 2014). The synergies that emerge from the combined effects of these stressors will be hard to predict with accuracy but are likely to magnify the negative consequences of climate change for coldwater fishes in North America. The range of climate change impacts will not be equally harmful across all salmonid species. Although, all salmonids tend to be dependent on cold, clean water supplies, some species, such as bull trout (Salvelinus confluentus), Arctic grayling (Thymallus arcticus), and Dolly Varden (S. malma), are particularly sensitive to increasing temperatures and sedimentation (Selong et al. 2001; Jones et al. 2013). Changes in winter precipitation from snow to rain may impact fall-spawning species such as brook trout or brown trout to a greater degree than spring-spawning trout because of increased scouring of their egg beds (Wenger et al. 2011; Goode et al. 2013). Other species, such as California golden trout (O. aquabonita) and Lahontan cutthroat trout (O. clarkii henshawi), may occur in regions that are in the midst of sustained drought and particularly vulnerable to loss because of increasing isolation and small population size. Despite our understanding of climate-driven impacts and known sensitivity of salmonids to warming conditions, predictions of future ecological conditions are complicated by the interactions among climate, biological, and geological processes. None of these factors act in isolation. The degree that warming and changes in disturbances impact particular habitats and species depends on the resilience of the habitat or species in question, including the interactions of biological, geomorphic and hydrologic systems. Impacts from climate change are likely to be more severe where stream and lake conditions are degraded or fragmented and less severe where habitats are robust and interconnected (Rieman and Isaak 2010). Unfortunately, many habitats of native salmonids have a legacy of pollution and fragmentation caused by dams, water diversions, agricultural runoff, and roads. The majority of native trout and charr species and subspecies occupy less than 25% of their historical habitat (Trout Unlimited 2015). The purposes of this paper are to 1) review existing and likely future climate change impacts to salmonids in North America, 2) provide a primary bibliography for these impacts, and 3) describe how restoration can help trout adapt to climate change. The reader should keep in mind that the conservation status of most native salmonids already has declined as a result of the legacy of agricultural development, hydropower development, and the introduction of non-native species (Behnke 2002; Trout Unlimited 2015). Some taxa already are classified as vulnerable, threatened, or endangered by state, provincial, and federal agencies. Furthermore, as occupied habitat becomes increasingly fragmented and isolated, risks to climate-driven disturbances increase as well. Conservation efforts such as building artificial barriers to protect native trout from upstream invasions of non-native trout and warmwater fishes may result in further vulnerability to climate change because of range restrictions. Thus it is important to view increasing risk not just from the perspective of one or two factors but from the full variety of impacts that may accumulate over space and time.
Submitted by Jack Williams in July 2015 to Elsevier as part of their Science Direct Reference Modules in Earth Systems and Environmental Sciences