Westslope cutthroat trout (Oncorhynchus clarkii lewisi)

Cutthroat trout spawn in the spring and are thus sensitive to the timing and magnitude of snowmelt and the accompanying flood pulse. Winter floods do not pose a risk to cutthroat embryos, but it is possible that increased severity of fall and winter floods could negatively affect overwintering juveniles (although quality data on this topic are lacking due to the challenge of monitoring survival in flood prone systems). Like many stream rearing salmonids, cutthroat can be vulnerable to sub-optimally warm temperatures during base flow periods in late summer and fall. During these low flow periods, terrestrial subsidies typically comprise the dominant food source for trout, and may be critical for enabling fish to offset the elevated metabolic costs caused by higher water temperatures. Factors that mediate the magnitude of terrestrial subsidies, such as land use practices in riparian areas, can in turn mediate the sensitivity of trout to altered thermal regimes. Recruitment of cutthroat trout in high elevation streams may be constrained by cold, rather than warm, summer temperatures. Warming may have some positive effects by increasing the amount of high-elevation habitat capable of rearing juvenile cutthroat. The primary source of decline for westslope cutthroat has been hybridization with rainbow trout. A key uncertainty is how climate conditions might facilitate hybridization. Genetically pure cutthroat often exist in cold tributary streams and cutthroat show subtle signs of being better adapted to cold temperatures than rainbow trout when studied in the laboratory. This suggests warming temperatures could increase hybridization by allowing rainbow trout to invade cold headwater streams. However, in an analysis across a large watershed, environmental factors were not as important as demographic factors in determining levels of hybridization. Westslope cutthroat are unique among the cutthroat subspecies in that they exhibit an anadromous, coastal-roaming ecotype. Populations with this life-history present may be less sensitive to altered flow and thermal regimes in freshwater because there is less cumulative exposure to freshwater conditions and individuals at sea can survive ephemeral climate-related disturbance such as thermal stress events or periods of low flow. Westslope cutthroat trout exihibit adfluvial (live in lakes and spawn in tributaries), fluvial (live in rivers and spawn in tributaries) and resident life-history forms, and their use of lakes and larger rivers may provide them with some resilience to higher temperatures and decreasing flows that may occur in smaller tributaries in summer months