Drought impacts to trout and salamanders in cool, forested headwater ecosystems in the western Cascade Mountains, OR

Climate change projections for the western United States suggest that many regions will experience increasing frequency and severity of droughts. In summer 2015, the Pacific Northwest experienced a drought with early onset of stream low flows, reduced summer discharge, and elevated temperatures. We evaluated population responses of two dominant stream predators—coastal cutthroat trout (Oncorhynchus clarkii clarkii) and coastal giant salamander (Dicamptodon tenebrosus)—across nine Cascade Mountain streams. Drought conditions impacted both trout and salamanders, but their responses differed. Adult trout abundance and biomass were significantly lower in 2015 relative to 2014, a year with discharge and temperature closer to historical norms. Juvenile trout abundance did not differ between years but juveniles were significantly larger in 2015. Salamander abundance and biomass were not significantly different between years but body condition was lower in all nine streams in 2015. Differences in temperature among streams did not explain trout or salamander responses. Habitat was important for trout responses with trout abundance and biomass experiencing smaller declines in systems with more deep pool area. Despite notable short-term drought impacts to trout and salamanders in 2015, populations recovered to pre-drought conditions within two years in all but the smallest stream.
Keywords: Drought; Disturbance; Cutthroat trout; Coastal giant salamander; Climate change