Effects of A Severe Drought On Summer Abundance, Growth, and Movement of Cutthroat Trout In A Western Oregon Headwater Stream

Winter snowpack depth, snowmelt timing, and snowmelt duration are projected to change in the future, leading to increased frequency and severity of drought in the Pacific Northwest. In summer 2015, stream flows throughout the Pacific Northwest were at record low levels because of low winter snowpack conditions consistent with these climate projections. We explored effects of the 2015 low-snowpack-associated drought on Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii) abundance, growth, and movement patterns in two 100m reaches (350 y old forests) of an unnamed perennial western Oregon headwater stream before (2 y) and during a severe drought. We found that the abundance of Cutthroat Trout declined substantially during the drought year, regardless of habitat availability, riparian forest age, or stream wood structure. Fish growth during summer was consistently negative during all 3 y of the study in both reaches. During the drought year, estimated abundance and total biomass of Cutthroat Trout declined in both reaches compared to the 2y prior. In all 3 y, the majority (76%) of fish in the reach with a young riparian forest stand moved >2 m from their release point. In contrast, across the 3 study years, only 26% of fish on average moved >2 m from their release point in the old-growth reach, which had more large wood and pool area. Across both reaches, in the non-drought years, most fish moved into pools (32.4%), but some moved to riffles (23.3%). During the drought year, of the fish that were recaptured, only upstream movement to pools were observed. There were no observed movements of recaptured fish to riffles. Overall, study results suggest that increasing severity of summer drought in the Pacific Northwest is likely to reduce the abundance of fish in small headwater streams, and the remaining fish preferentially use pool habitats such as those found in structurally complex streams.
Key words: Cutthroat Trout, drought, fish movement, growth, headwater stream, Oncorhynchus clarkii clarkii, Oregon