Vulnerability of Salmon to Climate Change in Rivers of the North Pacific Rim

Climate change is predicted to increase stream temperatures and shift hydrological regimes throughout the range of Pacific salmon.  We assessed the vulnerability of Chinook, coho and steelhead based on exposure to predicted changes in river flows and temperatures for a suite of 1500 rivers in North American and Kamchatka.  We modeled flows and temperatures out to the year 2099 by downscaling climatic forcings from ensemble IPCC GCMs and using the variable infiltration capacity (VIC) model to generate runoff.  We then used a novel routing model to produce flows for the 1500 Pacific Rim rivers.  The hydrology was coupled to a thermal energy budget simulator to produce stream temperatures associated with modeled air temperatures and flows.  The calibrated model produced reasonable coherence between modeled and historical data, providing confidence in the future casting. We then evaluated shifts in temperature and flow stress (duration of exposure above ecologically-relevant thresholds) for life stages (migration, spawning, egg incubation and rearing) for each species in relation to predicted river temperatures (generally increased) and shifts in flow patterns (earlier or more flashy peaks), averaging effects across life stage for historical and future time periods. We used a cluster analysis to combine dimensions of vulnerability: modeled exposure to climatic stress (temperature and flow) and potential sensitivity to climate stress (catchment human footprint index and habitat production potential measures in each river).  Clusters were used to map the relative vulnerability of each species throughout the North Pacific Rim and to identify potential future salmon strongholds within the suite of modeled basins. The analysis showed serious vulnerability for rivers south of the Skeena in NA and a substantial shift northward in stronghold rivers, with the least impacted rivers in Kamchatka and western Alaska.  Rivers on the north slope of Alaska, currently not considered suitable for salmon, emerge as potentially important rivers in the future climate regime.