Life at the Upstream Edge: Contemporary Controls and Future Predictions of Non-Native Smallmouth Bass Range Expansion in Pacific Northwest Rivers

The trifecta of climate change, land-use, and non-native species will have a fundamental influence on the persistence of aquatic species of conservation concern globally.  Using a combination of intensive field work and modeling, we developed a framework to assess the interactive effects of (1) climate-induced stream warming, (2) riparian land-use management, and (3) introduced smallmouth bass, on stream-rearing Chinook salmon in rivers the Pacific Northwest.  Smallmouth bass is a non-native predator that seasonally overlaps with Chinook salmon in their juvenile rearing habitats.  By linking downscaled climate projections, a fine-scale stream temperature model, and a field-derived fish-habitat model, we predict that bass will greatly expand their range upstream as salmon-bearing streams warm.  This is cause for concern given the known negative effects of predatory bass on native fish communities.  Our simulations of riparian restoration demonstrate that stream shading can reduce climate-induced stream warming, and therefore diminish the ability of bass to move into headwater Chinook rearing areas.  Additionally, we found a significant reduction in bass range expansion for a given level of investment when restoration was prioritized (versus opportunistic) based on a spatially explicit model of a river segments’ capacity to reduce thermal loading through re-vegetation.