Hot on the Heels of Chinook Salmon: Climate Change and Smallmouth Bass Invasions in Salmon Rearing Habitat

Smallmouth bass (Micropterus dolomieu) have been widely introduced throughout the Pacific Northwest and the potential for climate-related range expansions of bass, especially into juvenile salmon rearing grounds, is of increasing conservation concern.  Prior studies examining the impact of smallmouth bass on salmonids in the Pacific Northwest have largely focused on predation on salmon smolts as they move through the hydropower corridor of the Columbia River.  Little is known about the potential sympatry between bass and salmon that rear in rivers, such as spring Chinook salmon (Oncorhynchus tshawytscha).  The objective of this work was to characterize the extent and physical determinants (e.g., temperature, habitat, flow) of sympatry between smallmouth bass and juvenile spring Chinook salmon in the John Day River, a tributary to the Columbia River where both species occur.  We conducted extensive spatially continuous surveys (>50 km) of smallmouth bass nests and adults, juvenile Chinook salmon, and habitat in the John Day River over two seasons (June, August) and two years (2009, 2010).  The survey extent bounded the upstream extent of smallmouth bass in the system, and the typical downstream extent of rearing Chinook salmon.  Smallmouth bass were sympatric with rearing juvenile Chinook over a broader range in the early summer compared to the late summer (23 river km of overlap in June, 6 river km in August).  In June, the upstream extent of smallmouth bass spawning distribution corresponded well with adult bass distribution, and appears to be limited by minimum spawning temperature (~13° C).  By August bass had moved extensively upstream as water temperatures warmed, indicating their invasion front is temporally dynamic.  The downstream extent of juvenile Chinook salmon corresponded to their upper tolerance limit in the early summer (maximum temperature 19° C) and their upper lethal limit in the late summer (maximum temperature 25° C).  Our ultimate goal is to develop a bass-habitat model and link it to a fine scale stream temperature model to predict how climate-induced warming will alter the distributional range of smallmouth bass and the extent of overlap with rearing Chinook salmon within the John Day River system.