Humans have a rich history of introducing piscivorous non-native fishes to provide food subsidies and support recreational fishing opportunities in freshwater ecosystems throughout the world. Smallmouth bass (Micropterus dolomieu), a cool-water fish native to central North America, provide a compelling example of the potential ecosystem consequences associated with predatory fish introductions. Non-native bass have resulted in growth reductions of other native piscivores and induced changes in the behavior, distribution, and at times caused the local extirpation of prey fish species. In the Pacific Northwest U.S. range expansions of non-native bass, especially into salmon rearing grounds, is of increasing conservation concern, yet has received little scientific inquiry. We employed a spatially continuous sampling design to determine the extent of bass and subyearling Chinook salmon (Oncorhynchus tshawytscha) sympatry in the North Fork John Day River (NFJDR), a tributary of the Columbia River that contains an upstream expanding population of non-native bass. Extensive surveys were conducted over two seasons and two years to determine seasonal and annual variation in bass and Chinook salmon overlap. A recursive partitioning algorithm was applied to determine the primary habitat variables that influence bass distributions at reach and channel unit scales. We also identified potential factors that limit the upstream extent of bass in the NFJDR.
Results/Conclusions
Our study revealed that bass seasonally occupy up to 22% of salmon rearing habitat in the upper NFJDR, and their primary period of sympatry with subyearling Chinook salmon was in the early summer. Where these species co-occurred, bass occupied 60-76% of channel units used by subyearling Chinook salmon in the early summer and 28-46% of the channel units Chinook salmon used in the late summer. Given these salmon were well within the gape limitation of bass, the upstream range expansion of bass could greatly reduce their potential rearing habitat. Our multi-scale analysis suggests bass were selecting habitat based on antecedent thermal history at a broad scale, and if satisfactory temperature conditions were met, mesoscale habitat features (i.e., channel unit depth and type) played an additional role in determining bass abundance. Habitat determinants of bass abundance were consistent across years, despite marked differences in the magnitude and timing of spring peak flows. The overriding influence of water temperature on bass suggests that managers may be able limit future range expansions into salmon rearing habitat by concentrating on restoration activities that mitigate climate or land-use related stream warming.