Does Whirling Disease Mediate Hybridization between a Native and Nonnative Trout?

Abstract

The spread of nonnative species over the last century has profoundly altered freshwater ecosystems, resulting in novel species assemblages. Interactions between nonnative species may alter their impacts on native species, yet few studies have addressed multispecies interactions. The spread of whirling disease, caused by the nonnative parasite Myxobolus cerebralis, has generated declines in wild trout populations across western North America. Westslope Cutthroat Trout Oncorhynchus clarkii lewisi in the northern Rocky Mountains are threatened by hybridization with introduced Rainbow Trout O. mykiss. Rainbow Trout are more susceptible to whirling disease than Cutthroat Trout and may be more vulnerable due to differences in spawning location. We hypothesized that the presence of whirling disease in a stream would (1) reduce levels of introgressive hybridization at the site scale and (2) limit the size of the hybrid zone at the whole-stream scale. We measured levels of introgression and the spatial extent of hybridization between Rainbow Trout and Westslope Cutthroat Trout in four disease-positive streams and six disease-negative streams within the Blackfoot River basin of Montana. In addition to disease status, we considered habitat quality, stream slope, distance from the confluence, temperature, and elevation. Whirling disease presence was not associated with either the level of introgression at a site or the size of the hybrid zone. Temperature, elevation, and stream slope were all influential in determining levels of introgression at the site scale. Stream slope was the most influential factor determining the size of the hybrid zone, as longer, steeper streams contained smaller hybrid zones. Stream slope is a driver of many habitat characteristics that may provide refuge from invasive species in the coming decades. Although the multispecies interactions examined in this study did not alter the impacts of invasion on native species, community assemblages will continue to change with the spread of nonnative species, requiring continued assessment to determine their impacts on native species.

Received May 15, 2014; accepted January 7, 2015

ACKNOWLEDGMENTS

We thank W. Lowe, M. Schwartz, D. Patterson, and C. Muhlfeld for feedback on drafts of this work. We also acknowledge our funding sources for this study, including the National Science Foundation's Graduate Research Fellowship Program (DGE-0809127), the National Science Foundation's Integrative Graduate Education and Research Traineeship Program (DGE-0504628), the Westslope Chapter of Trout Unlimited, and the Winston Fly Rod Company. We are grateful to Craig Podner, Bradford Balis, and Julia Clymer for help in collecting samples and to Sally Painter, Angela Lodmell, and Taylor Wilcox for assistance with processing genetic samples in the laboratory. Finally, we thank the many private landowners of the Blackfoot Valley who allowed us to access many of our study sites through their properties.