Spatial and Temporal Variation of Whirling Disease Risk in Montana Spring Creeks and Rivers

Abstract

Spring creeks are important spawning and rearing areas for wild trout, but the stable flows, cool temperatures, and high nutrient levels that characterize these unique habitats may also make them highly susceptible to establishment and proliferation of the whirling disease pathogen Myxobolus cerebralis. We evaluated the spatial and temporal dynamics in whirling disease risk by using sentinel rainbow trout Oncorhynchus mykiss fry in nine different spring creeks and their conjoining rivers or reservoirs in Montana over a 20-month period. Whirling disease risk was high in five of the seven pathogen-positive spring creek study sites; at these sites, prevalence levels exceeded 90% and over 50% of sentinel fry had moderate to high infection severity scores. Spring creeks generally had higher disease prevalence and severity than paired river or reservoir sites. Fine sediment levels varied widely among springs creeks with high and low whirling disease risk, and we found no significant association between fine sediment level and infection severity. The low risk measured for some spring creeks was likely attributable to the pathogen invasion being in its early stages rather than to environmental characteristics limiting the severity of infection. High whirling disease risk occurred over a wide range of temperatures at spring creek sites (4.5–13°C) and river sites (1.7–12.5°C). There was an unusual seasonal cycle of infection in spring creeks, with peak infection levels occurring from late fall to early spring and declining to near zero in late spring to early fall. The low infection risk during spring suggests that spring-spawning trout would be at a low risk of infection, even in spring creeks with otherwise high disease severity. In contrast, fry of fall-spawning trout may be much more susceptible to infection in spring creek environments.

Received November 22, 2011; accepted May 7, 2012

ACKNOWLEDGMENTS

Funding for this study was provided by the Whirling Disease Foundation (WDF) and Montana Fish, Wildlife, and Parks. Sincere thanks are extended to Dave Kumlein, Susan Higgins, Wanda McCarthy, and Leon Hirsch for technical and financial support from the WDF grant. We also thank the many landowners who graciously allowed access to their properties. We acknowledge the following individuals for laboratory and field assistance: Ron Aasheim, Jeff Bagdanov, Jim Bowker, Dan Carty, Chris Clancy, Eve Davey, Dan Downing, Bonnie Elliot, Cal Fraser, Grant Grisak, Ray Heagney, Janet Hess-Herbert, Jody Hupka, Rob Jakubowski, Matt Jaeger, Rebecca Krueger, Jennie Miles, Andrew Munro, Lee Nelson, Ron Pierce, Molly Quinn, Sandy Pigeon, David Schmetterling, Ron Spoon, Linda Staton, Matt Toner, and Scott Opitz. We are especially grateful to Beth MacConnell and Eileen Ryce for invaluable suggestions and assistance. Dave Erdahl, Billie Kerans, Beth MacConnell, Al Zale, and three anonymous reviewers provided helpful comments on earlier drafts of the manuscript.