Potential Dispersal of the Non-Native Parasite Myxobolus cerebralis in the Willamette River Basin, Oregon: A Qualitative Analysis of Risk

Abstract

Myxobolus cerebralis, the myxosporean parasite responsible for whirling disease in salmonids, was first detected in the United States in 1958. It has since spread across the country, causing severe declines in wild trout populations in the intermountain west. This study qualitatively assesses the risk of introduction and establishment of the pathogen into the Willamette River basin, Oregon, by examining potential routes of dissemination and relationships among obligate hosts, the parasite, and the environment. The approach is a synthesis of historical data, literature, and original research. The risk of M. cerebralis introduction in the Willamette River basin is addressed as a function of three main elements of dispersal: (1) movement of infected fish by humans, (2) natural dispersal (via migratory birds and stray anadromous salmonids), and (3) recreational activities. Establishment of the parasite is dependent upon several environmental and biological factors, including water temperatures, density, and spatial/temporal overlap of hosts, and the distribution and genetic composition of the oligochaete host, Tubifex tubifex. This study finds the probability of introduction of the parasite to vary throughout the Willamette River basin. Areas with greater probability have been identified as the Clackamas and Santiam River subbasins. If the pathogen were introduced, probability of establishment is high in certain areas of the basin as conditions are appropriate for propagation of the parasite lifecycle.

Keywords:

• Tubifex tubifex
• Myxobolus cerebralis
• whirling disease
• Willamette River
• Oregon
• stray salmonids
• risk assessment

ACKNOWLEDGMENTS

This work was supported in part by the National Partnership for the Management of Wild and Coldwater Species, the Whirling Disease Foundation, the United States Fish and Wildlife Service, the Oregon Department of Fish and Wildlife (ODFW), and the United States Geological Survey. We are grateful to the following members of the Oregon State University Center for Fish Disease Research: Sascha Hallett and Stephen Atkinson for critical consultation on oligochaete sampling and genetic assays, Richard Holt for assays of stray fish, Don Stevens for assistance with worm identification, Harriet Lorz for her efforts in sampling and maintaining oligochaete cultures, and Lindsay Osborne and Richard Stocking for help in the field. We also thank a number of ODFW personnel for their assistance: hatchery personnel for collecting stray fish, Ken Kenaston and crew for assistance in the field, Mark Lewis and Nadine Hurtado for help using agency databases, and Antonio Amandi for providing essential consultation on study design and background and for his helpful comments on this manuscript. Genetic work on stray steelhead was done by Paul Moran and Melanie Paquin of NMFS. We acknowledge Paul Reno and Carl Schreck of Oregon State University, Department of Fisheries and Wildlife, for providing critical comments on this manuscript.

Notes

*Fin clips: AD = adipose; LV = left ventral; RV = right ventral; LM = left maxillary; RM = right maxillary.

^†NS = North Santiam River at Lower Bennett Trap.

^‡NSM = North Santiam River at Minto Pond.

*Total worms; 3 subsamples of 30 ml each per sample processed.

^†Total Tubifex tubifex; 3 subsamples of 30 m each per sample processed. Number reflects total number morphologically identified as T. tubifex subtracted by the proportion of false positives (worms genetically identified as another species).

*Current address: SWCA Environmental Consultants, Portland, Oregon, USA