An analysis of whirling disease risk in Western Montana

Abstract

Myxobolus cerebralis, the causative agent of whirling disease, has been a major contributor to the loss of young trout in numerous streams within the Intermountain West (Colorado, Idaho, Montana, Utah, Wyoming). Currently there are no effective management procedures for mitigating the effects of this disease because it is not fully known why the parasite has severe effects in some trout populations while remaining fairly benign in others. Characteristics of the parasite, hosts, environment, and their interactions, may partially explain varying responses of wild rainbow trout populations to whirling disease. The goal of this study was to examine possible contributors to, and indicators of, stream degradation and their relationship to whirling disease risk. Specifically, the objectives were to quantify relationships between whirling disease risk and 1) land use 2) biological stream integrity and 3) physicochemical parameters within four major drainages in western Montana. The hypothesis was that whirling disease risk is influenced by anthropogenic land use practices that create favorable habitat for the oligochaete worm host, Tubifex tubifex, which is reflected in the biological integrity and physicochemical features of the stream.

Whirling disease risk was quantified by the severity and prevalence of infection in caged sentinel trout. A geographic information system (GIS) was used to model land use (e.g., agriculture, mines) within drainages. Bioassessment metrics specific to Montana (e.g., total taxa richness, sensitive taxa richness) and those directly related to whirling disease (e.g., density of T. tubifex) were used to assess biological stream integrity. Physicochemical characteristics included those associated with favorable T. tubifex habitat (e.g., substrate), and those that have been associated with an increased incidence of whirling disease (e.g., temperature). Importance of predictor variables was assessed using Spearman's rank correlation and regression tree analyses. Final regression trees identified the proportion of riparian forest, road density, oligochaete density, Limnodrilus hoffmeisteri density, Plecoptera taxa richness, and the proportion of Plecoptera as the most important predictors of whirling disease risk among drainages. A greater understanding of the linkage between land use, biological stream integrity, physicochemical features and whirling disease risk is needed before effective management techniques can be implemented in Montana watersheds and elsewhere.