Occurrence and seasonal dynamics of the whirling disease parasite, Myxobolus cerebralis, in Montana spring creeks
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Date
2004
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Montana State University - Bozeman, College of Letters & Science
Abstract
Spring creeks provide a unique ecosystem for wild trout populations, characterized by high nutrient
levels, chemical stability, steady flow regimes, and near optimum temperatures for spawning and
rearing. However, several of these factors may accentuate whirling disease in spring creeks by
providing favorable Tubifex tubifex habitat, the alternate host for the whirling disease parasite, and
promoting the release of the infectious triactinomyxin (TAM) stage over prolonged periods. This study
evaluated the prevalence and severity of whirling disease in nine different spring creeks and adjoining
rivers or reservoirs in southwestern Montana. I investigated the influence of temperature, T tubifex
abundance, water quality, and habitat characteristics on infection severity. Infection severity was
measured with sentinel fish exposures in the spring and fall at all sites to evaluate the extent of
infection. Seasonal dynamics of infection severity was assessed through monthly sampling in three
spring creeks over a 20-month period. I found that infection severity was common (7 of 9 sites) but that
severity among infected sites varied widely. Only five of nine sites had high infection levels (> 50% of
sentinel fish exhibiting moderate to severe lesion scores ≥ grade 3) where population declines would be
expected. The seasonal cycle of infection in spring creeks was much different than observed in
non-spring fed streams as peak infection occurred during winter months, declined in late spring, and
remained at low levels until fall when infection began to rise again. No significant relationships were
found between infection severity and habitat features, T. tubifex abundance, and water quality, with the
exception of infection severity and phosphorus in sites with moderate to high infection levels. Peak
infections in spring creeks occurred at temperatures ranging from 6 to 12 °C. Redd counts and
estimated emergence periods, correlated with sentinel fish exposure results indicated that fry emerging
and rearing during late spring and summer are likely to avoid high infection, even in highly infected
spring creeks. In contrast, given the high infection in spring creeks in late fall and winter, fry of fall
spawning trout may be much more susceptible to infection in these systems.