Occurrence and seasonal dynamics of the whirling disease parasite, Myxobolus cerebralis, in Montana spring creeks
Anderson, Ryen Aasheim
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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.