Abstract:
Whirling Disease is a parasitic disorder causing skeletal deformities in susceptible
salmonids. Trout Unlimited, a non-profit conservation and sportsfishing organization, is
interested in the disease because it has been implicated in the population declines of inland trout
species that occurred in the early 1980s in the upper Big Lost River Basin of Mackay, Idaho.
Presence/absence and severity surveys were conducted in the basin previously, but not in the past
decade. Rainbow trout sentinel fry exposures and wild fish monitoring was conducted in
connected tributaries to determine the current distribution and prevalence of the disease, to
identify streams where trout populations are unaffected and reproducing successfully and to guide
future management. Distribution of the disease is similar to prior assessments, but severity of
infection seems to have declined. Waters testing positive for Whirling Disease have temperature
regimes that are significantly warmer than negative waters 36.5% of days surveyed. Infected
waters also have significantly larger brook trout fry, suggesting early emergence of trout fry may
coincide with high concentrations of the parasite that causes Whirling Disease.
