James Robison - Cox 2 , Jay Rotella 1 , Travis Horton 3 , and Billie Kerans
نویسندگان
چکیده
Whirling disease has spread rapidly throughout the United States the past 20 years, but predicting its impacts to trout populations has been problematic. Using a database that contained mark-recapture information for 384,938 trout during the years 1980-2007, a before-after control-impact study design was used to analyze data from infected river sections and non-infected reference sections on six blue-ribbon Montana rivers (Missouri, Blackfoot, Bitterroot, Gallatin, Ruby rivers and Rock Creek) having severe whirling disease infections. A Bayesian mark-recapture model indicated that disease had a strong negative effect on abundance of small (200-300 mm total length) rainbow trout Oncorhynchus mykiss, with abundance declining an average of 50% (range 30-69%) of pre-disease levels. This marked decline was consistent across all study rivers. In contrast, a parallel decline in larger fish was not observed; instead, the numbers of rainbow trout >300 mm long either remained the same or increased after whirling disease, with the magnitude of the changes varying by river. Rainbow trout of all size classes showed no reduction in growth or condition after a whirling disease outbreak, suggesting that those fish that survive initial infection do not suffer survival or performance deficits even in highly infected systems. As anticipated, there were generally few changes in brown trout density following whirling disease. The exception was in Rock Creek, where decline in rainbow trout from 90% of total trout density to 20-30% after whirling disease was met by a similar magnitude increase in brown trout. Across all rivers, high infectivity levels coincided with low stream flows since 2000, indicating drought may have exacerbated whirling disease impacts on rainbow trout.
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