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The western pearlshell mussel, Margaritifera falcata, was historically the most common freshwater mussel species in streams in forested watersheds in the Pacific Northwest.  Upper Bear Creek in King County, Washington supported an apparently healthy and stable population of freshwater mussels as recently as 10 years ago.  Within the last 5 years, observations suggesting general decline became more serious when mussel beds became dominated by empty shells, rather than living mussels.   Field surveys, a caged mussel relocation experiment, and lake toxicity screening was used to determine: (1) How much M. falcata mortality has occurred and what is the spatial extent of die-offs along Bear Creek? (2) Do healthy M. falcata become diseased when relocated to Bear Creek for an extended period of time? (3)  If healthy freshwater mussels become diseased when relocated to Bear Creek, what is the timeline for the onset of symptoms and is there a pathological sequence of events that lead to mortality? (4) Could an algal toxin be contributing to M. falcata mortality?  Results from this study indicate that heightened M. falcata mortality is occurring downstream, but not upstream, of Paradise Lake and that early fall is a critical time for this mortality.  Two lines of evidence support these statements.  The caged mussel relocation experiment supported observations made during the survey that suggested that non-predation related mortality occurred downstream, but not upstream of Paradise Lake.  Mortality observed in both the caged mussel relocation and the fathead minnow toxicity testing suggest that a toxic substance or stressor is occurring in Paradise Lake and the Bear Creek – downstream mussel relocation site starting in late August and early September.  While there is an indirect spatial and temporal-link between patterns of M. falcata mortality along Bear Creek and Paradise Lake toxicity, the link between conditions at Paradise Lake and the downstream relocation site has not been explicitly tested.  Further studies are needed to determined what is causing mortality, or if the cause of this mortality is part of a natural process (e.g., a freshwater algal toxin) or anthropogenic (e.g., pollution).

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