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Results of a Pilot Freshwater Mussel Survey in King County

Results of a Pilot Freshwater Mussel Survey in King County

In 2004, a pilot freshwater mussel survey was conducted to better understand the density, age and size distribution of the western pearlshell mussel, Margaritifera falcata, in three King County streams. One study site was located in Bear Creek, a stream in the Lake Washington Watershed known to support at least one large mussel bed thought to be in decline. The other two study sites were located in Covington Creek in the Green River Watershed and Stossel Creek in the Snoqualmie River Watershed. Concern regarding freshwater mussels in upper Bear Creek has increased over the past two decades with increasing reports of dead and dying mussels and a general overall decline in numbers observed by local residents and County staff. Evidence was observed of a similar decline at the Covington Creek site.

The goals of the survey were to locate three relatively dense beds of M. falcata and document size and age distribution within each bed, in addition to documenting in-stream and riparian habitat associated with the bed. The second goal was to determine the general condition of the mussels in these streams. This survey addressed four general questions regarding distribution and size, sex ratios, shell erosion, and histology.

25-cm wide transects were established at 5 m intervals across a 100 meter reach in each of the three streams. Each transect was divided into 25 cm by 25 cm quadrats. Mussels within the 25-cm-wide transects were hand picked from the stream bottom, counted, photographed and measured. To evaluate the accuracy of enumeration methods and to identify any sampling bias toward collection of larger animals, 10 percent of the sample quadrats were sieved (in addition to hand picking). Various habitat variables were also noted for each quadrat, including water type, substrate type, habitat structure, and riparian cover. An average of 22 mussels from each stream reach were sacrificed for sex determination, age determination, and shell characteristics. An additional 16 mussels were sacrificed for histological examinations.

Several results of this study suggest that the mussels within the Bear Creek study reach are declining and in poor health relative to Stossel and Covington creeks.

  • Bear Creek had the lowest mussel density (11/m2) compared to Stossel Creek (35/m2) and Covington Creek (19/m2), and comparison of these results to a previous study suggests a substantial decline in the mussel density in upper Bear Creek over the last 7 years.
  • Sex ratios in Bear Creek mussels were dominated by males (4:1), whereas sex ratios at Covington (1.2:1) and Stossel (1.4:1) creeks were somewhat similar to what would be expected (near 1:1).
  • Histological examination indicated that Bear Creek animals were in poor condition and severely stressed, as evidenced by gill lesions and severe loss of fat reserves.
  • Many hundreds of empty shells were observed in the upper Bear Creek study reach, while few if any were observed in Stossel and Covington creeks.

This study did not attempt to identify a specific cause for the decline in freshwater mussels in upper Bear Creek.

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Download the Results of a Pilot Freshwater Mussel Survey in King County report