Changes in density, age structure and age-specific mortality in two western pearlshell (Margaritifera falcata) populations in Washington (1995–2006)



  • 1.Two western pearlshell (Margaritifera falcata) populations in western Washington were surveyed in 1995 and re-surveyed in 2006. Significant declines in mussel numbers had occurred in both streams during the past decade.
  • 2.In Bear Creek, overall density dropped from 56.0–6.9 mussels m−2, and in Battle Creek, overall density dropped from 80.7–13.4 mussels m−2 in 2006.
  • 3.Large numbers of empty shells were found in Bear Creek in 2006, indicating high levels of mortality. A disproportionate loss of large, old mussels resulted in a forward shift in modal age class, from 51–60 years to 31–40 years.
  • 4.In Battle Creek, survival appeared to be higher, although significant numbers of shells were found in places. A lack of juvenile mussels indicated recruitment problems, possibly due to habitat degradation following colonization of the stream corridor by beavers and/or a lack of migratory host fish.
  • 5.Effective remedial actions for the Bear Creek and Battle Creek M. falcata populations are required within the next 5–10 years and 50 years, respectively, in order to ensure their long-term survival.

Copyright © 2007 John Wiley & Sons, Ltd.