Most freshwater mussel species in North America are imperiled. Life history traits of many species have been documented but information regarding western pearlshell mussels (Margaritifera falcata Gould) is scarce. Our goal was to improve understanding of western pearlshell mussel reproduction, growth, and movement. The study area was a 250 m reach in Merrill Creek, Oregon. We examined 1389 mussels for gravidity and examined water samples for glochidia during presumed spawning times over a 4-year period. We tagged 415 mussels for mark-recapture observations for growth and movement. No mussels showed signs of gravidity. However, four mussels near our study transects were observed releasing conglutinates. Glochidia were present from April to mid-June. Glochidia were not detected until maximum daily water temperature had reached 10.0 °C, and were no longer detected once minimum daily water temperature remained above 9.0 °C. There was a negative relation between growth rate and mussel size. Growth rate was not significantly different than 0.0000 mm d-1 for large mussels and 0.0011 mm d-1 for small mussels. For movement, 60% (n = 15) of mussels were recaptured at the same transect as originally marked, 32% (n = 8) were recaptured 3.7–115.6 m downstream and 8% (n = 2) were recaptured 12.0 m upstream. This basic life history information is essential to consider when developing management plans associated with the conservation of western pearlshell mussels and their habitat. Our results indicate they are slow-growing, slow-moving, long-lived, and thus likely slow to adapt to environmental change and respond to habitat perturbations.
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Vol. 91 • No. 1