Spatial distribution and habitat use of the western pearlshell mussel (Margaritifera falcata) in a western Washington stream

We investigated the spatial distribution and habitat associations of western pearlshell mussels (Margaritifera falcata) in a southwest Washington stream. Variation in mussel occurrence differed with the scale of the observations, being lower among study reaches and higher within reaches. Additionally, mussels exhibited a highly aggregated, non-random spatial distribution pattern. The distribution of mussels at large scales (across reaches) was associated with dissolved oxygen and shear stress. Mussel distribution at small scales (with the 50 m reaches) was associated with wetted width, canopy, abundance of small gravel substrate, and distance from the stream bank. Mussels were found in locations having reduced shear stress, turbulence, and gradient and increased wetted width, abundance of small gravel, dissolved oxygen, and conductivity. Optimum water depth was 0.2 - 0.6 m and optimum current velocity was 0.23 - 0.30 m/sec. Mussels preferred substrates where boulders increased bed roughness, allowing small gravel and sand to create a stable, heterogeneous substrate.