Freshwater Mussels Pacific Northwest - The Xerces Society

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rents 105 . Banks and pools are often favorable habitats because the currents are
weaker, shear stress is lower, and the substrates are more stable 51,92 . M. falcata
does not tolerate sedimentation. In Idaho’s Salmon River, M. falcata covered
with a substantial amount of sand and gravel were unable to move to the surface
and perished 105 . In environments where host fish are abundant, physical
habitat is ideal, and human threats are minimal, M. falcata can attain very
high densities (>300 per square yard), often carpeting the stream bottom. In
1981, Clarke wrote, “In favourable localities in British Columbia the mussels
may be so abundant and closely packed that they completely obscure the
stream bottom.” 18
Conservation
Recent conservation concerns about M. falcata closely mirror well-known
stories of the decline of Pacific salmon fisheries. Both need clean cold streams
and rivers, and M. falcata reproduction requires salmon and trout hosts. The
greatest threats to western pearlshells come from loss of host fish species and
water diversion projects for irrigation, power generation, and water supply,
particularly in Washington, Oregon, Idaho, and California. Dams destroy
many miles of free-flowing rivers, disrupt native fish communities, and may
have contributed to the demise of many populations of western pearlshells.
Agriculture and rapid urbanization are affecting aquatic ecosystems throughout
the West through nutrient enrichment, siltation, and chemical pollution,
all of which may negatively impact western pearlshells. Climate change
has been implicated in the decline of the closely related M. Margaritifera in
North America and Europe 40 and it is likely that M. falcata will be affected
in similar ways.
Invasive species that compete with native fish may affect M. falcata.
In some locations where western pearlshells are still abundant, native cutthroat
trout are being replaced by nonnative rainbow, brown, and brook
trout 27,28,48,81 . The long-term effects of increasing nonnative fish populations
on native mussels, albeit with fish species that may also serve as hosts, are
illustration: Ethan Nedeau
The cutthroat trout is an important host species for Margaritifera falcata
Freshwater Mussels of the Pacific Northwest