Freshwater Mussels Pacific Northwest - State Water Resources ...

DistributionWestern ridged mussels inhabit Pacific drainages from southern California tosouthern British Columbia. They are found east to Idaho and Nevada, and inthe northern part of their range are mainly distributed east of the Cascades.They have a limited distribution west of the Cascades in Oregon and Washington.There are historical records in the upper Columbia River watershedin western Montana, perhaps the Clark Fork River or Kootenai River, butthese have not been confirmed recently. Its stronghold is large tributaries ofthe Snake River and Columbia River in Washington, Idaho, and Oregon.Life HistoryLittle is known about the life history of this species. Fish hosts are unknown,but its habitat preference suggests that it parasitizes coldwater stream fishsuch as trout and salmon. It is a relatively slow-growing, long-lived species.HabitatWestern ridged mussels occur in streams of all sizes and are rarely foundin lakes or reservoirs. They are found mainly in low to mid-elevation watersheds,and do not often inhabit high elevation headwater streams wherewestern pearlshells can be found. They often share habitat with the westernpearlshell throughout much of the Pacific Northwest. They are more tolerantof fine sediments than western pearlshells and occupy depositional habitatsand banks. They can withstand moderate amounts of sedimentation, but areusually absent from habitats with unstable or very soft substrates.ConservationWestern ridged mussels have been extirpated throughout their original rangein California, particularly in southern California and the Central Valley. Theyhave also been extirpated from many sites in the Snake and Columbia watersheds.Elsewhere, they are thought to be declining, mainly because the perceivedenvironmental threats to this species are universal throughout its range.However, the magnitude and geographic extent of the declines are unknown.Lack of information on life history, reproduction, and ecology of westernridged mussels will hinder effective conservation and management.For now, the only measures are to protect the ecosystems where they live.40Freshwater Mussels of the Pacific Northwest