Distribution, Abundance and Population Age Structure of Freshwater Mussels in the Mid Klamath Subbasin, CA

Freshwater mussels (Bivalvia: Unionoida) are an integral component of freshwater ecosystems. North America harbors the world’s most diverse and imperiled mussel fauna, which until recently formed an important part of the diets and material culture of indigenous peoples, including the Karuk Tribe of California. This study, a collaboration between the Karuk Tribe and Whitman College, represents the first systematic survey of freshwater mussels in the Klamath River Basin of northwestern California, where little is known about mussel distribution, abundance, habitat requirements or conservation status. We snorkel surveyed 80 sites on the mid Klamath and 17 sites on its tributary the Salmon to assess abundance, distribution, habitat and population age structure of mussels.  We assessed physical habitat factors at macro, meso and micro scales.  We identified three mussel genera (Margaritifera, Gonidea, and Anodonta) in the Klamath, with Gonidea abundant and widely-distributed within the Klamath, and Anodonta and Margaritifera present in low numbers and restricted in range to upriver reaches with infrequent scour and downriver reaches with better water quality, respectively. In the Salmon River, we recorded only Margaritifera. At the mesohabitat level, both reach geomorphology and bank type were good predictors of mussel distribution.  At the microhabitat level, G. angulata were found significantly more often in bedrock, sand and gravel substrates than would be expected given substrate availability.   Mussels were situated in flow refuges located in areas of low shear stress and high substrate stability protecting them from scour during high flows. The G. angulata population was dominated by mussels 5-8 years old, with few younger and older mussels. The lack of young mussels can be partly attributed to the difficulty of finding juvenile mussels.  Continued study of Klamath mussel distribution, habitat and ecology will be essential to their conservation in the watershed.