Changes in the biodiversity of freshwater mussels in the Canadian waters of the lower Great Lakes drainage basin over the past 140 years

Severe declines in the diversity and abundance of freshwater mussels have b een documented over the past century in the United States. Although similar trends might be expected in Canada, mussels have received little attention to date. The Committee On the Statics of Endangered Wildlife In Canada (CO SEWIC) expanded its mandate in 1994 to include invertebrates, thus providin g the impetus for assessing the health of Canada's freshwater mussel fauna. The purpose of this study was to determine if there have been changes over time in the richness and composition of freshwater mussel communities in t he lower Great Lakes drainage basin, which historically supported the most diverse and unique mussel fauna in Canada. Over 4,100 occurrence records fo r 40 species of mussels collected from approximately 1,500 sites between 18 60 and 1996 were compiled and examined together for the first time. Compari sons of historical and recent data revealed a pattern of species losses and changing community composition throughout the basin, particularly in the s pecies-rich Lake Erie and Lake St. Clair drainages. River systems that once supported numerous species characteristic of a wide variety of habitats ar e now dominated by fewer siltation- and pollution-tolerant species of the S ubfamily Anodontinae. A detailed examination of the data for the Grand, Tha mes, and Moira rivers confirmed that the same trends are occurring in widel y-separated systems throughout the basin. The results of this study provide compelling evidence that the steady decline in freshwater mussel diversity that has been documented for the United States is also occurring in Canada .