Changes over time in the diversity and distribution of freshwater mussels (Unionidae) in the Grand River, southwestern Ontario

The Grand River, a major tributary To Lake Erie in southwestern Ontario, hi storically supported a diverse and abundant freshwater mussel fauna, with 3 4 species recorded from the system since 1885. A recent study suggested tha t the number of species inhabiting the Grand River has declined over rime. The present study provides a detailed assessment of changes over rime in th e diversity and distribution of mussels throughout the Grand River, by comp aring the results of surveys conducted at 94 sites in 1995 and 1997-98 with those from a surveys conducted 25 years earlier and with the historical da ta. Timed searches were conducted in both 1995 and 1997-98, using sampling efforts of 1.5 and 4.5 person-hours, respectively. Only 17 species had been found alive in 1970-72, probably because of the impacts of sewage from a r apidly-growing human population. At that rime, only six species occurred in the lower reaches of the main stem. Mussel populations have since rebounde d, with 25 species found alive throughout the system in 1995/1997-98 includ ing 21 in the previously impoverished lower reaches. This recovery is attri buted to significant improvements in water quality over the past 25 years. The recent addition of fishways to some of the dams and weirs on the river should improve the reproductive success of mussels, by eliminating barriers to the movement of host fishes. Although environmental conditions in the G rand River appear more favorable now for mussels than they have in decades, there is concern that the growing pressures of urbanization and agricultur e may slow, stop, or even reverse these hard-won gains.