Changes Over Time in the Diversity and Distribution of Freshwater Mussels (Unionidae) in the Grand River, Southwestern Ontario

Abstract

The Grand River, a major tributary to Lake Erie in southwestern Ontario, historically supported a diverse and abundant freshwater mussel fauna, with 34 species recorded from the system since 1885. A recent study suggested that the number of species inhabiting the Grand River has declined over time. The present study provides a detailed assessment of changes over time in the diversity and distribution of mussels throughout the Grand River, by comparing the results of surveys conducted at 94 sites in 1995 and 1997–98 with those from a survey conducted 25 years earlier and with the historical data. 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 rapidly-growing human population. At that time, only six species occurred in the lower reaches of the main stem. Mussel populations have since rebounded, with 25 species found alive throughout the system in 1995/1997–98, including 21 in the previously impoverished lower reaches. This recovery is attributed 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 Grand River appear more favorable now for mussels than they have in decades, there is concern that the growing pressures of urbanization and agriculture may slow, stop, or even reverse these hard-won gains.