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COSEWIC assessment and update status on the round hickorynut Obovaria subrotunda in Canada - 2003


Global Range

The round hickorynut was found throughout the Tennessee and Cumberland river systems and in the Ohio River system from western Pennsylvania and peninsular Michigan west to eastern Illinois; it also occurred in Lake Erie and Lake St. Clair and their drainages in Canada and the United States (Parmalee and Bogan 1998). It was historically known from Alabama, Georgia, Illinois, Indiana, Kentucky, Michigan, Mississippi, New York, Ohio, Pennsylvania, Tennessee, West Virginia and Ontario (Figure 2).

Figure 2: North American Distribution of Obovaria subrotunda (based on information provided by jurisdictions)

Figure 2: North American distribution of Obovaria subrotunda (based on information provided by jurisdictions).

Canadian Range

In Canada, O. subrotunda is known only from southern Ontario. The National Water Research Institute’s Lower Great Lakes Unionid Database was used to identify occurrence records for O. subrotunda in Ontario. At the time of writing, the database consisted of approximately 6000 records for 40 species collected from over 2000 sites in the lower Great Lakes drainage basin since 1860 (see Metcalfe-Smith et al. 1998a for a detailed description of the database and its data sources). The earliest record of the round hickorynut in Canada is two fresh whole shells collected in 1890 from Lake Erie at Kingsville by J.T. McQueen (specimens held by the Canadian Museum of Nature; cat. no. 002448). Since then it has been found in other locations in the western basin of Lake Erie (particularly around Pelee Island), Lake St. Clair, and the Welland, Grand, Detroit, Thames and Sydenham rivers. Figure 3 shows the historical distribution of the round hickorynut in Ontario, based on 44 records collected between 1890 and 1986, and the current distribution, based on 30 records (live animals and shells) collected over the past decade (1991 - 2001). Live specimens were most recently collected from the East Sydenham River and the St. Clair delta in Lake St. Clair in the summer of 2001. Appendix I contains detailed information for all known records of the round hickorynut in Canada.

Lake Erie, Lake St. Clair and the Detroit river (and presumably also the Welland River) are now infested with the exotic zebra mussel, Dreissena polymorpha, which has nearly destroyed native freshwater mussel populations in these waters -- leaving only isolated pockets of surviving animals in some nearshore areas of the lakes (Zanatta et al. 2002). Approximately 64% of historical records for O. subrotunda are from waters now infested with zebra mussels. The round hickorynut has apparently been lost from the Grand and Thames rivers (Metcalfe-Smith et al. 1998b, 1999), and has declined dramatically in the Sydenham River (Metcalfe-Smith et al. 2001). However, 53 live specimens were collected in Canadian waters of the St. Clair delta in Lake St. Clair between 1999 and 2001 (Zanatta et al. 2002). The St. Clair population appears to be the only significant population of O. subrotunda left in Canada. Overall, the round hickorynut has been lost from over 90% of its historical range in Canada. It is now restricted to the East Sydenham River and a portion of the Lake St. Clair delta. The current extent of occurrence is approximately 1750 km2and the area of occupancy is 8 km2.