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

Summary of Status Report

Obovaria subrotunda historically occurred in 12 states and the province of Ontario. In the United States, it 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. Almost every jurisdiction has reported declines, and some of these are quite severe -- especially in the Tennessee River system. In Canada, the round hickorynut was historically found in the western basin of Lake Erie (particularly around Pelee Island), Lake St. Clair, and the Welland, Grand, Detroit, Thames and Sydenham rivers. It has been lost from Lake Erie, the Detroit River, and the offshore waters of Lake St. Clair due to impacts of the zebra mussel. A significant population was discovered in shallow waters of the St. Clair delta in Lake St. Clair in 1999, but most specimens were large and it is not certain that recruitment is still occurring. The round hickorynut has apparently been lost from the Grand and Thames rivers, and has significantly declined in the Sydenham River where only three live specimens have been found in recent years.

Obovaria subrotunda may be very sensitive to human disturbance, since there is evidence that it was lost decades ago from many systems. For example, it has been found in only one river in Illinois since 1969; and it was last seen alive in Lake Erie in 1950, in the Clinton River, Michigan in 1935, and in the Grand River, Ontario at the turn of the century. Only a few subfossil shells have been found in New York and in the Thames River, Ontario in recent years. The round hickorynut was not among the 16 species of unionids that have recolonized the lower Grand River over the past 25 years as a result of significant improvements in water quality. Furthermore, O. subrotunda has significantly declined in the Sydenham River, where the endangered northern riffleshell (Epioblasma torulosa rangiana), snuffbox (Epioblasma triquetra), rayed bean (Villosa fabalis), and mudpuppy mussel (Simpsonaias ambigua) still persist and are reproducing.

The round hickorynut is currently listed as endangered in Illinois, Michigan, and Alabama, threatened in Tennessee, and special concern in Indiana, and is therefore afforded some protection in these states (it is not federally listed in the U.S.). Most land along the reach of the East Sydenham River where a few live specimens of O. subrotunda were found in recent years is privately owned and in agricultural use. The only significant population of O. subrotunda left in Canada is located in the waters of the St. Clair delta, within the territory of the Walpole Island First Nation. The area is undeveloped and is under the control of the First Nation, which means there is excellent potential for protecting the population from human disturbance. However, it may not be possible to protect it indefinitely from the zebra mussel. The most significant threats to the continued existence of the round hickorynut in Canada are zebra mussels and agricultural impacts.