Native Clams Survive in Isolated Pockets

From May, 1997 (Update #30)

Native clam populations can coexist with zebra mussels in specialized habitats such as wetlands, according to S. Jerrine Nichols of the U.S. Geological Survey. Nichols reported at the Seventh International Zebra Mussel Conference in late January that live native clams have been found at four wetlands with direct connections to Lake Erie. Native clams, also referred to as unionids, have been severely reduced and almost eliminated by zebra mussels in the open waters of Lake Erie. Nichols' observations began at Metzger Marsh, a 908-acre embayment at the western end of Lake Erie, east of Toledo, Ohio. Her discovery of this unintentional native clam refuge occurred largely by accident. In 1996, as part of a wetlands restoration project at the marsh, a 2.4 km stone dike was built across the mouth of the bay to allow marsh water levels to be lowered, thereby aiding the restoration of native vegetation. Nichols found extensive beds of native clams during a survey of the dewatered mudflats and sandflats, including 21 species, three of which are threatened species in Ohio. Nichols also reported signs of excellent native clam reproduction. Multiple year classes were collected for 18 of the 21 species, and many clams appeared ready to spawn. Less than 1% of the 7,000 clams collected showed any evidence of zebra mussel colonization. Nichols believes native clams are surviving at this site - in spite of the presence of zebra mussels - due to sediment characteristics and water temperatures. Native clams were found most frequently in soft, silty sediments, instead of sandy areas, and clams were often found burrowed deep in the muck (up to 40 cm) where attached zebra mussels could not survive. "Trails" left by the movement of native clams were evident during daylight surveys, indicating the clams moved up to the surface at night, when water temperatures are more favorable than daytime water temperatures that can approach 86 F. Nichols noted that she's encouraged to find some wetlands still contain healthy native clam populations with few zebra mussels. "These refuges aren't ideal, but they're better than nothing," said Nichols.

ID: 199705-3.

The Zebra Mussel Update was a 4- to 8-page quarterly national newsletter published by the University of Wisconsin Sea Grant Institute from May 1990 through May 1997. The ZMU documented the spread of the zebra mussel -- an exotic nuisance mussel -- through North America's freshwater environments, especially the Great Lakes, and on efforts to control it.