Project Status Reports

PSR 2000-04  July 2000

Native Freshwater Mussels of the Upper Mississippi River System

by Marian E. Havlik and Jennifer S. Sauer

Native freshwater mussels are one of the most endangered groups of animals in North America. In the United States, 69 of 304 mussel species are listed as federally endangered or threatened. Surveys conducted over the past few decades have documented significant declines in mussel populations across the continent. Among the factors thought to be respon sible for the decline are dams, pollution, siltation, commercial navigation, over harvest, and mortality caused by zebra mussel encrustation. Mussels are an important food source for muskrats, raccoons, minks, and bottom feeding fishes. Commercially, shells of certain native mussel species are made into beads that are inserted into oysters as nuclei for cultured pearls.

Historically, 51 species have been documented in the Upper Mississippi River System (UMRS, which includes Mississippi and Illinois River mainstems), but only 44 species have been documented in surveys conducted within the past 35 years. This loss in species richness may be linked to habitat changes after the locks and dams were built. Nearly all of the species (7) not recently found in the UMRS were considered infrequent inhabitants of the UMRS mainstem by biologists in the late 19th and early 20th century, but were more commonly found in the tributaries of the UMRS.

The current conservation status of UMRS native mussels is summarized in Table 1. This table represents an update to Table 11-1 in the “Ecological Status and Trends of the Upper Mississippi River System 1998: A Report of the Long Term Resource Monitoring Program.” In the table, we have included all 51 species of mussels historically found in the UMRS. The conservation status of native mussels varies from state to state. Each state describes the status of a species population only within that particular state, not the UMRS as a whole. It is often difficult to interpret such a table because of the different definitions of the conservation status for each species and the variability in ranking procedures among the states.

Some species in Table 1 are not presently found in the UMRS mainstem. Species such as scaleshell and slippershell have usually been found in UMRS tributaries but only rarely in the UMRS itself. In 1913, upstream from Lock and Dam 19, mussel composition changed in part because some fishes that are obligatory hosts for mussels could not migrate past the dam. Other navigation dams built in the 1930’s also affected mussels by changing the character of the river. The percent abundance of many mussel species has changed especially in pooled portions upstream of dams. For instance, the threeridge mussel is now the most abundant mussel species in the UMRS. The ebony shell (formerly composing 80% of the mussel fauna) and elephantear almost disappeared from the UMRS because populations of their primary host fish—the skipjack herring—declined sharply. Populations of other species such as the washboard, mapleleaf, flat floater, and lilliput mussels have increased in the pooled portions of the river.

Forty-four mussel species still exist in the UMRS proper and an additional 7 species survive in the immediate tributaries (within 100 miles of the UMRS). These include winged mapleleaf, snuffbox, ellipse shell, and cylindrical papershell. The UMRS and tributaries contain three species that are federally endangered (winged mapleleaf, Higgins eye, and fat pocketbook), and one species presently under federal review (scaleshell).

This report is a product of the Long Term Resource Monitoring Program for the Upper Mississippi River System.

For further information, contact

Marian E. Havlik
Malacological Consultants
1603 Mississippi Street
La Crosse, Wisconsin 54601-4969
havlikme@aol.com
608-782-7958 (voice and fax)

Jennifer S. Sauer
USGS Upper Midwest Environmental
Sciences Center
2630 Fanta Reed Rd.
La Crosse, Wisconsin 54603
608-783-7550 #64
jennie_sauer@usgs.gov

Project Status Reports (PSRs) are preliminary documents whose purpose is to provide information on scientific activities. Because PSRs are only subject to internal peer review, they may not be cited. Use of trade names does not imply U.S. Government endorsement of commercial products.

All Project Status Reports are accessible through the Upper Midwest Environmental Sciences Center’s website at http://www.umesc.usgs.gov/reports_publications/psrs/umesc_psr.html