Summary

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Havilk, M.E. 1993. Unionids and Margaritiferids (Molusca: Bivalvia) in the Saint Croix River bordering Minnesota at Afton and Wild River State Parks, 8-17 June 1992. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 41 pp.

Summary:

Over 40 unionid and margaritiferid species (Mollusca: Bivalvia) have been reported from the St. Croix River in recent times, but large areas have never been sampled. From 8 to 17 June 1992 we did random, semi-quantitative, and quantitative sampling by wading and SCUBA diving near the Minnesota shoreline of Wild River and Afton State Parks to determine the presence and habitat of rare bivalve mollusks.

A total of 1560 living unionid and margaritiferids were found (20 species). Four additional species were among 391 empty shells. One dead federally endangered Lampsilis higginsi (Lea, 1857), Higgins' Eye, was found near Afton State Park's north boundary. Elliptio dilatata (Rafinesque, 1820), Spike (32.0%) and Actinonaias ligamentina carinata (Barnes, 1823), Mucket (30.8%) dominated the fauna. No Dreissena polymorpha (Pallas, 1771), Zebra mussel, were found.

Areas sampled at Wild River State Park were up to two meters deep, and had a diverse, abundant mussel fauna, particularly in sand, gravel, and cobble areas that also served as habitat for submergent aquatic vegetation. Thirteen sites (32 sub-sites) along the 18.7 mile border of Wild River Park yielded 1480 living unionid and margaritiferid mollusks with densities to 74/m2 representing 19 living species. Two additional species were represented by empty shells only.

Proposed as federally endangered, 40 large, old Cumberlandia monodonta (Say, 1829), Spectacle Case, were found at Wild River Park. The largest, 235 mm long, was about 70 years old (independently aged three times). Their unique habitat of large boulders was limited. Since the smallest C. monodonta was 165 mm long, the absence of young adults strongly suggests a host fish problem. C. monodonta is reproducing below the Northern States Power dam, St. Croix Falls, WI.

Other living rare unionids found at Wild River were Cyclonaias tuberculata (Rafinesque, 1820), Purple Warty-Back (23 specimens up to 50 years of age), Alasmidonta marginata (Say, 1818), Elk Toe, (29 specimens) and Pleurobema sintoxia (Rafinesque, 1820), Round Pig-Toe (17 specimens). At least six species showed evidence of recruitment.

Afton Park's unionid habitat was limited, and the fauna sparse, both in numbers and species diversity. Depths were up to six meters, or more, near the shoreline. Twelve sites (14 sub-sites) along the two mile border of Afton Park yielded 80 living unionids representing eight species. Recruitment was minimal, and some specimens were very slow growing (stunted length and height for age). Three additional species were represented by empty shells only.

This project was supported by the MNDNR, Division of Parks and Recreation's Working Capitol Account, and by the Minnesota Nongame Wildlife Tax Checkoff, through the MNDNR Nongame Wildlife Program.

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