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Chapter 4 ECOLOGICAL TRENDS OF SELECTED FAUNA IN THE UPPER MISSISSIPPI RIVER By Jon Duyvejonck' INTRODUCTION The Mississippi River flood of 1993 was a significant event for river flora and fauna as well as for people. How- ever, unlike the devastation caused to human resources, the effects on river organisms were both good and bad. For some organisms, whether flood impacts were beneficial or harmful may not be known for years. For example, contpo- sition of the river floodplain forest will change significantly in coming decades due to the virtual elimination of certain tree species from the forest canopy and understory. In order to understand the effects of the flood upon the river's ecosystem, one must have a good understanding of the eco- logical status of that system prior to the flood, Such a back- ground is crucial to understanding the potential impacts associated with any changes in floodplain management pol- icies considered as a result of the flood. This paper focuses on historical trends for two groups of upper Mississippi River (UMR) organisms, freshwater mussels and fishes. At the turn of the century, the unregulated harvesting of millions of tons of mussels provided the base for a multi- million dollar pearl and buttonmaking industry. Mussel populations declined significantly as a result. The economic utilization of mussel resources experienced a resurgence in the 1960's to satisfy the cultured pearl industry. The mussel resource has an uncertain future because of threats from zebra mussels, poor water quality, and impacts related to navigation and floodplain development. Like mussels, UMR fishes were an important economic resource for many decades. Commercial utilization of UMR fish resources continues but is not as significant to the economy as it was previously, Common carp (Cyprinus carpio) eventually replaced native fishes as the dominant species in the com- mercial catch. The species composition of the UMR fish I Upper Mississippi River Conservation Committee. U.S. Fish and Wildlife Service. assemblage does not appear to have changed significantly in the last 100 years. However, human-induced changes have caused marked alterations in abundance for some species. MUSSEL FAUNA The UMR, which extends from the Twin Cities, Min- nesota, to the mouth of the Ohio River near Cairo, Illinois, once harbored one of the most diverse and abundant mussel populations in the United States. Grier and Mueller (1922- 1923) listed 63 species of mussels inhabiting the Missis- sippi River main stem. Ellis (1931) found 39 species of mussels at 254 locations between Lake Pepin, Minnesota, and Quincy, lIlinois. Smith (1899) estimated that more than 400 native species of unionids were present on the Missis- sippi, but Carlander (1954) attributed this high number to multiple varieties of the same species. These surveys appear to be the definitive investigations for determining a list of native UMR mussel fauna. According to Van der Schalie and Van der Schalie (1950), the river below St. Louis had a rather poor mussel population because of the tremendous silt loads delivered to it by the Missouri River, They also reported that 14 of those species reported by Grier and Mueller (1922-1923) appeared to be more common in smaller streams and should not be considered main stem species. Three additional species reported by Grier and Mueller were found only in sloughs and backwaters and not the ntain stem proper. In addition, 15 of the species reported by Ellis (1931) were not true species according to Van der Schalie and Van der Schalie (1950). After considering the above, Grier and Mueller's original list of 63 species could be reduced to 38 species originally inhabiting the main stem proper, which is more in line with the 39 species found by Ellis (1931). More recently, Fuller (1980) recognized 50 species of UMR mussels on the basis of a comparison of historical and current distributions of UMR freshwater mus- sels. Fuller justified his "expanded" list of species on the 41