Fish assemblages and biotic integrity of a highly modified floodplain river,the Upper Mississippi,and a large,relatively unimpacted tributary,the Lower Wisconsin

The Upper Mississippi River is a dynamic floodplain river that has been largely transformed by navigational levees and dams since the 1930s. The pools upstream of each dam are lake‐like and only about the upper third of each reach retains a riverine character. In contrast, the Wisconsin River is not managed for commercial navigation and today its lower 149 km represent one of the least‐degraded large river reaches in central North America. Riverine reaches in both the Mississippi and Wisconsin rivers have similar macro‐habitats including numerous islands, large side channels, and connected backwaters and floodplain lakes. In this study, shoreline electrofishing samples were collected during summer 2002 and 2003 to characterize resident fish assemblages. We compared fish species abundance, biomass, and biotic integrity along main and side channel borders between the Upper Mississippi River and the Lower Wisconsin River. We expected that, in the absence of environmental degradation, fish composition and structure would be similar between the Mississippi and Wisconsin rivers, and between channel types within each river. Nonmetric multidimensional scaling and redundancy analysis revealed that fish species in the Mississippi River, unlike in the Wisconsin River, were characteristic of non‐riverine habitats. We consider non‐riverine fish assemblages indicative of environmental impairment. The main and side channel sites in the Mississippi River had more variable fish assemblages than the Wisconsin River. Analyses of fish index of biotic integrity scores showed that environmental condition was excellent for both channel types in the Wisconsin River, whereas in the Mississippi River the side channel was rated good and the main channel only fair. We conclude that differences between the two rivers and between channel types of the Mississippi River are consistent with direct and indirect effects of navigation. This study demonstrates the utility of a fish index of biotic integrity, an inexpensive and rapid bioassessment tool, for detecting change in ecological health on one of the world's largest rivers. Copyright © 2006 John Wiley & Sons, Ltd.