Historical Changes in Large River Fish Assemblages of the Americas

<em>Abstract.</em>—The Wisconsin River is a lowland warmwater river located entirely within the state of Wisconsin. It is the largest river within the state, with a length of 676 km, a drainage basin of 31,800 km², and an estimated mean annual flow at the mouth of 292 m³/s. The middle part of the river has been heavily modified by dams and pollution, but the lower portion is relatively nondegraded. A total of 110 native fish species have been recorded from the river, a high number for the upper Mississippi River basin. Only two alien species occur and only one, common carp <em>Cyprinus carpio</em>, is common. Five stateendangered, five state-threatened, and 10 state-special-concern (=vulnerable) fishes are known from the river. Populations of the endangered black redhorse <em>Moxostoma duquesnei</em>, threatened paddlefish <em>Polyodon spathula</em>, blue sucker <em>Cycleptus elongatus</em>, black buffalo <em>Ictiobus niger</em>, and vulnerable western sand darter <em>Ammocrypta clara </em>in the Wisconsin River are among the largest in the state for these species. Historical data are scarce, but it appears that no fish species have been eliminated from the river. However, several species have been extirpated from discrete reaches of the river. Fish species richness decreases from mouth to headwaters, but reach-specific fish assemblage structure and composition and biotic integrity vary in a more complex pattern, most likely as a consequence of habitat fragmentation and isolation by dams and industrial and municipal pollution. In particular, the Prairie du Sac Dam, lowermost on the river, prevents upstream fish movement and causes a sharp drop in species richness and biotic integrity upstream.