Correspondence Between the Distribution of Fish Assemblages in Wisconsin Streams and Omernik's Ecoregions

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Abstract
Recently, Omernik divided the conterminous United States into ecoregions, areas with similar land-surface form, soils, potential natural vegetation and land use, for management of aquatic resources. Most of Wisconsin is in four of these ecoregions. I used multivariate analyses of fish and habitat data from three size-classes of streams in western and southern Wisconsin to evaluate the correspondence between ecoregions and geographic patterns in the distribution of fish assemblages. Overall, correspondence was better than expected by chance, and improved with increasing stream size. For all three size-classes of streams, agreement between the ecoregion classification of sampling stations and a classification based on a nonhierarchical cluster analysis of fish species abundance data was statistically significant. Detrended correspondence analysis ordinations of fish data showed differences among sampling stations that were associated with ecoregion membership. Catches of several fish species differed significantly among ecoregions, even with conservative significance levels. However, the ecoregion classification was fairly imprecise, and some sampling stations were more similar to stations in other ecoregions than they were to stations in their own ecoregion. Thus, in Wisconsin, ecoregions will be most useful in setting management policies for broad geographic areas containing many streams, and less useful in dealing with limited geographic areas and individual streams. Several analyses suggested that data on four general habitat characteristics, water temperature, stream gradient, substrate composition and shoreline vegetation characteristics, could be used to develop a more precise classification of Wisconsin streams. However, unlike the ecoregion classification, this classification would probably be applicable only to Wisconsin and only to fish assemblages.