Habitat associations of small fishes around islands in the upper Mississippi River

In large rivers, islands provide a variety of habitat types and increase habitat heterogeneity. Creating or modifying islands with dredged sediments from channel maintenance operations provides an opportunity to enhance habitat features that might promote certain fish communities or general fish abundance. To determine associations between fish species and habitat features of islands, we sampled fish by seining at 62 sites around 20 islands in the upper Mississippi River from Winona, Minnesota, to Prairie du Chien, Wisconsin (180 km). Habitat characteristics were divided into macrohabitat features associated with islands, such as island shape, location, or maximum depth around the island, and mesohabitat features of sites, such as depth, sediment type, and vegetation abundance. Cluster analysis of islands based on macrohabitat features identified four clusters distinguished primarily by water depth and distance from the main channel. Mean fish density did not differ among island clusters. Cluster analysis of sites based on mesohabitat features produced four clusters distinguished primarily by vegetation abundance. Mean densities of most fish taxa were highest in clusters with moderate or dense vegetation and lowest in the cluster with no vegetation. For the eight most abundant fish species, multiple-regression analysis of density on mesohabitat features across all sites indicated that all species were positively correlated with vegetation abundance, which explained 7-49% of variation in density. Our results suggest that mesohabitat features of sites were more important than macrohabitat features of islands in determining density of small fishes and that modifications that increase the abundance of vegetation around islands are most likely to increase fish density.