The Biological Importance of Middle Mississippi River Islands on Fish Assemblages, Missouri, USA

Alterations to vital river ecosystem functions cause decreases in habitat diversity, and as a consequence, result in a loss of biological diversity.  Historically, islands were natural features in large rivers.  In the Middle Mississippi River (MMR), the river is no longer free to migrate and produce new islands due to navigation features such as flow regulation, channel straightening, bank stabilization, dikes, revetments, and closing structures.  Additionally, levees disconnect the main river channel from the floodplain, wetlands, and associated backwaters.  In the MMR, such activities have contributed to the stabilization, narrowing, and deepening of the navigation channel, and to the loss of backwaters, islands, and side channels.  We investigated the ecological and biological significance to fish assemblages of created islands in a dike field, in relation to dike fields without islands.  We hypothesized that there would be no difference in fish communities between island and non-island (reference) sites, or among habitat types within islands and reference sites; and that physicochemical factors would have no correlation with fish assemblages.  Fish assemblages at five island and five non-island sites located within dike fields were studied over a two-year period.  Habitat characteristics including velocity, water depth, water temperature, dissolved oxygen concentration, conductivity, turbidity, and pH were measured at each sample site.  Fishes were collected using trawling, electrofishing, and mini fyke nets.  NMDS, ANOSIM, indicator species analysis, and vector fitting were used to reveal differences in fish assemblages and environmental factors among islands and non-island sites, and habitat types.  Species richness was greater at islands than at reference sites.  At habitat types, species richness was lowest at tip habitat, but similar among inside, outside, and reference.  Fish assemblages differed significantly between islands and references sites for total standardized count and for adult standardized count.  The fish communities differed significantly among each of the habitat types, with the exception of outside and reference habitat, for total standardized count; and among the habitat types, with the exception of tip and reference sites, for adult standardized count.  Additionally, average depth, conductivity, pH, velocity, water temperature, and Secchi visibility were significantly related to differences in fish community assemblages between islands and reference sites, and between habitat type and reference sites.  These results demonstrate that created islands increase local habitat diversity by creating shallow backwater-like habitat, which is limited in the MMR, and support a fish assemblage which is distinct from that found in conventional dike fields.