Spatial patterns of fish communities in the Upper Mississippi River System: Assessing fragmentation by low-head dams

We assessed the similarity of fish communities among river reaches to assess community-level fragmentation by low-head dams in the Upper Mississippi River System (UMRS). The spatial coverage of standardized electrofishing sampling used in the Long Term Resource Monitoring Program (LTRMP) was extended for three of the six regional trend areas (RTA; pools 4, 13, and the Open River Reach) to include river reaches (outpools) immediately upstream and downstream from the standard RTA from 15 June to 31 October 2000. Additionally, pools 19 and 20 were sampled in September 2000. Cluster analysis and non-metric multidimensional scaling of community composition and structure data revealed two major groups, upper and lower reaches, and four (for composition) or five (for structure) sub-groupings of river reaches. In general, all outpools grouped with the nearest RTA for both community composition (no exception) and community structure (one exception). This suggests that fragmentation of fish communities from low-head dams is minimal. Mantel correlations demonstrated strong inverse association between the similarity of fish communities with the distance between reaches. Habitat variables measured during electrofishing collections were significantly correlated with spatial variation of fish composition and community structure, but provided only marginal improvements to correlations with distance between reaches alone. Furthermore, habitat variability among river reaches also was related to distance between reaches. Determining the extent to which variation of fish communities is related to habitat or demographic processes (e.g. migration, larval drift, source-sink dynamics) will be challenging for this system. Although low-head dams on the UMRS may restrict movements for individuals and populations of certain fish species, we found little evidence that these effects have led to substantial, community-level fragmentation. Copyright (c) 2006 John Wiley & Sons, Ltd.