Comparison of fish community composition and structure among river reaches of the Upper Mississippi River: Determining the effects of Lock and Dam 19 in structuring fish communities

Completed in 1913, Lock and Dam 19 (RM 364.2) separates navigation Pool 19 (74.5 km) from Pool 20 (35.2 km) and created the first artificial impoundment on the Upper Mississippi River (UMR). Lock and Dam 19 is unique among most other dams on the UMR in that it is a hydroelectric dam with a significant hydraulic head (~10 m) that creates the largest impoundment (pool) on the system (74 km as opposed to a median of 42 km for the other pools on the mainstem). This dam likely acts as a significant barrier to upstream migration for fish. In 2013 and 2014, standardized pulse-DC electrofishing was conducted in Pool 19 (n=171 sites) and Pool 20 (n=112 sites) to assess local and system scale variation in fish community composition and structure among reaches above and below Lock and Dam 19. Sampling was consistent with standardized protocols from the Long Term Resource Monitoring program (LTRM) allowing for comparisons among the reaches we sampled and those sampled by the LTRM (i.e., Pools 4, 8, 13, 26, the LaGrange Reach of the Illinois River, and the open Mississippi River). Cluster analysis and non-metric multidimensional scaling of community composition data revealed two major groups, upper and lower reaches, with Pool 19 grouping with upper reaches and Pool 20 grouping with lower reaches. Cluster analysis and non-metric multidimensional scaling of community structure data revealed groupings of upper and lower reaches, with Pools 19 and 20 grouping together as a group between upper and lower reaches. Pool 19 shared species unique to both upper and lower reaches for community composition. The Pools 19 and 20 grouping between upper and lower reaches shared abundances of species unique to both upper and lower reaches for community structure. Pool 19 had the highest abundance of centrarchid species among all sampled reaches (mean weighted CPUE 91.5) in 2013. Our findings suggest that Pools 19 and 20 act as a transition for fish community composition and structure between upper and lower LTRM reaches in the Upper Mississippi River. Seventy-five fish species were collected in Pool 19 (n=30,836) and 59 collected from Pool 20 (n=14,962) in 2013 and 2014. Future and consistent standardized sampling of Pools 19 and 20 is critical to understand long-term changes in fish assemblages and the effects of Lock and Dam 19 on the ecology of the Upper Mississippi River.