A Fish Community Comparison Upstream and Downstream a High-Head Dam on the Upper Mississippi River

Abstract Standardized sampling is useful for detecting and monitoring variations in fish communities and the establishment and spread of invasive species. Lock and Dam 19, a high-head dam with a 10-meter hydraulic head, creates the largest impoundment on the Upper Mississippi River. Upstream, Pool 19 is characterized by static water levels and expansive macrophyte beds. Downstream, Pool 20 is characterized by lotic conditions and sparse vegetation. Lock and Dam 19 serves as barrier to upstream migration of fishes, particularly Bigheaded carp (Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molotrix). There is limited standardized fisheries data available for Pools 19 and 20, particularly in off-channel areas. In 2013 and 2014, we conducted two years of standardized electrofishing in Pools 19 and 20 to 1) detect a presence of adult or juvenile Bigheaded carp, 2) quantify differences in fish community structure (i.e., relative abundance of species) between Pools 19 and 20 and among strata (i.e., backwater, side channel, main channel, and impounded reaches), and 3) quantify hydrological differences (i.e., water quality variables) between Pools 19 and 20 and among strata. We detected two Bigheaded carp upstream Lock and Dam 19, the first to be documented using a standardized collection method. We used permutational multivariate analysis of variance using distance matrices (vegan R package version 2.6-4) to determine that fish community structure differed significantly between pools (P < 0.01) and among strata (P < 0.01), except between main and side channel borders. We determined water quality variables differed significantly between the pools and among strata demonstrating strata proportional differences may drive the fish community differences between Pools 19 and 20. We have demonstrated that expanding standardized sampling efforts to Pools 19 and 20 is important to understand long-term changes in fish assemblages and the effects of Lock and Dam 19 on the ecology of the Upper Mississippi River.