Upper Mississippi River Silver Carp (Hypophthalmichthys molitrix) demographics.

Silver Carp (Hypophthalmichthys molitrix) have invaded Midwestern USA river systems, including the Upper Mississippi River, where this population poses broadly documented ecological and economic threats. To assess this population, we conducted a standardized fisheries-independent investigation along nearly 700 river kilometers of the Upper Mississippi River using electrified dozer trawls with a stratified-random sampling design. Data collected included Silver Carp relative abundance, size, and age at 14 locations between Pool 18 and the Ohio-Mississippi River confluence. We used empirical data to estimate demographic baselines and test whether relative abundance, body condition, length, age distribution, and growth varied within and among locations fragmented by physical structures. We found that demographic rates varied broadly across the sample area but were similar in several adjacent locations. Demographic rates supported aggregating locations with similar demographic characteristics into Meaningful Monitoring Units (MMUs), where demographic variability was relatively low within each unit, but differed among MMUs. Specifically, the Upper Mississippi River can be divided into four MMUs: the Open River, Pool 26, Pools 20-25, and Above Lock and Dam (L&D) 19. Demographic rates for these MMUS serve as baseline estimates for assessing population change and these MMUs represent a reasonable spatial scale for Silver Carp management and control, as well as fisheries-independent monitoring to measure population changes from ongoing management efforts. Although a definitive characterization would require further investigation, our findings suggest patterns consistent with source-sink dynamics within the study area: the Open River and Pool 26 exhibited traits consistent with source populations, whereas Pools 20-25 and Above L&D 19 exhibited traits consistent with sink populations. The observed variability in demographic rates for Silver Carp across a broad geographic area provides a comprehensive baseline for Silver Carp demography and reveals potential management implications in a river system fragmented by physical structures.