Contrasting Population Demographics of Invasive Silver Carp between an Impounded and Free-Flowing River

Abstract: Extensive research has been conducted on Silver Carp Hypopthalmicthys molitrix in the heavily modified Illinois River in the midwestern United States due to the potential for populations to enter the Great Lakes; however, little research has been conducted on populations in unimpounded rivers. The Wabash River is an unimpounded large river in the U.S. Midwest that may be a good model for studying Silver Carp in unregulated river ecosystems that could potentially be invaded if reproducing populations in the Great Lakes are established. In this study we compared population demographics of Silver Carp in the Wabash and Illinois rivers. Abundance was over three times greater in the Illinois than in the Wabash River, although Wabash River Silver Carp had significantly greater mean length, age, and condition and a higher growth rate than did Illinois River Silver Carp. Although the heavily modified and degraded environment in the Illinois River may have aided in the rapid expansion of Silver Carp, intraspecific competition, increased commercial fishing, and competition with native species have likely reduced condition, decreased growth rate, and increased mortality rate of the population. Differences between Silver Carp populations in the Illinois and Wabash rivers suggest that the impacts of invasion may be reduced in ecosystems with little modification.