Larval Fish and Zooplankton Dynamics of An Impounded and Free-Flowing River

Navigation dams reduce hydrologic connectivity and alter flow regimes in river systems. Effects on communities include interfering with fish reproductive cues and altering zooplankton dynamics, an essential food source for larval fishes. However, there is a lack of research on navigation dams and their effects on fish communities. Studies simultaneously examining fish spawning and zooplankton communities are critical because phenological changes in either fish or zooplankton have the potential to cause a mismatch between spawning and larval food availability. Conversely, increased water residence time within dammed rivers may increase zooplankton abundances and benefit larval fishes. To investigate these processes, we collected larval fish and zooplankton throughout the spawning season in the Illinois (dammed) and Wabash (undammed) Rivers from 2014 to 2018. We compared larval community composition and peak spawning times for fish families between rivers. Larval fish and zooplankton densities peaked simultaneously in June in both rivers every year. However, zooplankton abundances within the Illinois River were much higher than the Wabash. There was also high interannual variability in larval production. The most productive years within the Wabash River were 2014 and 2018, where 7 times more larvae were captured relative to other years. However, in the Illinois River 3 – 4 times more larvae were captured in 2015, 2017, and 2018 than other years. Cyprinidae were the most abundant family in both rivers, largely due to the abundance of Asian Carp. The second most abundant family in the Illinois River was Clupeidae, while the second most abundant in the Wabash River was Catostomidae. Both Clupeidae and Catostomidae showed drastically different abundances between rivers. Pairing larval fish with zooplankton data will improve our understanding of ecosystem dynamics within these large rivers and the effects of navigation dams.