Navigation Dams and Larval Fish Dynamics

Navigation dams, such as those on the Illinois and Mississippi Rivers, reduce hydrologic connectivity and alter flow regimes in river systems. Effects on fish community dynamics include interfering with reproductive cues and altering zooplankton community dynamics, an essential food source for larval fishes. However, there is a lack of research on navigation dams, and their effects on fish communities are not clearly understood. 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. On the other hand, increased water residence time within dammed rivers may lead to increased zooplankton and be beneficial to larval fishes. To determine if navigation dams have altered the spawning cues of fishes, we compared spawning synchrony between the Illinois (dammed) and Wabash (undammed) Rivers by comparing larval community composition and using occupancy models to determine peak spawning times for individual fish families. These data were then compared to the zooplankton community to determine how spawning time corresponds to food availability. We collected larval fish and zooplankton throughout the spawning season in the Illinois and Wabash Rivers from 2015 – 2018. Larval fish densities peaked in June in both rivers every year, typically around the timing of peak zooplankton abundance. Cyprinids were the most abundant family in the Wabash River, while Clupidae were the most abundant in the Illinois River. Our research will improve our understanding of ecosystem dynamics within these large rivers, and the effects that navigation dams have on them.