Asian Carp Hypophthalmichthys spp. at the Edge of Their Invaded Range: Population Changes in the Upper Illinois River and Implications for Management and Suppression

The threat posed to uninvaded ecosystems by Asian carp has focused control efforts on preventing further range expansion. In the Illinois River, their upstream progression is a major concern due to the connection with Lake Michigan. An Asian carp harvest program in the upper river aims to reduce upstream propagule pressure. To evaluate this effort and develop appropriate future management strategies, characterizing the population response of Asian carp to anthropogenic (and environmental) drivers is critical. We conducted annual hydroacoustic surveys (2012–2014) in the upper reaches where harvest occurs. Overall Asian carp density was higher in 2012 (a low flow year) but stabilized in 2013 and 2014 (moderate to high flow years). Species-specific annual densities within reaches were more variable; in most cases density was highest in 2012, but only the uppermost reach showed clear consecutive declines (for bighead carp). Species composition (bighead and silver carp) and size structure shifts were also apparent among years. We discuss the possible causes of Asian carp population changes in the Upper Illinois River (e.g. harvest, hydrological conditions, immigration from the lower river source population) and the importance of fishery-independent population estimates to track Asian carp invasion dynamics.