Demographic Differences of Silver Carp, Hypophthalmichthys molitrix, Populations Between Impacted and Unimpacted Midwestern River Ecosystems

The silver carp, Hypophthalmichthys molitrix, is an invasive species that has received increased attention due to the species proximity to the economically important Great Lakes.  We sought to estimate the population demographics of silver carp on two different river ecosystems, the altered Illinois River and the more pristine Wabash River.  The Illinois River is channelized and dammed for transport of cargo on barges, while the Wabash River is the longest free flowing river east of the Mississippi River and is still connected to its natural flood plain.  Silver carp were captured using DC electrofishing, weighed (g), measured (mm), and a postcleithrum bone was removed for age estimation.  Each postcleithrum was cut into 640 mm cross-sections using a Beuhler ® Isomet Low Speed Saw, and we estimated age at 10-70x magnification with transmitted light.  Relative density (fish/hour) of silver carp in the Illinois River (56.25) was over six times that of the Wabash River (8.37, p<0.001).  Average length was higher in the Wabash River (615.28 mm) than in the Illinois River (517.14 mm, p<0.0001).  In both rivers, carp exhibit quick growth reaching lengths over 300 mm in the first year.  The total annual mortality for the Illinois was higher than on the Wabash (0.662 and 0.351). Furthermore, the hybridization rate was higher in the Illinois (65%) than the Wabash (27%, p<0.0001).  Although silver carp in the Wabash River showed fast growth and low adult mortality, the density was several times lower possibly due to increased resilience of the more pristine ecosystem.