Monitoring the Population Demographics of Invasive Silver Carp in the Illinois River

The silver carp, Hypophthalmichthys molitrix, is an invasive species that has received increased attention as the species moves closer to the economically important ecosystem of Lake Michigan.  We sought to estimate the population demographics of silver carp on the Illinois River and assess their invasion potential.  We sampled 250 silver carp using pulsed DC electrofishing.  All fish were weighed to the nearest gram and measured to the nearest millimeter.  A cleithrum bone was extracted from each fish for the purpose of age determination.  Each cleithrum was cross-sectioned to 640 micrometers using an Isomet Low Speed Saw, and age was estimated using under a dissecting microscope using transmitted light.  Silver carp ranged in size from 376 mm to 717 mm, with the average being 489.9 mm.  Ages ranged from 0 to 6 years old, with an average of 1.65 years old.  One and two year old fish represented the majority of the population, 44.8% and 41.6% respectively.  This population shows rapid growth during the early years of life reaching an average of 460 mm for age 1 fish.  Adult mortality was found to be high after age 2, but this may be due to insufficient sampling methods for larger individuals.  Regular collection of  age, growth, and mortality statistics for the silver carp provide state and federal agencies with up to date information to make management decisions.  We will continue sampling the Illinois River silver carp population over the next 1.5 years and will increase our effort to include the Wabash River.