Assessing Spawning and Hatch Locations of Naturally Spawned Grass Carp Eggs in a Great Lakes Tributary

Invasive Grass Carp (Ctenopharyngodon idella) have been stocked for decades in the United States for vegetation control. Adults have been found in all of the Great Lakes except Lake Superior, but no self-sustaining populations have yet been identified in Great Lakes tributaries. Previous research suggested natural reproduction has occurred in the Sandusky River; hence we sampled ichthyoplankton using paired bongo net tows June through August 2015 to determine if Grass Carp are spawning. We identified and staged eight eggs that were morphologically consistent with Grass Carp. Five eggs were confirmed as Grass Carp using quantitative PCR and DNA sequencing, while three were retained for future analysis. All eggs were collected during high flow events, either on the day of or 1-2 days following peak flow, supporting a suggestion that high flow conditions favor Grass Carp spawning. From our findings, we used hydrologic modeling to calculate the most likely spawning and hatch locations for these eggs. Preliminary model results suggest eggs were most likely released near the hypothesized spawning site near Fremont, Ohio. Hatch locations ranged from river km 15 into Muddy Creek Bay. These locations will help guide future sampling efforts, inform risk assessments and aid targeted control efforts.