Population Demographics and Early Life History of Grass Carp in the Upper Mississippi River

Despite having a longstanding presence within the Upper Mississippi River (UMR) basin, there is a paucity of information regarding Grass Carp demographics within this and other introduced areas.  Our objectives were to evaluate Grass Carp demographics in populations inhabiting the UMR basin. Adult Grass Carp were sampled with boat electroshocking and trammel nets while egg and larval Grass Carp were sampled with ichthyoplankton tows in the Mississippi, Des Moines, Skunk, Iowa, and Cedar rivers. In all, 202 adult Grass Carp were collected ranging from 421 to 996 mm. Grass Carp below Lock and Dam 19 (L&D19) were more abundant, slower growing, lower condition, consistently recruiting, and contained a higher proportion of females compared to Grass Carp collected above L&D19. Gonadosomatic index in all locations peaked during May but females with developed gonads were present across all months. Grass Carp larvae were only captured at tributary UMR confluence sites while eggs and larvae were present from late May to late June with peak densities of larvae observed during late May. Grass Carp demographics in the UMR appear to be linked to density-dependent interactions and natural reproduction was documented throughout the region, suggesting populations are self-sustaining.