Invasive Carp Reproduction Phenology in Tributaries of the Upper Mississippi River

AbstractInvasive Bighead Carp Hypophthalmichthys nobilis, Silver Carp H. molitrix, and Grass Carp Ctenopharyngodon idella are expanding throughout the upper Mississippi River (UMR) basin. Spawning has occurred in the main‐stem UMR but could be limited to high‐discharge events when dam operations create a free‐flowing river. Alternatively, naturally free‐flowing tributaries could offer alternative habitat for successful reproduction. Our objectives were to examine temporal and spatial variation in adult gonad development, ichthyoplankton densities, and back‐calculated spawn dates of invasive carp collected in three UMR tributaries. We compared ichthyoplankton densities between the upstream and mouth reaches of tributaries and densities between the UMR and adjacent tributaries. Ichthyoplankton samples were collected every 10 d, and adult invasive carp were sampled monthly at nine sites throughout the Des Moines, Skunk, Iowa, Cedar, and Mississippi rivers during April–October 2014 and 2015. Peaks in adult gonadosomatic index, an increase in postspawn females, and ichthyoplankton collections suggested that peak spawning occurred during late May through June, when water temperatures were 18–30°C and channel velocities were at least 0.7 m/s. However, reproduction occurred as late as August in some tributaries. Ichthyoplankton densities were highest in the Des Moines River among tributaries, but tributary densities were similar to the UMR densities, suggesting that both tributaries and main‐stem sites provide suitable spawning locations. Invasive carp reproduction in UMR tributaries provides additional sources of potential recruitment for further population expansion upstream in the UMR and should be considered when devising plans for controlling populations along the leading edge of the invasion.