Current Status of Reproducing Populations of Grass Carp in the United States

Grass carp reproduction in the Mississippi River Basin was first documented in 1975.  Since then, it has become apparent that the fish are well established in the central and southern portions of the basin.  Adult Grass Carp are at least occasionally captured throughout the basin as well as in much of the rest of the United States.  Both sterile triploid and fertile diploid Grass Carp are produced in aquaculture, and are broadly stocked (where legal and sometimes where illegal) for control of vegetation in ponds and reservoirs.  It is thus normally difficult to determine whether a captured Grass Carp originated in aquaculture or if it is the progeny of natural reproduction.   Recent egg and larvae collections, as well as advancements in ploidy measurement, otolith microchemistry, and stable isotopes have allowed a better understanding of where Grass Carp are now currently reproducing.   These methods have not been deployed in all areas where grass carp have been collected, but unexpected results have been found.  We here endeavor to explain what this new information tells us about the current status of reproducing populations of Grass Carp in the United States, including in the Mississippi River, the Great Lakes, and reservoirs.