A Novel Field Technique to Assess Ploidy in Introduced Grass Carp (Ctenopharyngodon idella, Cyprinidae)

Grass Carp, Ctenopharyngodon idella (Cyprinidae), are commonly produced as triploid, sterile, individuals and stocked as a biological control for aquatic vegetation. Diploid fish can result in unwanted fertile populations. Current methods of determining ploidy of an individual require laboratory equipment (Coulter Counter) and are not possible under field conditions. Here we introduce a rapid, inexpensive technique to distinguish diploid fertile versus triploid sterile Grass Carp under field conditions using a compound microscope. We compared blood smears of known diploid and triploid Grass Carp individuals and found that the frequency of abnormally shaped erythrocyte nuclei (dumbbell or teardrop shaped) was significantly higher in triploid animals. This difference was accompanied by larger cell and nuclear volumes and was significantly correlated with Coulter Counter values (the standard measure of ploidy used by the United States Fish and Wildlife Service). Additionally, we field tested our method on a managed, presumably triploid, standing stock of Grass Carp and found that all individuals exhibited proportions of abnormal nuclei typical of triploids. Finally, we used a blinded survey to confirm diagnostic reliability of our method. Examination of blood smears for abnormally shaped nuclei may become a powerful tool in the management of invasive Grass Carp.