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

Invasions by Asian carps have become a high profile topic due to the threat they pose to native ecosystems. One species, Grass Carp, Ctenopharyngodon idella, is commonly produced as triploid, sterile, individuals and stocked for biological control of unwanted aquatic vegetation. Sale and transport of diploid fish can result in unwanted fertile populations. Current methods of determining ploidy require the use of laboratory equipment and are not possible to perform under field conditions. Here we introduce a rapid 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 individuals, finding that the frequency of abnormally shaped erythrocyte nuclei is significantly higher in the latter. This difference is accompanied by larger cell and increased nuclear volumes, and it is significantly correlated with Coulter Counter values. Additionally, we field tested our method on a triploid stock of Grass Carp and found that all individuals tested exhibited proportions of nuclei typical of triploids. Finally, a blinded study was used to confirm reliability of our visual assessment of ploidy. Examination of blood smears for abnormally shaped nuclei may become a powerful tool in the management of invasive Grass Carp.