Assessing the risk of diploid grass carp Ctenopharyngodon idella in the certified triploid supply chain in Ohio

Non-native grass carp (Ctenopharyngodon idella) have been stocked in the United States for vegetation control since the 1970s, and recent evidence suggests some natural reproduction in the Great Lakes basin. Despite all states and provinces bordering Lake Erie either banning grass carp or requiring imports of only sterile, U.S. Fish and Wildlife Service (USFWS)-certified triploids, the majority of grass carp captured and analyzed from Lake Erie are diploid, or reproductively viable. Potential sources of diploid grass carp include illegal importation, compromises in the USFWS-certified triploid shipments, migration from legal diploid states, or natural reproduction resulting from diploid stockings prior to the 1988 Ohio law requiring only certified triploids. The goal of this study was to explore the risk that diploid grass carp occur in the USFWS-certified triploid supply chain destined for Ohio. During 2015 and 2016, undercover Ohio Department of Natural Resources-Division of Wildlife law enforcement purchased 1200 grass carp from 16 distributors, and overnighted 80 dissected grass carp head and eyeball sample shipments (n = 15 fish per shipment) for ploidy analysis by flow cytometry. Standardized methods for both field and laboratory processing were established. No diploid grass carp were detected in these collections, indicating fidelity of the USFWS-certified triploid grass carp supply chain in Ohio. Thus, these shipments are not a likely source of diploid grass carp in Lake Erie. This study is the first large-scale evaluation of the potential for ecological risk from diploid grass carp occurrence in USFSW-certified shipments of triploids for national distribution.