Results of a U.S. National Ecological Risk Assessment for the Grass Carp

The Grass Carp Ctenopharyngodon idella is established in areas outside of its native range in four continents.  Diet of adult and large juvenile grass carps is dominated by macrophytes and invertebrates.  Ecological effects of established and stocked Grass Carp populations vary depending on: abundance and biomass of that species, macrophyte community structure, composition, and biomass; and fish and benthic community structures and compositions within the ecosystem of concern.  Carefully managed triploid Grass Carp populations have benefitted many lake systems by reducing abundance and biomass of nuisance and invasive macrophytes.  However, Grass Carp populations that are either established at, or stocked at, high densities have resulted in ecological impacts.  Negative ecological impacts of some Grass Carp populations include: development of nuisance phytoplankton blooms; decrease in invertebrate abundance and diversity; food web disruption; and reduction in spawning of, and spawning habitat for, centrarchid, esocid, and percid fishes.  The climate match is high between areas where the Grass Carp is native/established as nonnative and the Continental U.S.  Based on the Grass Carp climate niche in the Continental U.S., and the documented history of invasiveness, that species was assessed as high risk of establishment in, and ecological impacts to, much of the Continental U.S.