Re-Evaluating the Invasion Risk of a Biological Control Agent (grass carp, Ctenopharyngodon idella): Reducing Ecological Uncertainty With Improved Risk Assessment

Despite the wide use of grass carp as an effective control strategy for nuisance aquatic macrophytes, there has been a great deal of uncertainty in terms of its perceived risk to North American aquatic ecosystems. Recent observations of feral (and sometimes diploid) individuals in the Great Lakes (GL) Basin have spurred interest in re-evaluating the mechanisms by which grass carp interact with ecosystems and a revisitation of national management and control plans. Here we assess the current perception of ecological risk of grass carp to the GL via questionnaire of fisheries experts. We provide an updated ecological risk assessment using ploidy assessment, eDNA surveillance, national occurrence databases, species distribution models and meta-analysis. We found that diploid individuals occur in multiple GL waterways and eDNA surveillance suggests that grass carp are abundant in tributaries of Lake Michigan. Species distribution models predict that grass carp establishment is likely in all 5 of the Great Lakes. Meta-analysis results show that grass carp have altered ecosystem structure and function through direct impacts to macrophytes and water quality and indirect impacts to fish, amphibians, invertebrates and fowl. We recommend a re-evaluation of the use and distribution of grass carp in North America.