Reducing uncertainty in the perceived risk of grass carp (Ctenopharyngodon idella) invasion in the Great Lakes: Ploidy, distribution, and ecosystem impact

Background/Question/Methods

The perceived ecological impact of a species motivates policy and management. In the 1960’s, herbivorous grass carp (Ctenopharyngodon idella) was imported to the U.S. as a biocontrol tool for aquatic weeds prior to a thorough assessment of its potential risk to ecosystems. Shortly after its introduction, observations of substantial macrophyte removal and escapes into open systems led to the use of triploid (sterile) individuals, to reduce the risk of reproducing diploids. While this has likely served to reduce potential grass carp abundance in open systems, there remains great uncertainty with respect to its distribution and its impact in open systems. Recently, a number of diploid individuals have been discovered in rivers leading to the Great Lakes. Using recently developed quantitative tools we measure the current perception of C. idella risk by Great Lakes fisheries experts and compare this to quantitative assessments of C. idella potential distribution, known distribution, and ecosystem impacts. We used a questionnaire to evaluate the perceived ecological impact of grass carp by Great Lakes fisheries experts, machine learning to produce environmental niche models, environmental DNA to detect presence, and a random-effects meta-analysis of C. idella to predict the impact of C. idella on the Great Lakes ecosystem.

Results/Conclusions

Expert questionnaire results (n=27) indicate a great deal of uncertainty in the perceived risk of C. idella to the Great Lakes by academic and resource management experts: 30% indicated the perceived ecological impact was “unknown”, 30% of responses were “low”, and the remaining 40% reported “moderate”, “high” or “very high” impacts. The environmental niche model’s mean accuracy (based on 10 established North American populations) was 0.88 (SD±0.05) resulting in C. idella’s potential range in North America encompassing most habitats in the Great Lakes ecosystem. eDNA samples (N = 1646) collected in the southern basin of Lake Michigan and western basin of Lake Erie showed C. idella presence in locations not previously reported in the USGS NAS database, and the capture of 8 diploid individuals show establishment in regions where only triploid distribution is permitted. Meta-analysis results showed C. idella presence and abundance significantly impacted community structure and community process. Our results show that grass carp present a threat to the ecosystem functioning of the Great Lakes, which is not the common perception amongst Great Lakes experts.