Tracking ghosts: combined electrofishing and environmental DNA surveillance efforts for Asian carps in Ontario waters of Lake Erie

Effective control or eradication of invasive species depends on their early detection at low abundance to enable timely management responses. In aquatic environments, the detection of rare or low-abundance species is challenging at best, and discriminating between detection failure (null) and true absence (zero) can be difficult. We developed and applied environmental DNA (eDNA) markers to screen for occurrences of bighead carp (Hypophthalmichthys nobilis), silver carp (H. molitrix), and grass carp (Ctenopharyngodon idella) in Ontario waters of Lake Erie, Lake St. Clair, and tributaries, as part of ongoing efforts to prevent these Asian carps from becoming established in the Great Lakes. A network of 180 sites in nearshore and tributary habitats, selected based on perceived risk of fish access or habitat suitability, were sampled using both boat electrofishing and/or eDNA sampling (948 water samples) throughout the 2012 open-water season. Electrofishing efforts did not capture any Asian carps, and no positive detections of environmental DNA for any of these species were obtained using both conventional and quantitative PCR. These combined results suggest that these Asian carp species are not yet established in Ontario waters of Lake Erie or Lake St. Clair, and highlight the value of combining conventional fisheries assessment and environmental DNA surveillance for assessing the potential presence of invasive species in freshwater systems.