Using eDNA, sediment subfossils, and zooplankton nets to detect invasive spiny water flea ( Bythotrephes longimanus )

In light of the ongoing spread and adverse impacts of invasive species, there is an urgent need to develop more effective monitoring and management strategies. Such efforts are constrained by our limited capacity to efficiently detect invasive species. Here, we present the case of Bythotrephes longimanus (spiny water flea) invasion into Wisconsin lakes. Detecting Bythotrephes has proven to be challenging due to its capacity to persist at low densities and its highly seasonal population dynamics. We use Bythotrephes to explore detection using three monitoring methods: zooplankton net tows, environmental DNA (eDNA), and sampling of Bythotrephes tail spine subfossils in sediments. Detection probabilities were highly seasonal for both the net tow and eDNA sampling methods—though detections occurred one to two weeks earlier in net tows—and seasonal targeting substantially improved detection by both methods. Conversely, Bythotrephes spine subfossils were found in all 10 lakes with confirmed Bythotrephes populations and in all five samples taken from each lake, except for a single lake where four of the five samples had subfossils. This method was insensitive to seasonally varying population densities as sediments integrate over variation in population densities. In this case, detection and abundance estimation were well covered by sediments and zooplankton nets, respectively, and eDNA provided little additional benefit to surveillance. Our work highlights the importance of choosing methods that address both species life history and monitoring objectives when designing surveillance programs.