Factors influencing detection of invasive Red Swamp Crayfish Procambarus clarkii (Girard, 1852) in Michigan ponds

Red Swamp Crayfish Procambarus clarkii (Girard, 1852) (RSC) are a highly invasive species established on five continents. Recent detections of RSC populations in several locations in Michigan, USA spurred action to develop trap-based early detection and response plans based on the best available science, however, few recommendations are available concerning important sampling logistics such as timing and density of deployed traps. We surveyed 231 waterbodies (mainly lentic systems with relatively few small lotic systems) for RSC in two invasion zones in southern Michigan. We used Bayesian hierarchical models to evaluate baited traps for detection of RSC invasions, and qualitatively evaluated feasibility of eDNA metabarcoding for enhancing trap-based detection strategies. While overall occupancy was low, predicted detection probabilities with baited traps ranged from 27 to 98% and depended on season, density of traps, and relative crayfish abundance. There was high agreement between trapping and eDNA detections (94%). However, an improved eDNA sampling design that increases the number of samples is needed. We conclude that trap-based detections of RSC in Michigan are most likely to occur mid-summer, trapping densities should be increased in the spring and fall to offset lower detection probabilities, and eDNA could feasibly supplement trapping in spring and fall.