Abstract: Control of invasive crayfish populations has been successful in a small number of ponds and lakes but few studies have been conducted in flowing stream or river systems. Procambarus clarkii (red swamp crayfish) is a species of great concern in the Laurentian Great Lakes area due to the recent discovery of established populations in inland and Great Lakes-connected waters. In other systems, introduction of P. clarkii has resulted in reduced abundance of macrophytes and macroinvertebrates and increased competition with native crayfish species. A high-density population is established is the North Shore Channel of the Chicago Area Waterway System (CAWS). The CAWS is a system of canals that connects Lake Michigan to the Mississippi River Basin. We have implemented intensive trapping and removal of P. clarkii to reduce population size in this system. Our goals in this work are to reduce population size so that the chance of spread is lessened and to develop more efficient trapping methods. We have conducted intensive trapping of the population during five consecutive summers (2018-2022) using baited traps. To determine the effects of trapping on population size we compared catch per unit effort (CPUE) in removal reaches to a control reach and used linear regression of the ratio of CPUEs to determine if our removal efforts were significantly affecting population size. The ratio of CPUE did increase significantly in 2018 (two trap reaches; p = 0.001, p = 0.007), 2020 (two trap reaches; p < < 0.005 for both), 2021 (four trap reaches; p < < 0.005 for all), and 2022 (four trap reaches; p < < 0.005 for one reach). For these years, our trapping efforts led to a reduction in P. clarkii CPUE in removal reaches relative to the control reach. We have also worked to improve trapping methods to maximize P. clarkii CPUE. We found that using hotdogs as bait increased catch rates by ~2x compared to other commonly used baits (p = < < 0.005, Dunn test). We have also developed a simple modification to traps that prevents crayfish from leaving them and increases trap catches by ~2x compared to Gee crayfish traps (p< < 0.001, Tukey's HSD). Our study demonstrates that although eradication of an established population of invasive crayfish such as P. clarkii is unlikely in a river system, control efforts can reduce population size and may reduce chance of further spread and ecosystem impacts.
