COS 304-4 - How does altering size-selectivity of removal gears impact control potential for a prolific invasive species, common carp (Cyprinus carpio)?

Abstract: Invasive species are one of the greatest threats to native biodiversity and ecosystem function and are frequently the targets of control efforts to mitigate their negative impacts. Control programs for invasive fishes are frequently challenged by density-dependent processes that allow the invasive population to recover from harvest. While density-dependent processes make sustainable harvest of commercial species possible, invasive species control efforts must overcome density-dependent increases in survival and recruitment to effectively reduce target species. Control efforts were able to reduce the population of common carp (Cyprinus carpio) in Utah Lake, UT, by approximately 75% between 2009 and 2017. However, highly age-selective gears and compensatory recruitment have allowed the population to recover, prompting efforts to increase the efficacy of control efforts. Here, we evaluate how much gear selectivity and fishing effort would need to change for mechanical removal to achieve carp control targets in Utah Lake. We use an age-structured population model fit to empirical carp removal and standardized survey data to simulate the response of the carp population to scenarios of alternative gear selectivity and increased removal effort. Simulation results suggest that achieving the management goal of a 75% reduction in carp biomass is unlikely at current effort levels even with improvements in gear selectivity. Achieving the management goal likely requires both improving gear selectivity and maintaining effort greater than the historical maximum levels. Interestingly, our results indicate that current removal efforts are fishing at lower rates than would produce maximum sustainable yield, a frequent target of commercial fisheries incentivized for long-term population maintenance. Results of this research will inform an overall carp management strategy for Utah Lake, identifying the need for expanded efforts in terms of mechanical removal and alternative approaches, and contribute to the growing global effort to improve the outcomes of invasive fish control programs.