Modelling the response of common carp (Cyprinus carpio) to natural and managed flows using a stochastic population model

Common carp (Cyprinus carpio; hereafter carp) is a highly invasive freshwater fish that has colonised river basins across the world causing ecological degradation. In regulated rivers, restoration of natural flow regimes to rehabilitate aquatic ecosystems is a primary focus of environmental management, yet some actions (e.g., environmental water delivery or artificial floodplain inundations) may have unintended consequences including favouring carp. We developed a stochastic population model to represent carp population responses to hydrology and management actions. The model uses current understanding of carp life history, including different survival rates of early life-stages for a range of habitat types, to estimate population responses to various flow/inundation scenarios. Using case studies from two major regions of a large, regulated dryland river, we demonstrate that large natural floods and prolonged droughts accounted for the greatest amplitude of projected carp population increases and declines, respectively. Within-channel flow pulses and small floods, which are a primary focus of environmental flows, appeared to have little influence on long-term carp population trajectories. Artificial inundations generated by floodplain infrastructure, however, caused significant carp recruitment compared to baseline scenarios. Large natural floodplain inundations also maintained high carp biomasses and thus likely high impacts on natural ecological values. The carp population model captures the response of carp to flows and artificial floodplain inundation and will inform environmental managers on the likely trade-offs between achieving restoration of natural ecological values and suppressing a globally invasive fish.