Incorporating antecedent conditions into environmental flow delivery can improve restoration outcomes for native freshwater fish

Flow regulation is a major threat to freshwater ecosystems. Managers use periodic water allocations, termed environmental flows, to migrate the effects of flow regulation on fish. The effectiveness of environmental flows has been studied in some systems, but the role of flow conditions prior to water allocation (i.e. antecedent conditions) is rarely quantified. We evaluated the effects of floodplain inundation due to variable river flows on fish abundance in wetlands of the mid-Murray River, Australia, using a hierarchical multi-species model parameterized to estimate abundance while accounting for variable detection. We evaluated the effects of antecedent conditions on three time scales including patterns in long-term (five years prior), medium-term (one year prior) and short-term (three months prior) wetland inundations. We found species-specific differences in the importance of antecedent flows, but found a general divergence in the response of native and non-native species. Native species tended to respond positively to consistent frequent inundation of wetlands while non-native species tended to respond positively to long / medium-term dry periods and short-term wet periods. We also found that detection probability varied among species, sampling gears, wetland areas, and wetlands indicating that accounting for variable detection can be critical when evaluating patterns in fish abundance. Our study highlights the importance of frequent floodplain inundation for the recruitment of native fishes and the control of non-natives. Future research of environmental flows for managing fish assemblages should consider antecedent conditions and apply models that account for incomplete detection to better inform the management process.