Changes in prey selection and fitness of non-native Erythroculter erythropterus following estuarine restoration in the Nakdong River, South Korea

Estuarine ecosystems globally are being restored through re-naturalization efforts, including the Nakdong River Estuary (NRE) in South Korea, which partially re-opened in 2019 after decades of disconnection. We investigated how this restoration affected the feeding ecology and fitness of Erythroculter erythropterus (Skygager), a non-native cyprinid that became dominant in the upper NRE following barrage construction. We surveyed fish populations, analyzed stomach contents using Next Generation Sequencing, and assessed morphological changes from May 2018 to June 2022. Following NRE opening, zooplankton and fish diversity indices increased, while phytoplankton and benthic invertebrate diversity decreased. E. erythropterus showed altered prey selection patterns, shifting from diverse prey items to predominantly zooplankton after NRE opening, coinciding with altered vertical migration patterns of plankton communities. While length distribution did not change significantly, the condition factor K decreased significantly, indicating reduced fitness. Body shape parameters also shifted toward more elongated forms, suggesting morphological responses to changed hydraulic conditions and feeding ecology. Our findings demonstrate that controlled estuarine reconnection can affect non-native species through cascading trophic effects, potentially serving both restoration and non-native species management objectives. This study highlights the complex ecological consequences of partial estuarine restoration and the importance of considering multiple trophic levels when evaluating restoration outcomes.