Responses of fish population structure to barrage re-opening in an estuarine ecosystem

Background Re-openings of estuary barrages for ecosystem restoration are rare globally. The Nakdong River Estuary (NRE), South Korea, provides a unique opportunity to assess the effects of temporary barrage re-opening on fish population structure. Methods We conducted monthly fish population surveys both upstream and downstream of the estuary barrage from 2021 to 2023, categorizing results into closing period (CP) and opening period (OP). Results During the OP, we observed shifts in fish distribution, including decreased proportions of non-indigenous species and altered habitat distributions. Additionally, we observed an increase in brackish water species populations and their upstream movement. Monthly frequency analysis revealed that brackish and non-indigenous species exhibited high frequencies mainly during OP (July to October), while freshwater (demersal) species showed low frequencies during the same period. Discussion Our study highlights the significant impact of temporary estuarine ecosystem restoration on fish distribution and emphasizes the need for careful consideration of both frequency and duration of estuary barrage re-openings.