Change in fish community composition following weir removal, field observations, and physical habitat simulations

The number of untended weirs has increased in Korea in recent years due to land use changes. The removal of abandoned weirs in streams has been attempted following an agreement between local government and residents. The Gongneung Weir-2 was built for irrigation in the 1970s and was removed in 2006. The objective of this study was to investigate the impact of the removal of Gongneung Weir-2 on the composition of the fish community. The study area was a 900-m-long reach in the Gongneung-cheon River in Korea, and Gongneung Weir-2 was located in the middle of the reach. Before weir removal, field monitoring revealed that five fish species were dominant and accounted for more than 80% of the entire fish community. The composition of the fish community changed significantly after weir removal. Lotic fish became dominant after weir removal, whereas lentic fish were dominant prior to weir removal. Physical habitat simulations (PHSs) for individual dominant fish species were carried out. For the PHS, the River2D model and the adaptive neuro-fuzzy inference system method were used for hydraulic and habitat simulations, respectively. The distributions of the highly suitable portion for each fish species were identified before and after weir removal. The PHS successfully predicted changes in the composition of the fish community after weir removal. The PHSs for the entire fish community and for the lotic and lentic guilds were undertaken and the simulated results were compared with each other. The PHSs for the entire fish community could not account for the less dominant fish species in the fish community.