Evaluation of Human Impact on Estuarine Habitat of Endangered Fish Larvae and Effect Prediction for the Restoration

The aim of this study was to examine the influence of human development of an estuary on an endangered freshwater fish, Plecoglossus altivelis ryukyuensis. Human development that decreases the estuarine area can increase the temperature and the salinity of the habitat of the larval stage of the fish and change the distribution of the food resources of copepods. To investigate the impact of such development on the physical environment, multipoint field observations and 3D flow simulation were conducted in two contrasting estuaries: an estuary that had lost its tidal flats and a conserved estuary. Ecological impacts were determined by estimating the field survival rate of the larva in an experimental model and the density of the copepods statistically. The results of the field investigation of the estuary that had lost its tidal flats showed that a brackish water area with a temperature less than 20 degrees C, which is critical for larvae, was significantly reduced compared to the conserved estuary. According to the model estimates, the area that can be considered as available in the artificial estuary is only one-quarter of that of the conserved estuary. Analysis of the predicted effect of tidal flat restoration showed that the restoration would result in a two-fold increase in the size of the suitable habitat area.