EFFECTS OF DAMS AND WEIRS ON THE LONGITUDINAL DISTRIBUTIONS OF PHYSICAL HABITAT AND FISH FAUNA IN THE MIYARA RIVER WATERSHED ON ISHIGAKI ISLAND

Abstract

 Since rivers on an island in general face water scarcity, grey infrastructures have been developed for water use and flood control, of which anthropogenic impact has resulted in severe degradations of aquatic and terrestrial ecosystems therein. It is therefore necessary to conduct comprehensive studies considering multiple aspects including physical environment and natural ecosystems. This study aims at understanding the longitudinal distributions of instream physical habitat conditions and fish species with consideration to water infrastructures in the Miyara river watershed on Ishigaki island. The watershed was segmented into six study zones based on the locations of water-related infrastructures such as dams and weirs as well as tidal effects on river flows. As a result, we observed 29 families, 72 fish species (namely, 7 genuine freshwater fishes, 17 diadromous fishes, 37 peripheral freshwater fishes, and 11 marine fishes) in the Miyara river watershed. Instream physical habitat conditions such as water temperature and water depth showed statistically significant differences among the study zones. The number of fish species was largest in the estuarine zone, and decreased with the distance from the river mouth and the presence of grey infrastructures. The effects of such infrastructures on the longitudinal distributions of fish species were found to be deep, stagnant water flows and the movement barrier at the upstream boundary of tidal dynamics in the river system.