Abstract
Tidal fluctuations in salinity, water temperature, turbidity and distributions of three dominant fish species (the freshwater demoiselle Neopomacentrus taeniurus, cardinalfish Fibramia amboinensis, and jungle perch Kuhlia rupestris) detected by visual transect surveys were investigated in the Udara River estuary of Amitori Bay around Iriomote Island, southern Japan in March and June 2013. The estuarine species N. taeniurus frequently occurred at several depth levels in the lower and middle estuaries with high salinity (29-34‰) at high tide, and at deeper water depths (below 50 cm) with high salinity (ca. 29‰) at low tide. The estuarine species Fibramia amboinensis was restricted to the middle estuary with a salinity range of 19-29‰ at both tides. The catadromous Kuhlia rupestris was restricted to the upper estuary with low salinity (ca. 3‰) at both tides. These results suggest that responses to tidal salinity fluctuations were species-specific in the tropical estuary. Some species actively relocated to select the most suitable salinity levels in accordance with the tides, while others remained in a spatially limited area at both tides, tolerating a broad range of salinity or adapting to particularly low salinity.
