Abstract
Effects of river mouth closure on fish fauna were studied in 11 rivers in the northern region of Okinawa Island. We investigated the occurrence of fish species and recorded physical environmental variables, i.e., water level, flow velocity, water temperature, and the width of the river mouth at high water, in the tidal river areas over four seasons. There were significantly fewer fish species in rivers with completely closed mouths than in those with open or incompletely closed mouths, particularly those species depending strongly on the sea and tidal river areas. This was presumed to be due to low salinity and obstruction of fish migration. Among the rivers with completely closed mouths, marine fish species occurred only in the Zatsun River, where the salinity changed with the tide. This shows that marine fish species are able to survive in tidal rivers, even when the mouth is completely closed. In two rivers with completely closed mouths, no larvae or juveniles of Rhinogobius species were found in the spring season, suggesting that the closed mouths obstructed the migration of these species. There were significant positive correlations between the number of fish species and the range of water-level fluctuation and the width of the river mouth at high water, two indications of the degree of river mouth closure. We suggest that the water-level range and the width of the river mouth at high water provide valid indices of the effects of river mouth closure on fish fauna.
