SOME ISSUES OF RIVER INPROVEMENT SUGGESTED BY CONSERVATION PLAN OF ENDANGERED MOLLUSK Ellobium chinense IN UPPER TIDAL ZONE OF ESTUARY - THE EXAMPLE OF YASAKA RIVER IN OITA PREFECTURE

Abstract

Ellobium chinense habits in estuary and is one of the endangered molluscan species. The relations between the habitat conditions such as distribution and density, and physical factors as salinity, morphological features, bed materials and vegetation were investigated in Yasaka River in Oita Prefecture. Creating new alternative habitats was planned as conservation. This species habits in the upper tidal zone of 0.8-1.2 m above the MSL where is often lost by river improvement. The upper tidal zone in estuary should be conserved from the standpoints of its endemic species, food web and biodiversity.