Ti 皿 e variations of coastal bathy 皿 e 亡 ry and sand grain size were investigated at the north Kashimanada Coast, a 40km long section o 工 coast located be 亡 ween Oarai Port and Kashima Port. From the change of coastline position obtained from aerial photographs in 1975 and l984 and bathymetrical surveys from 1984 to 2004 , it was found that the beaches located just next to Oarai Port and Kashima Port had been depositional and the areas next to those depositional beaches had been heavily eroded

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... The central area of the Kashimanada Coast was shown to be slightly depositional . Grain size distribu 亡 ions at the shoreline obtained from surface layer samplings in 1982,1998 and 2003 showed a tendency that grain size becarne coarser in the eroded areas and that the areas were expanding . From analyses of the bathymetricaL surveys , it could be considered that in a ! most 2 /30f the coast , offshore longshore sandbars diminished 丘 rom 1984 to 2004 . From the long − term change of distributions of juxJeniles of Hard Clam . Meret 」 ' ix tamai ' ekii , along the Kashimanada Coast, it was found that thc number of juveniles bad been remarkabLy reduced since l996 and tha匸 juvenlles are now distributed at only the depositional beaches next to Oarai Port and Kashima Port, whereas in l993 and earlier , juveniles were distributed in wide areas of the Kashimanada Coast , especially at the central area . From the coincidence of the areas of beach erosion , grain − size increase , sandbar extinction and decrement of number of luveni 亘 es , those changes of physical environments were considered te be responsible for the ioss of juveniles at the Kashimanada Coast. The continuous change of the environment suggests that the su 正 table habitat area for juveniles of the Hard Clam may be further decreased ,