Changes of Shoreline Position and Sand Grain Size at Kashimanada Coast and Their Effects to Distribution of Hard Clam Juveniles, Meretrix lamarckii

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  • 鹿島灘砂浜域の地形と底質粒径の経年変化およびそれらがチョウセンハマグリ稚貝の分布に及ぼす影響について
  • カシマナダ スナハマイキ ノ チケイ ト テイシツ リュウケイ ノ ケイネン ヘンカ オヨビ ソレラ ガ チョウセンハマグリチガイ ノ ブンプ ニ オヨボス エイキョウ ニ ツイテ

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Time variations of coastal bathymetry and sand grain size were investigated at the north Kashimanada Coast, a 40km long section of coast located between Oarai Port and Kashima Port. From the change of coastline position obtained from aerial photographs in 1975 and 1984 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. The central area of the Kashimanada Coast was shown to be slightly depositional. Grain size distributions at the shoreline obtained from surface layer samplings in 1982, 1998 and 2003 showed a tendency that grain size became coarser in the eroded areas and that the areas were expanding. From analyses of the bathymetrical surveys, it could be considered that in almost 2/3 of the coast, offshore longshore sandbars diminished from 1984 to 2004. From the long-term change of distributions of juveniles of Hard Clam, Meretrix lamarckii, along the Kashimanada Coast, it was found that the number of juveniles had been remarkably reduced since 1996 and that juveniles are now distributed at only the depositional beaches next to Oarai Port and Kashima Port, whereas in 1993 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 juveniles, those changes of physical environments were considered to be responsible for the loss of juveniles at the Kashimanada Coast. The continuous change of the environment suggests that the suitable habitat area for juveniles of the Hard Clam may be further decreased.

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