Distribution expansion of Gray mangroves, Avicennia marina and A. alba, in Haneji Inland Sea and around Yagaji Island, and consideration on the eradication of these species.

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  • 羽地内海及び屋我地島周辺におけるヒルギダマシ Avicennia marina 及びウラジロヒルギダマシ A. alba の分布拡大と駆除に関する考察

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Abstract

Avicennia marina is a domestically introduced mangrove species in Okinawa Island and its natural northern limit distribution is Miyako Island. Propagation of this mangrove in Okinawa Island was induced by plantation activities as well as by natural reproduction from the parent trees. Planted A. marina was found in Haneji Inland Sea, around 2000, and rapidly expanded over the wetlands in the vicinity. Another Avicennia mangrove, A. alba, was also confirmed in the same area as well. In this study, distribution of these two species was compared between 2016 and 2021, and inhibition of expansion of these mangroves, namely eradication, is considered. The volume of distribution increased 1.55 folds from 2016 to 2021 and the newly distributed areas were also added to this research. Mainly increased areas were in the vicinity of Warumi Strait, in the west of the inland sea. The increased volume of these areas were more than 9.47 folds, which is profoundly higher if compared to the value less than 1.92 folds in the main distribution areas of Yohena wetland and vicinity areas. Three categories of eradication difficulty levels, A to C, were conceived. Newly added areas of this study were categorized levels B and C, where the areas could be eradicated by small scale manual labor. Distribution of these areas showed rapid expansion, which underlines the importance of these areas because the inhibition of distribution expansion is these areas serves as a key to control the overall expansion.

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