The expanding wall and the shrinking beach: Loss of natural coastline in Okinawa Island, Japan

Okinawa is the largest and most populated island of the Ryukyu Archipelago in southern Japan and is renowned for its natural resources and beauty. Similar as to what has been happening in the rest of the country, Okinawa Island has been affected by an increasing amount of development and construction work. The trend has been particularly acute after reversion to Japanese sovereignty in 1972, following 27 years of post-war American administration. A coastline once characterized by extended sandy beaches surrounded by coral reefs now includes vast portions delimited by seawalls, revetments, and other human-made hardening structures. Additionally, a significant part of coastal Okinawa Island is now constituted by artificially reclaimed land. Nevertheless, the degree of severity of the current situation is unclear, due to the lack of both published studies and easily accessible and updated datasets. The aims of this study were to quantify the extension of coastline alterations in Okinawa Island, including the amount of land-filling performed over the last 51 years, and to describe the coastlines that have been altered the most as well as those that are still relatively pristine. Our analyses were performed using a reference map of Okinawa Island based on GIS vector data extracted from the OpenStreetMap (OSM) coastline dataset, in addition to satellite and aerial photography from multiple providers. We measured 431.8 km of altered coastline, equal to about 63% of the total length of coastline in Okinawa Island. Habitat fragmentation is also an issue as the remaining natural coastline was broken into 239 distinct tracts (mean length = 1.05 km). Finally, 21.03 km 2 of the island’s surface were of land reclaimed over the last 51 years. The west coast has been altered the most, while the east coast is in relatively more natural condition, particularly the northern part, which has the largest amount of uninterrupted natural coastline. Given the importance of ecosystem services that coastal and marine ecosystems provide to local populations of subtropical islands, including significant economic income from tourism, conservation of remaining natural coastlines should be given high priority.