Cumulative Modifications of the Endangered Horseshoe Crab Tachypleus tridentatus Habitat by Small-Scale Developments at Tsuyazaki Cove in Fukuoka, Japan

Possible reasons for the decline of Tachypleus tridentatus in relation to cumulative developments of its habitat were examined by the estimation of the changes in potential spawning sites, combined with field investigation results in Tsuyazaki, Fukuoka, Japan. The field investigation was conducted from 2005 to 2013. During every high tide from late June to early August each year, we counted the number of T. tridentatus pairs visiting the shallow water of the cove to spawn. The changes in potential spawning sites, or sandbanks, from 1948 to 2010 were estimated by interpretation of aerial photographs. A significant negative relationship between the area of the potential spawning sites and the area of development from 1948 to 2010 was found. The breeding pairs of T. tridentatus declined rapidly over the sampling period from 2005 to 2013. The results of this study suggest that the decline of T. tridentatus can be explained in relation to the development of the bay mouth causing poor sand supply to potential spawning sites, which in turn resulted in the habitat degradation of local populations of T. tridentatus.