CONSERVATION PLANNING FOR ENDANGERED SPECIES OF HORSESHOE CRAB IN TERMS OF GEOMORPHOLOGY OF HABITATS AND LIFE HISTORY

摘要

The relationship between the habitat conditions of the horseshoe crab Tachypleus tridentatus, one of the endangered species in Japan, and the artificial changes in a bay was investigated, taking Moriye Bay in Oita Prefecture as an example. A mitiga-tion method for restoring the spawning sites of the horseshoe crab was proposed. Since the tidal flat has a vast area and is exposed for only a short period during the ebb tide, comprehensive field observation is difficult. Therefore, an investigation method using high-resolution aerial photographs taken at low altitude was developed. A precise classi-fication of the surface of the tidal flat in terms of its micro-geomorphology was carried out. The impact of river flooding on the tidal flat was investigated by detailed surveys of the tidal flat, and the habitat conditions of the horseshoe crab were classified. In the lower Yasaka River, the straightening of the meandering part was carried out in 2000, causing an increase in the flood velocity at the river mouth bar, where the spawning site of the horseshoe crab was located, resulting in its disappearance. An appropriate alterna-tive location of the spawning site was selected, and beach nourishment was carried out using the materials obtained from the nearest rivers to minimize the environmental im-pact due to the mixing of different species from other regions. New conservation and in-vestigation methods for the horseshoe crab were proposed, taking their whole life history into account. The survey methods of the habitat were reconsidered, focusing on the fact that large-scale landform changes have entirely altered the geomorphologic features of the habitat.