Tatara beach is a part of the coast line of lmari port in Saga. It is the famous place where hundreds of horseshoe crabs come to spawn every year. There is a bridge being built just outside Tatara beach, connecting the Seto district and the Kubara district. The construction of this bridge has raised concerns over the possible direct and indirect impact on the lifecycles and behavioral patterns of the horseshoe crabs. We assumed that the dredging of the sea would directly effect the migration of horseshoe crabs spawning in this area, so we conducted a survey on the number of horseshoe crabs at all dredging site. These crabs were released back into the sea after the survey. The construction schedule was also arranged to decrease the impact on spawning. As for the indirect impact, it was assumed that the dredged-up sea bed would hamper the mobility of the horseshoe crabs. To examine this, we attempted investigations, which consisted of tagging and tracking horseshoe crabs by supersonic wave transmitters, estimating state of horseshoe crabs by gill net capture.
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