Abstract
The damselfly, Mortonagrion hirosei, was designated as an endangered species by the Environment Agency in 1991, because its habitat. is vulnerable to the effects of land reclamation and river improvement. Preand post-assessments of the habitat of the damselfly were taken at the site of the Tone Kamome Ohashi Bridge project spanning the Tonegawa River, which marks the border between Hasakimachi, Ibaraki Prefecture, and Choshi, Chiba Prefecture. Before the project, the larvae of the damselfly were found abundantly in the reed-covered area of this bridge on the Ibaraki side of the river in October 1998. However, there were no larvae found on this side after the project in December 2001. Also, the adults of the damselfly were not observed in July 2002. The larvae of the damselfly were collected only from the puddle of the depressed ground covered with dead leaves of reed plants. However, the puddles were filled with the alluvial deposits. The level of the habitats of the damselfly after the project was higher than before the project from 20 to 100 mm. Because the water supply from the watercourses and springs was decreased remarkably, the habitats of the damselfly were dried and the salinity of the habitats also became high. It was supported that the extinction of the damselfly on the Ibaraki side of the Tone Kamome Ohashi Bridge was caused by the reconstruction of the watercourse, which was extended and changed into boarded.
