Impact of the opening of Moriyama Dike on water quality and aquatic fauna in Honjo Area of Lake Nakaumi, Japan

In May 2009, Moriyama Dike was opened in Honjo area, which lies near Lake Nakaumi along the coast of San-in District, Japan, and is isolated by two dikes for land reclamation. This study was conducted to examine the impact of the opening of the dike on water quality as well as fi sh and crustacean life. We carried out a statistical analysis using monthly water-quality observations and a small catch net survey through 2004 to 2014. The analysis results showed that seawater likely flowed into Honjo, and salinity stratification developed. Furthermore, the oxygen deficient layer became thicker and progressed with prolonged duration. From the catch net survey, we found that the number of species of marine fi sh greatly increased; however, no signi fi cant increase in the catch per net was observed. In terms of fi sh and crustacean types, the number of horse mackerel ( Trachurus japonicus ) and Taiwan blue crab ( Portunus pelagicus ) significantly increased. This was considered to be due to the easy access provided by the opening to the Sea of Japan. However, the catch of rock fi sh ( Sebastes schlegeli ) and spiny goby ( Acanthogobius flavimanus ) , decreased after the opening. Because these species prefer benthic life, it was considered to have been a ff ected by oxygen depletion in the lower layer of the Honjo area.