Species composition of fish in the shallow water at Daecheon beach was
determined using monthly samples collected by a beach seine from April 2010 to March 2011. These data were compared to the data obtained from the same water using the same sampling method in 1984~85 just after dike construction in Cheonsu Bay and in 1995~96 in order to assess the impact of the environmental change by dike construction on the fish assemblage. A total of 30 species, 1,464 individuals and 17,808 g of fish were colllected during the study. Fish collected were mainly composed of small-sized fishes and juveniles. Dominant species were Chelon haematocheilus (49.3% of the total number of individuals), Takifugu niphobles (13.7%) and Paralichthys olivaceus (10.0%). These 3 dominant species comprised 63% in the total number of individuals and 52% in biomass. The number of species and abundance of fish were high between May to October and low in cold sampling months. Pelagic fish such as Sardinella zunasi and Thryssa kammalensis were abundantly collected in 1984-85, but their catch was gradually decreased as time and few pelagic fish were collected in 2010~11. The benthic fish were most abundant in 1984~85, and Favonigobius gymnauchen and Kareius bicoloratus were predominated in abundance. Benthic fish was gradually decreased in abundance as time. Its species composition did not greatly change by 1995~1996, but it was significantly changed in 2010~11. In 1984~85 samples, Sillago japonica and Konosirus punctatus were predominated among semi-benthic fish. Species composition of this group was slightly changed by 1995~96, but it was also significantly changed in 2010~11 samples, in which Chelon haematocheilus and Takifufu niphobles were predominated. After construction of dike in Cheonsu Bay, water quality has been gradually degraded by eutrophic freshwater input during rainy season, which impacted the nursery function of the pelagic migrant fish. The benthic fish was decreased by degradation of benthic environment by the accumulation of fine sediments and organic matter. However, euryhaline estuarine fish such as Chelon haematocheilus tolerating the poor water quality increased recently.