Bull. Jpn. Soc. Fish. Oceanogr. 78(1), Page 1-12, 2014   Seasonal variation in the fish community and their prey organisms in the Yura River estuary Takashi Aoki1†, Akihide Kasai2, Taiki Fuji1, Masahiro Ueno3 and Yoh Yamashita2 1 Graduate School of Agriculture, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan 2 Field Science Education and Research Center, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan 3 Field Science Education and Research Center, Kyoto University, Nagahama, Maizuru, Kyoto 625-0086, Japan † E-mail: aoki1103@kais.kyoto-u.ac.jp To clarify spatial and temporal variations in the biomass of fishes and their prey organisms in the Yura River estuary, monthly surveys were conducted from April 2011 to March 2012. A total of 3121 fishes from 36 species, from 18 families, were collected. Fish diversity was high from spring to summer, while it was low from autumn to winter. Gobies were most abundant in the estuary, as in other estuaries in Japan. From gut content analyses, copepods, whose abundance was highest in summer, chironomids, amphipods and mysids were important prey items for fishes. Food consumption by fish, calculated from the amount of biomass and growth of fish, was larger in spring and summer than in autumn and winter. It is estimated that the consumption of copepods provisionally exceeds abundance in summer. These results suggest that the high production of zooplankton, such as copepods, contributes to fish growth in the Yura River estuary from spring to summer. Key words: biomass, fish community, gut contents, prey organisms, Yura River estuary