Bull. Jpn. Soc. Fish. Oceanogr. 78(3), Page 169-175, 2014   Fish Community in Shallow Waters of Tidal Reach of the Ohta River, Southwestern Japan: Comparison between a Drainage Channel and a Natural River Kazuki Mishiro1a, Yuji Iwamoto1, 2, Shintaro Inoue1, 3, Takuma Morita1, 4, Ken-ichiro Mizuno1, 2, Yasuhiro Kamimura1, 5, Kotaro Hirai1, 6 and Jun Shoji1† 1 Takehara Marine Science Station, Hiroshima University, 5-8-1 Minato-machi, Takehara, Hiroshima 725-0024, Japan a Oita Prefecture, Eastern Promotion Branch, 768-1 Ankokuji, Kunisaki, Oita 873-0504, Japan 2 Hiroshima Prefectural Technology Research Institute, Ondo, Kure, Hiroshima 737-1207, Japan 3 Customs and Tariff Bureau, Ministry of Finance, 3-1-1 Kasumigaseki, Chiyoda, Tokyo 100-8940, Japan 4 TTC Group Co., Ltd., 686 Kamitaga, Atami, Shizuoka 413-0101, Japan 5 Fisheries Research Agency, National Research Institute of Fisheries Science, 2-12-4 Fukuura, Yokohama, Kanagawa 236-8648, Japan 6 Nagano Broadcasting Systems, Inc., Suwa Branch Office, 1-6-1 Suwa, Suwa, Nagano 392-0004, Japan † E-mail: jshoji@hiroshima-u.ac.jp Fish collection and environmental surveys were conducted in the tidal reach of the Ohta River (a drainage channel: DC, and a natural river: NR) from March 2007 to February 2008. Mean temperature ranged between 8.7 and 28.5°C with inter-river differences <1.0°C in most months. Mean salinity ranged between 3.3 and 20.5 in DC and between 3.8 and 12.3 in NR. Monthly mean salinity was usually higher in DC than in NR, while salinity differences between the two rivers were minimal in January, June, July and December when there was heavy precipitation prior to the surveys. A total of 7,453 fishes belonging to 23 or more species in 16 families were collected by the monthly surveys. Numerically dominant fish species through the survey were Gymnogobius breunigii, Acanthopagrus latus and Cottus reinii in DC and Lateolabrax japonicas, G. breunigii and Gambusia affinis in NR. The percentage of freshwater spawners (i.e. G. affinis and Tribolodon hakonensis) was higher in NR than in DC, indicating that freshwater or low-salinity zones were continuously provided in the tidal reach of NR. Upriver migration of seawater spawners (A. latus and L. japonicus) is suggested to highly contribute to the seasonal increase of fish production in the tidal reach of the Ohta River. Key words: habitat, Hiroshima Bay, occurrence, seasonal change