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.