Within the mid-western zone of the Seto Inland Sea including Hiroshima Bay and Aki-nada Sea, environmental changes such as the recent appearance of tropical/warm temperate fish have been observed. This phenomenon is associated with the warming of the water temperature especially during winter. To obtain basic data on the present conditions of the fish fauna, we conducted a year-long underwater fish census on two shallow water sites around Kashima Island from September 2011 to August 2012. A total of 50 species (24 families) were recorded in our monthly surveys, including three species (Plotosus japonicus, Abudefduf vaigiensis, Oplegnathus punctatus) normally occurring in tropical/warm temperate waters. These three species appeared during the warm water season and subsequently disappeared in winter. We found a positive correlation between the number of fish species and water temperature recorded each month (r = 0.85, p < 0.05). The diversity of fish recorded in the present study was very similar to the diversities of fish in temperate waters. Because of the rapid decrease in species number during winter, we suggest that the low water temperature during winter strongly restricts the number of survivors in the fish community even in the temperate waters of the mid-western area of the Seto Inland Sea.