Although the ecosystem structure of dam reservoirs has been extensively studied in order to understand water quality in Japan, the habitat and behavior of the fish living in these reservoirs have been neglected. For that reason, studies of these ecosystems based on behavioral ecology have been insufficient. This study aimed to clarify the relationship between the summertime vertical distribution of Japanese dace ( Pseudaspius hakonensis ) and environmental factors in the Miharu dam reservoir (Lake Sakura), Fukushima, Japan. To this end, an acoustic telemetry survey was conducted to track the dace. At the same time, a temperature-dissolved oxygen analysis (hereafter Temp-DO analysis) was conducted to document environmental factors. These surveys were conducted for two periods - from 19 August to 9 September 2019 and from 18 August to 6 September 2020. Six fish with inserted acoustic tags were released, and continuous behavior data were collected for two individuals. The tracked individuals had a similar habitat, inhabiting a depth of approximately 17 m. The Temp-DO analysis revealed that a strong summer stratification was present. Surface temperatures were above 26 °C and hypoxic water was widespread beneath the density cline. These observations demonstrate that the dace were present in 17~18 m depth due to avoidance of both high temperatures and hypoxic water. In dam reservoirs with hypoxic water, dace may reduce oxygen consumption by utilizing depth with lower temperature. In many lake environments, including brackish lakes, restrictions of fish habitat similar to those in this study may occur. Therefore, if ecosystem condition is to be considered in the management of lakes, in addition to water quality surveys, it is necessary to understand the details of fish ecology as is demonstrated in this study.