The effect of water temperature on white-spotted conger activity was examined using acoustic accelerometer transmitters to understand thermal effects on the behavior of a target fishery species. We recorded swimming activities of 21 congers using video cameras and compared the body undulation frequency with acceleration values measured using accelerometer transmitters in the body cavity. We found a significant, positive, linear relationship between the acceleration values and activity (body undulation frequency) of congers. We conducted a calibration trial to evaluate the relationships between conger activities and acceleration values measured using accelerometer transmitters, which confirmed the body undulation frequency of congers was associated with its swimming activity. We also examined the effect of water temperature on conger activity levels. The activity levels of eight congers at water temperatures from 10 to 28 °C were tested using accelerometer transmitters. We found a positive relationship between the conger activity levels and water temperatures between 10 and 26 °C, whereas the activity declined between 26 and 28 °C. These results demonstrate that accelerometer transmitters can quantitatively measure changes in the activity of white-spotted conger with water temperature in nature.