The Heart rate and the activities of a red sea bream were measured simultaneously in a saltwater pond. The heart rate was measured using an ultrasonic transmitter. The activities including 2 axes acceleration, swimming speed and depth were measured using a data logger. After sunrise, the fish became active and the heart rate showed 8% increase. Applying the relationship between metabolic rate and ambient water temperature obtained in laboratory trials, the oxygen consumption and the out put volume were estimated to have 18 and 10% increase, respectively. The estimated metabolic rate by applying the equation of motion was an overestimate considering the energy efficiency. While the fish dashed and cruised with tail beat frequencies at 3.8 and 1.8 Hz, respectively, the heart rate was stable.
