In order to correlate the metabolic rate and locomotor activity level of fish, a calorimetric system (Setaram GF 108) was supplemented with a video tracking and motion analysis system. The motion analysis system is based on digital image processing. Once every second, two images (with an interval of 120 ms) are digitized and subtracted. The difference between the two images is used as a measure of the animal’s movement. The activity of different complex movements, like turning, accelerations, fin movements, and branchial ventilation, were thus quantified, The combination of motion analysis with calorimetry gave some interesting results, Firstly, during extreme hypoxia, tilapia does not become lethargic as has been described for the anoxia-tolerant crucian carp [1]. The locomotor activity level during severe hypoxia corresponded to the locomotor activity level during normoxia in the restricted area of the calorimetric vessel. This implies that the calorimetrically determined reduction of the heat flux by 50% under these conditions can be ascribed to a reduction in the cellular energy metabolism - metabolic depression. Secondly, the metabolic rate under constant light conditions was elevated from 11-18%, and the animals showed strong fluctuations in the heat flux; periods of aerobic metabolism alternated with periods of anaerobic metabolism. This was in contrast to the experiments under constant dark conditions in which the metabolic rate was around the standard metabolic rate (SMR). Under the applied conditions, no correlation was observed between heat production measurements and locomotor activity. This may possibly be ascribed to the limited size of the calorimetric vessel in which the animals’ metabolic rates were around SMR. The observed oscillations in metabolic rate under light conditions could be another disrupting factor; oscillations in the circulation and ventilation could be responsible for this phenomenon.