Energy costs of spontaneous activity in horse mackerel quantified by a computerised imaging analysis

The metabolic cost for spontaneous activity of horse mackerel Trachurus trachurus was studied in an intermittent flow respirometer, characterised by a high temporal resolution of oxygen consumption rates, which made it possible to measure the oxygen consumption rate of fish over short time intervals of 5 min. The swimming behaviour was recorded on video, analysed with a PC-based image-analysis system and related to the observed oxygen consumption rates. The experiments were carried out at 13 °C. Metabolic rates did not increase when fish were swimming at low velocity. However, strong bends of the fish body, mainly when turning around in the respirometer, caused an increase of the metabolic rate, which declined gradually over the next 20 to 30 min. A linear relationship was found between the number of turns of 180° and subsequent increased oxygen consumption. The energetic cost for one turn of a 360 g horse mackerel equals the cost of constant swimming at 0.2 m·s^-1 over a distance of 73.7m.