Heart rate and muscle performance during forced swimming of jack mackerel Trachurus japonicus at different temperatures

The physiological condition of jack mackerel [18.3 ± 1.2 cm fork length (FL), n = 24] was simultaneously monitored using electrocardiography (ECG) and electromyography (EMG) techniques during swimming exercise in a flume tank at different temperatures of 10, 15 and 22 °C. To monitor the effect of temperature on the threshold level of the maximum sustainable speed, EMG demonstrated that the initiation of ordinary muscle activity at the lowest temperature (10 °C) occurred at swimming speeds of 3.2–3.5 FL/s, which was slower than the activation speeds at 15 and 22 °C of 4.0–5.4 FL/s. Ordinary muscle became active with increasing swimming speed which subsequently resulted in the frequency, amplitude and duration of burst discharge. ECG and EMG monitoring showed that the heart rate started to increase at the sustained speed level before the threshold and markedly increased up to the maximum level of heart rate at the higher speed. The index of ordinary muscle power output at the lowest temperature of 10 °C was lower than at higher temperatures of 15 and 22 °C. The heart rate increased with the increasing power output of ordinary muscle for all temperatures.