Ultra sonic transmitters (cylindrical, 1.5 cm diameter × 7 cm long) with three‐week battery life were used to examine the heart rate and electrocardiogram (ECG) of cod in swimming experiments. The maximum heart rate was observed when the period between the recovery (T) wave and the initial wave (P) of the ECG was zero and was limited by the P to T period which was not observed to vary in an individual. Low heart rates were observed in resting fish and maximum rates were associated with exhaustion and cruising swimming. Maximum rates continued during periods of oxygen debt repayment and during anaesthesia as long as oxygen was supplied to the gills. Brief inhibition of heart rate was associated with sudden stimuli and during maximum swimming exertion. The significance of rate change and its relation to the aerobic and anaerobic states is discussed.
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Wardle et al. (1973) studied this question.
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