Metabolism, feeding and cardiac function in pike, Esox lucius
Aspects of the physiological ecology of pike from an upland Scottish loch have been studied. A small laboratory stock was maintained in a healthy, feeding condition to allow measurements of a range of physiological and behavioural parameters. Resting and maximum metabolic rates increase with body size and the relationship between weight and oxygen consumption can be described by regression equations of the form: Y = ax6. The ratio of resting : maximum oxygen consumption (the metabolic expansibility coefficient) increases with body-weight. Heart rate increases during activity and feeding, accurately reflecting fluctuations in metabolic rate; so it can be used as a measure of metabolic rate in the field using heart rate telemetry techniques. For pike in the laboratory, the relationship between heart rate and oxygen consumption (VO2, mg/h standardised to a 500g fish weight) can be described by a significant regression valid for heart rates below 55 beats/min. Post-prandial oxygen consumption was measured directly in small pike (< 300g) or calculated in large pike from heart rate. Peak post-prandial oxygen consumption was found to utilise the total metabolic scope in small pike but not larger individuals. Weight specific peak post-pradial oxygen consumption decreases with increasing pike size; consequently smaller pike have the physiological capability to grow more quickly. In adult pike metabolic scope cannot be fully utilised by a combination of the maximum power demands of food processing and aerobic swimming; there seems to be a generous allowance for the metabolic demands of recovery from burst activity. Analysis of the electrocardiogram (ECG) indicates that pike show a reflex bradycardia in response to most sensory stimuli. Amplitude of the ECG increases after feeding events and following burst activity. The interval between Q and T waves of the ECG decreases as heart rate increases reflecting changes in the time for ventricular systole. A telemetry system for monitoring heart rate and movements of free-swimming wild pike has been developed and field tested. A preliminary track of movements by a pike over several days indicated that activity levels in the wild are low, however the fish was by no means sedentary. Using data from this and other studies of pike, a hypothesis is put forward that whilst young pike tend to maximise growth rates, adults feed well within their physiological limits for feeding and growth over much of the year, even in the presence of excess prey. It is suggested that sub-maximal feeding may have a selective advantage through decreasing mortality probability and increasing lifetime reproductive output.