Swimming behavior and energetics of sharks

Abstract

In a field study of blue shark swimming behavior, acoustic telemetry was used to record depth, swimming speed and tailbeat frequency from free-ranging blue sharks (Prionace glauca) in northwestern Atlantic slope waters. Records obtained from five sharks show a consistent pattern of vertical migration between the surface and depths as great as 450 meters, with the deepest dives occurring during the daytime and shallower dives at night. Mean swimming speed was 44.5 ± 1.6 (X±S.E.) cm.s-1 (0.179 ± 0.014 lengths.s-1) for three sharks, with short bursts up to 180 cm.s-1. Mean tailbeat frequency was 0.335 ± 0.021 beats.s-1. Measurement of swimming speed and rate of vertical movement during dives permits calculation of angles of ascent and descent. For 84 dives deeper than 50 m, the descent angle averaged 8.0 ± 0.7 degrees from the horizontal while the ascent angle was 6.4 ± 0.5 degrees. Tailbeat records indicate that blue sharks actively swam downward during most of the descent, with brief periods of gliding which appear to be associated with the most rapid descent rates. The observed diving behavior does not match that predicted by theory to be energetically optimal for migration, and may instead represent a strategy for encountering and capturing prey. Heart rate, metabolic rate and activity were simultaneously recorded in the laboratory from lemon sharks (Negaprion brevirostris) during rest and spontaneous exercise, and from leopard sharks (Triakis semifasciata) during steady swimming at controlled speeds to evaluate the usefulness of heart rate as a measure of field metabolic rate. Heart rate was monitored by acoustic telemetry using a frequency modulated ECG transmitter, and metabolic rate was measured as oxygen consumption rate. For seven lemon sharks at 25°C, mean resting values for heart rate and oxygen consumption rate were 52.0 + 0.4 (S.E.) beats.min-1 and 162.0 ± 2.0 (S.E.) mg02.kg-1hr-1, respectively. Both parameters increased significantly (p<.001) during swimming, to means of 55.9 ± 0.2 beats.min-1 and 233.6 + 2.3 mg Oz.kg-1hr-1, at a mean swimming speed of 0.400 ± 0.003 body heart rate and oxygen consumption rate were 36.6 + 1.8 beats.min-1 and 105.3 ± 35.6 mg Oz.kg-1.hr-1. While swimming-at the maximum sustained speed (0.84 ± 0.03 lengths.s-1) for 30-60 minutes, these rates were 46.9 + 0.9 beats.min- and 229.3 + 13.2 mg Oz.kg-1.hr-1. The observed elevations in heart rate from rest to exercise account for 20% of the increase in oxygen uptake in the lemon shark and 32% in the leopard shark, leaving the remainder to be brought about by increases in stroke volume and/or arteriovenous oxygen difference. Significant linear regressions of oxygen consumption rate on heart rate were obtained for both lemon sharks and leopard sharks; separate regressions were obtained for individual lemon sharks . Heart rate was approximately as closely correlated to oxygen consumption rate as was swimming speed.