Bioenergetics and swimming efficiency of juvenile scalloped hammerhead sharks, Sphyrna lewini, in Kaneohe Bay, Oahu, Hawaii

Abstract

The goal of this study was to determine the energetic requirements of juvenile scalloped hammerhead sharks (Sphyrna lewini) in Kaneohe Bay, Oahu, Hawaii using a multidisciplinary approach. A large flume/respirometer (635 1) was constructed to determine the critical swimming speeds (Ucrit )' swimming kinematics (tailbeat frequency, tailbeat amplitude, and stride length), and oxygen consumption rates (V02 ) of juvenile sharks over a range of swimming speeds (U). Swimming kinematics were also compared with unconstrained sharks in a seawater pond. These experiments indicated that tailbeat frequency (TBF) and water temperature could be used as a predictor of U and V02 for free-swimming sharks in the field; however, the flume affected the sharks' swimming kinematics at slow speeds. The flume and pond kinematic comparisons were used to correct for flume effects on sharks' V02. An acoustic tailbeat-sensing transmitter was designed and constructed to quantify activity and energy consumption of free-swimming hammerhead shark pups in Kaneohe Bay. Sharks with transmitters behaved similarly to uninstrumented sharks, but incurred a 28% increase in cost of transport due to increased drag from the transmitters. These data were used to correct for the effects of the transmitter on freeswimming sharks in the field. Sharks tracked in Kaneohe Bay with tailbeat transmitters exhibited increased U during dawn and dusk, while sharks tracked in the warmer summer months had higher activity rates and metabolic rates (MR) than a shark tracked during the winter. Sharks tracked in this study had higher MR than those measured for other species of tropical sharks and, as a result, require higher daily rations. Low and negative growth rates determined from sharks in the Bay and declining catch rates over the season suggest that a large percentage of the pups in Kaneohe Bay may starve as the result of their high metabolic requirements. Although prey do not appear to be li~itingi lack of foraging experience compounded by a high daily metabolic demand may explain why sharks lose weight during summer months. Those pups that survive the winter experience lower MR due to seasonal temperature decline and less competition as the result of high neonatal mortality.