Bioenergetics of free-ranging juvenile scalloped hammerhead sharks ( Sphyrna lewini) in Kāne'ohe Bay, Ō'ahu, HI

Abstract

Metabolic and activity rates determined from free-ranging juvenile scalloped hammerhead shark pups ( Sphyrna lewini) in Kāne'ohe Bay, Ō'ahu, HI, were used to develop a simplified energy budget for this population. Five shark pups were tracked using a specially designed acoustic tail beat transmitter. Previous laboratory experiments determined that tail beat frequency (TBF) and water temperature could be used as predictors of instantaneous swimming speed ( U) and oxygen consumption rates (V̇o 2). Sharks carrying transmitters had higher cost of transport than uninstrumented sharks, but because the difference was quantifiable, appropriate corrections could be made for transmitter effects on energy consumption of instrumented sharks. Sharks tracked in Kāne'ohe Bay had an overall average TBF of 70±10 beats min −1 and a U of 0.81±0.1 body lengths s −1, but swam significantly faster at night than during the day. These sharks also exhibited high average metabolic rates (MR) (96±15 kJ kg −1 day −1) compared with other species of sharks previously studied. Sharks tracked during warmer summer months swam slightly faster and exhibited higher MR than one tracked in December. The high MR measured for sharks in the bay indicate that these sharks require a high daily ration. The relatively low caloric value of the most common prey (snapping shrimp, Alpheus malabaricus) suggests that high numbers must be consumed to meet the sharks' daily requirements. Low and/or negative growth rates of shark pups in the field and declining population size over the summer season suggests that a significant percentage of pups in Kāne'ohe Bay may starve as the result of their high MR requirements. Although availability of A. malabaricus may not be limiting in Kāne'ohe Bay, foraging skills of some neonates may be inadequate to enable them to meet their daily energetic needs. However, sharks that are successful in surviving through the winter may actually grow faster in the colder months due to the temperature dependent decrease of their MR and a reduction in conspecific competition as the result of high summer seasonal attrition due to starvation and emigration.