Effects of an extreme temperature event on the behavior and age structure of an estuarine top predator, Carcharhinus leucas

Abstract

The frequency of extreme environmental events is predicted to increase in the future. Understanding the short- and long-term impacts of these extreme events on large-bodied predators will provide insight into the spatial and temporal scales at which acute environmental disturbances in top-down processes may persist within and across ecosystems. Here, we use long- term studies of movements and age structure of an estuarine top predator — juvenile bull sharks Carcharhinus leucas — to identify the effects of an extreme 'cold snap' from 2 to 13 January 2010 over short (weeks) to intermediate (months) time scales. Juvenile bull sharks are typically year- round residents of the Shark River Estuary until they reach 3 to 5 yr of age. However, acoustic telemetry revealed that almost all sharks either permanently left the system or died during the cold snap. For 116 d after the cold snap, no sharks were detected in the system with telemetry or captured during longline sampling. Once sharks returned, both the size structure and abundance of the individuals present in the nursery had changed considerably. During 2010, individual long- lines were 70% less likely to capture any sharks, and catch rates on successful longlines were 40% lower than during 2006�2009. Also, all sharks caught after the cold snap were young-of-the- year or neonates, suggesting that the majority of sharks in the estuary were new recruits and sev- eral cohorts had been largely lost from the nursery. The longer-term impacts of this change in bull shark abundance to the trophic dynamics of the estuary and the importance of episodic distur- bances to bull shark population dynamics will require continued monitoring, but are of consider- able interest because of the ecological roles of bull sharks within coastal estuaries and oceans.