Predation Risk Influences the Diving Behavior of a Marine Mesopredator

Exploring factors that influence diving behavior is critical to understanding energy budgets, habitat use, and exploitation rates of prey. Optimal diving behavior studies have focused primarily on trade-offs between oxygen recovery at the surface and energy intake at depth. General predictions from these models are often supported by empirical data, but a mismatch exists between theory and data that has led to increasingly complex models. Despite the importance of nonconsumptive predator effects in terrestrial and marine communities, the possibility that predation risk induces changes in diving behavior has only recently been recognized. We tested whether pied cormorants (Phalacrocorax varius) modify their diving behavior in response to spatio-temporal variation in tiger shark (Galeocerdo cuvier) abundance in the relatively pristine seagrass ecosystem of Shark Bay, Australia. As theory predicted, cormorants reduced the duration of the most dangerous component of the dive cycle by reducing the proportion of time spent at the surface as predation risk increased, but only in the most dangerous habitat. Contrary to model predictions, cormorants accomplished this reduction by increasing dive durations while maintaining similar post-dive surface intervals (leading to lower diving rates). By implication, foraging cormorants may be working harder during high-risk periods and in high-risk habitats to minimize their exposure to predators at the surface. Our finding that cormorants modify their diving behavior in response to spatial and temporal variation in predation risk suggests that the effects of predators on diving species may be greater, and manifest through more pathways, than is currently appreciated. Future studies of diving species, including those considered “top predators,” must explicitly consider the potential importance of predation risk. Furthermore, diving behavior as an index of patch quality should be used cautiously when divers are threatened by predators, which is often