Predator-induced modifi cations to diving behavior vary with foraging mode

Breath-hold divers are strongly interacting species whose top – down infl uence on aquatic communities is shaped by factors governing their diving decisions. Although some of these factors (e.g. physiological constraints, energetic needs) have been scrutinized, the possibility that predation risk infl uences diving behavior has been largely overlooked, and no study to date has asked if anti-predator responses by divers depend on foraging mode. We contrasted dive cycle changes by herbivorous dugongs Dugong dugon using two foraging tactics – cropping, which always permits anti-predator vigilance, and excavation, which limits surveillance at depth – in response to temporal variation in tiger shark Galeocerdo cuvier abundance. Dugongs responded to increasing shark abundance (one component of predation risk) by diving more frequently without changing their surface times and thereby spending a greater proportion of time at the surface, but only while excavating. When threatened, in other words, excavating dugongs sacrifi ced foraging time at depth to facilitate shark detection. In contrast, cropping dugongs at risk from sharks were able to continue diving and foraging normally. By implication, future studies should consider the infl uence of predation risk on diving decisions, even by large-bodied species, and the possibility that behavioral responses by divers to predators may vary with foraging mode.