Habitat use decisions by bottlenose dolphins (Tursiops aduncus) and tiger sharks (Galeocerdo cuvier) in a subtropical seagrass ecosystem

Understanding the factors that determine the spatial distribution of animals is critical to making predictions about how animals will respond to environmental change. Despite this, few studies have attempted to identify factors influencing habitat use decisions of animals in complex natural communities. In this thesis, I use behavioral ecological theory to make specific predictions about how bottlenose dolphins (Tursiops aduncus) should respond to natural variation in food availability and the risk of predation from tiger sharks (Galeocerdo cuvier). I begin by reviewing shark-dolphin interactions worldwide and show that in some regions sharks and dolphins are intraguild predators. Because there is no theoretical basis for making predictions about the spatial distribution of intraguild predators, I created a game theoretical model of intraguild predation. This model shows that the community context of interactions can influence the predicted distributions of predators and their prey. During field studies in Shark Bay, Western Australia, I found that tiger sharks are the greatest threat to dolphins, and over 74% of dolphins bear scars from unsuccessful attacks. However, dolphins are a minor component of tiger shark diets, which are composed mainly of dugongs (Dugong dugon), sea snakes, and sea turtles. Tiger shark catch rates were high in warm months and low in cold months and appear to be influenced by both water temperature and seasonal changes in primary prey density. When present, tiger sharks are found most often in shallow habitats where prey density is highest. Dolphins were found to trade-off food and safety. When sharks were absent, dolphins matched the distribution of their food, but when tiger sharks were present, dolphins foraged in productive shallow habitats less than expected by food availability alone. However, juvenile male dolphins foraged in dangerous habitats more often than other age/sex classes. Interestingly, dolphin habitat use may be influenced indirectly by habitat use decisions of other tiger shark prey species because of their influence on sharks. This thesis shows that behavioral ecological theory can be used in natural communities to understand the factors influencing habitat use decisions and that future research will benefit from considering the community context of behavioral decisions.