Shark attacks on bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia: Attack rate, bite scar frequencies and attack seasonality.

Shark predation may have been a central factor influencing the evolution of sociality in dolphins, as well as a determinant of dolphin habitat use and behavior. To understand the role of predation in driving interpopulation d ifferences in behavior and sociality, it is important to quantify differenc es in predation risk among populations. This study describes the frequency of shark-inflicted scars and estimates the shark attack rate on bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia. Shark bite sca rs were found on 74.2% (95 of 128) of non-calves, and most of these scars w ere inflicted by tiger sharks (Galeocerdo cuvier). Although there were no d ifferences among age/sex classes in the frequency of scarring, significantl y more adult males than adult females bore multiple scars. The rate of unsu ccessful shark attack was estimated to be between 11% and 13% of dolphins a ttacked each year. Large sharks (>3 m) were responsible for a disproportion ate number of attacks. However, bites from small carcharhinid sharks on 6.2 % of dolphins suggest that some of these small sharks may be dolphin ectopa rasites. Both the scar frequencies and attack rate suggest that Shark Bay d olphins face a greater risk of predation than bottlenose dolphins in other locations.