Evidence of Partial Migration in a Large Coastal Predator: Opportunistic Foraging and Reproduction as Key Drivers?

Understanding animal movement decisions that involve migration is critical for evaluating population connectivity, and thus persistence. Recent work on sharks has shown that often only a portion of the adult population will undertake migrations, while the rest may be resident in an area for long periods. Defining the extent to which adult sharks use specific habitats and their migratory behaviour is essential for assessing their risk of exposure to threats such as fishing and habitat degradation. The present study used acoustic telemetry to examine residency patterns and migratory behaviour of adult bull sharks (Carcharhinus leucas) along the East coast of Australia. Fifty-six VR2W acoustic receivers were used to monitor the movements of 33 bull sharks in the central Great Barrier Reef (GBR). Both males and females were detected year-round, but their abundance and residency peaked between September and December across years (2012-2014). High individual variability in reef use patterns was apparent, with some individuals leaving the array for long periods, whereas others (36%) exhibited medium (0.20-0.40) or high residency (> 0.50). A large portion of the population (51%) undertook migrations of up to 1,400 km to other coral reefs and/or inshore coastal habitats in Queensland and New South Wales. Most of these individuals (76%) were mature females, and the timing of migrations coincided with the austral summer (Dec-Feb). All migrating individuals (except one) returned to the central GBR, highlighting its importance as a potential foraging ground. Our findings suggest that adult bull sharks appear to be highly dependent on coral reef resources and provide evidence of partial migration, where only a portion of the female population undertook seasonal migrations potentially to give birth. Given that estuarine habitats face constant anthropogenic pressures, understanding partial migration and habitat connectivity of large coastal predators should be a priority for their management.