Ontogenetic shifts in movement and habitat use of juvenile pigeye sharks Carcharhinus amboinensis in a tropical nearshore region

Tropical nearshore regions provide habitat for a diverse assemblage of shark species, and although a range of age classes may use these environments, ontogenetic shifts in movement and habitat use remain relatively unclear. The hypothesis of this study was that home range size and range of depths used by juvenile sharks in a tropical nearshore region will increase with age. An array of 58 acoustic receivers deployed in Cleveland Bay, north Queensland, Australia, passively tracked 43 juvenile pigeye sharks Carcharhinus amboinensis of 3 age classes from 2008 to 2010. Individuals were present in the study site for long periods, ranging from 4 to 587 d (median = 134). Juvenile C. amboinensis associated strongly with shallow turbid habitats, and core home ranges of all monitored individuals consistently remained in areas adjacent to creek and river mouths. Significant differences in minimum convex polygon measures of home range revealed that older juveniles used larger areas and undertook excursions from core ranges more frequently than younger juveniles. Movements of all C. amboinensis were related to the tidal cycle, but changes in water depth associated with the tide had the strongest influence on the youngest juveniles. Young-of-the-year individuals constrained their movements to shallower depths than older (2-yr-old) individuals (medians = 205 and 283 cm, respectively), presumably as a refuging strategy and to decrease intraspecific competition. By defining transitions in habitat use behaviour among juvenile sharks, this study provides a better understanding of the implications associated with coastal shark species of multiple age classes using a shared environment.