Biological and environmental effects on activity space of a common reef shark on an inshore reef

Proximity to land and sources of freshwater expose fishes residing on inshore reefs to fluctuating environmental conditions (e.g. high freshwater run-off/low salinity events). However, site attachment in many reef residents, such as reef sharks, could mean that relocation in response to unfavourable environmental conditions may not be feasible. Passive acoustic telemetry was used to monitor movement and space use of 18 blacktip reef sharks Carcharhinus melanopterus on an inshore reef off the coast of Queensland, Australia, to determine their response to environmental change. Activity space of sharks was modelled against combinations of environmental (wind speed, rain, salinity and water temperature) and biological (size, sex) factors. Size was the most influential predictor of space use, with larger sharks having larger activity spaces. Sex also appeared in top-performing models, showing that juvenile males use more space than juvenile females, although effects were marginal. Model results also indicated a relationship between shark activity space and salinity, where space use increased with decreasing salinity. A similar but weaker relationship was observed with water temperature. These results show that blacktip reef sharks respond to minor changes in salinity, suggesting that they may be able to relocate when conditions are unfavourable, and help define the resilience of this species to disturbance and change.