Importance of environmental and biological drivers in the presence and space use of a reef-associated shark

As coral reef ecosystems come under increasing pressure from fisheries and climate change, understanding how species that rely on these habitats respond to changes within their environment is increasingly important. The presence and movement of 28 grey reef sharks Carcharhinus amblyrhynchos were monitored for 15 mo in the southern Great Barrier Reef in relation to environmental conditions and biological factors. Twenty-eight models including water temperature, atmospheric pressure, wind speed, rainfall and time (week or month), size and sex were tested against presence and activity space data and compared using Akaike information criterion (AIC). Model results for presence of individuals in relation to environmental conditions indicated that temperature and wind speed had weak effects, likely related to detectability. Models using 3 different measures of activity space indicated no relationship between shark activity space and environmental parameters. Week was a significant factor in the top performing activity space models, with males showing increased activity space during weeks in September, when mating is believed to occur. These results suggest changes in movement were biologically rather than environmentally driven. It appears that C. amblyrhynchos are adapted to a wide range of environmental conditions in coral reef habitats and that movement between or away from reef platforms is likely related to biological factors such as prey density, competition, reproduction or dispersal rather than environmental or seasonal changes.