Residency patterns and movements of grey reef sharks (Carcharhinus amblyrhynchos) in semi-isolated coral reef habitats

The degree of reef isolation may limit the frequency of long-range dispersals in reef-associated sharks. Therefore, understanding how the behaviour and spatial ecology of a species differs across reef habitats is essential for developing sound conservation approaches. The present study examined the residency, movement and activity space of grey reef sharks (Carcharhinus amblyrhynchos) in the central Great Barrier Reef (GBR). An array of 56 acoustic receivers covering 17 semi-isolated coral reefs across 150 km was used to monitor shark movements. Forty C. amblyrhynchos were tagged with acoustic transmitters and monitored from 251 to 821 days. Most sharks were detected on a single reef; however, some individuals (4 females; 10 males) moved to up to five reefs. Residency index ranged from 0.02 to 1.0, with a mean +/- A SD of 0.78 +/- A 0.26. Mixed-effect models showed that weekly and monthly residency was mainly influenced by shark size, with little or no effect of environmental parameters. Although C. amblyrhynchos were present year-round, juvenile sharks had lower residency to their tagging reef than adults. In addition, mature females were detected less between November and mid-February, which coincides with reported parturition in the central GBR. Long-term monitoring data revealed that C. amblyrhynchos exhibited high residency to their tagging reef, and therefore, even in systems with semi-isolated reefs such as the GBR, this species may benefit from spatial management approaches at the reef level. However, behavioural differences between sexes and life-stages of C. amblyrhynchos reported in this study suggest marine reserves may provide lower protection relative to more remote and isolated coral reefs.