Reef shark movements relative to a coastal marine protected area

Abstract

Marine protected areas (MPA) are one management tool that can potentially reduce declining shark populations. Protected-area design should be based on detailed movements of target animals; however, such data are lacking for most species. To address this, 25 sharks from three species were tagged with acoustic transmitters and monitored with a network of 103 receivers to determine the use of a protected area at Mangrove Bay, Western Australia. Movements of a subset of 12 individuals (Carcharhinus melanopterus [n=7]), C. amblyrhynchos [n=2], and Negaprion acutidens [n=3]) were analysed over two years. Residency for all species ranged between 12 and 96%. Carcharhinus amblyrhynchos had <1% of position estimates within the MPA, compared to C. melanopterus adults that ranged between 0 and 99%. Juvenile sharks had high percentages of position estimates in the MPA (84–99%). Kernel density activity centres for C. melanopterus and C. amblyrhynchos were largely outside the MPA and mean activity space estimates for adults were 12.8 km2 (±3.12 SE) and 19.6 km2 (±2.26), respectively. Juveniles had smaller activity spaces: C. melanopterus, 7.2 ±1.33 km2; N. acutidens, 0.6 km2 (±0.04). Both C. melanopterus and C. amblyrhynchos had peaks in detections during daylight hours (1200 and 0900 h, respectively), whereas N. acutidens had a peak in detections at 0200 h. Long-distance movements were observed for adult C. melanopterus and C. amblyrhynchos, the longest being approximately 275 km. These migrations of C. melanopterus might be related to reproductive behaviours, because they were all observed in adult females during the summer months and provide links between known in-shore aggregation and possible nursery areas. The MPA at Mangrove Bay provided some protection for juvenile and adult reef sharks, although protection is likely greater for juveniles due to their more restricted movements.