Abstract
Coastal marine environments are recognised as key ecosystems that support the feeding, reproduction, and development of a myriad of species including sharks. Terrestrial reef flats, such as those in Mo’orea, French Polynesia, provide ecologically important habitat for elasmobranchs including the blacktip reef shark (Carcharhinus melanopterus). These reef flats act as nurseries for juvenile C. melanopterus by providing a high food environment, coupled with purported low predation risk. Yet, despite the high ecological value of these habitats, they are increasingly threatened by anthropogenic activities resulting in habitat modification and overexploitation that has driven the considerable decline of coastal shark populations, including C. melanopterus. Given the high ecological and cultural importance of C. melanopterus in Mo’orea, French Polynesia, understanding how they interact with their environment is important if we want to better manage terrestrial reef flats. To this end, this study aimed to investigate the spatiotemporal distribution patterns of C. melanopterus and the predation risk of C. melanopterus juveniles within the terrestrial reef flats in Mo’orea, French Polynesia, and the environmental drivers behind observed patterns. Traditional sampling methods, such as the deployment of BRUVs or direct, in-water observations, can over or underestimate observed occurrences of elasmobranchs due to altered natural behaviours. Therefore, this study employed a non-invasive and undetectable sampling technique by using an unmanned aerial vehicle (UAV). The UAV was flown in a series of strip transects over five terrestrial reef flats surrounding Mo’orea at three discrete time points over the diel cycle in November 2022. Results reveal an asymmetric distribution of both juvenile and adult C. melanopterus. Juveniles were found to preferentially reside in areas with a mosaic of complex habitat and open sand cover, likely as a trade-off between access to food and maintaining visual cover from predators. Adult blacktip reef sharks, however, were found in highest densities in reef flats with highly complex habitat, possibly using these reef flats as a predator refuge. Furthermore, the use of terrestrial reef flats by adult C. melanopterus increased throughout the day, potentially to aid in foraging and to avoid predators. This study also quantified the predation risk faced by juvenile C. melanopterus, which was found to be consistent across the diel cycle, indicating juvenile C. melanopterus may not be avoiding reef predators through a temporal partitioning of space use. Furthermore, juveniles exhibited high spatial overlap with predators in areas with high habitat complexity, potentially due to shared food resources in such areas. I suggest juvenile C. melanopterus can survive regular interactions with predators due to their increased manoeuvrability in shallow, complex habitats when compared to their larger, less manoeuvrable predators. Here, I highlight the importance of protecting reef flat habitats to enable the survival of threatened reef sharks, whilst illustrating novel uses of a non-invasive shark surveying technique. This study furthers our understanding on how blacktip reef sharks use and interact within ecologically important terrestrial reef flats, which are susceptible to anthropogenic modification, to provide managers with crucial information to assist in targeted and effective conservation efforts.