Background/Aims
Coastal sharks are key contributors to reef ecosystem stability, yet are increasingly threatened by overfishing and habitat degradation, particularly in nursery habitats. Understanding their spatiotemporal ecology is essential for effective conservation. This study aimed to assess the seasonal patterns of abundance, size structure, and habitat use of Negaprion acutidens (sicklefin lemon shark) around Dongsha Island, a remote atoll in the northern South China Sea, using UAV-based aerial surveys.
Methods
We conducted UAV surveys at 20 fixed sites spanning lagoon and reef-edge habitats during winter and summer. Shark abundance, body size, and spatial distribution were extracted from aerial imagery and analysed using generalised linear mixed models (GLMMs) and non-parametric statistics. Key environmental covariates included tide phase, water direction, substrate type, and human impact level. Offshore distance from the island and anthropogenic disturbance zones were incorporated to evaluate habitat use across ecological and human gradients.
Results
Overall abundance did not differ significantly between seasons; however, distribution patterns shifted, with sharks concentrated in southern and lagoonal areas during winter. GLMMs identified water direction, tide phase, and substrate type as significant predictors of abundance, with a direction × season interaction indicating localised seasonal shifts. Most individuals were immature, with only one mature shark observed. Shark size increased in winter and with offshore distance, and larger individuals were more often found in seagrass habitats and during neap tides. GLMMs confirmed tide and substrate type as consistent predictors of size across seasons. Shark abundance, size, and offshore distance varied significantly across human impact zones, with smaller individuals and lower maturity levels found in high-disturbance areas.
Conclusion
Our results provide the first UAV-based evidence that Dongsha Island functions as a critical nursery ground for N. acutidens, influenced by fine-scale seasonal and anthropogenic gradients. Protecting key habitats—particularly southern and lagoonal areas—through seasonal zoning and spatially explicit management is vital. UAV surveys offer an efficient, non-invasive method for long-term shark monitoring and should be integrated into conservation frameworks for coastal shark populations.