Monitoring juvenile sicklefin lemon shark Negaprion acutidens in remote marine nurseries using unmanned aerial vehicles (UAVs)

Understanding spatial and demographic patterns in threatened coastal sharks is essential for effective conservation, yet remote reef systems remain understudied due to logistical constraints. We used dual unmanned aerial vehicles (UAVs) to monitor juvenile Negaprion acutidens around Dongsha Atoll, a no-take marine reserve in the northern South China Sea. Thirteen synchronized UAV surveys were conducted during summer and winter, covering 20 locations categorized into three zones representing different levels of human impact. We quantified seasonal variation in shark abundance, body size, spatial distribution, and environmental drivers using Generalized Linear Mixed Models (GLMMs). Results revealed stable overall abundance but strong fine-scale shifts between summer and winter. Northeastern sites showed sharp declines in shark sightings during winter, likely due to monsoonal exposure, while sheltered southern zones supported increased winter presence. Neonates were concentrated in lagoon habitats, whereas larger individuals occurred farther offshore and in seagrass areas, indicating ontogenetic habitat expansion. Human impact shaped demographic structure: low-impact areas hosted more and larger sharks, while smaller individuals and nearshore aggregation dominated high-impact zones. These findings confirm Dongsha's role as a critical nursery habitat for N. acutidens and highlight the utility of UAV surveys for capturing spatiotemporal ecological patterns in remote ecosystems. By integrating UAV monitoring with conservation planning, managers can prioritize seasonal protection of nursery zones and respond adaptively to climate and anthropogenic pressures.