Revealing Habitat Use and Anthropogenic Impacts on Sicklefin Lemon Sharks at Dongsha Atoll

Abstract

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 Island 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.

Keywords: sicklefin lemon shark, UAV, Dongsha Atoll, nursery habitat, human impact, seasonal habitat use

Methodology and Implementation Steps

The study site was Dongsha Island (approximately 2.6 km in length), located on the northwestern rim of Dongsha Atoll and the primary habitat of juvenile sicklefin lemon sharks. Surveys were conducted during summer (August 2021, N=6) and winter (February 2022, N=7), totaling 13 survey sessions. Each survey deployed two UAVs simultaneously, departing from the western lagoon of Dongsha Island. One UAV covered the northern coast while the other covered the southern coast, ensuring no individual was counted twice. Flight altitude was kept consistent throughout to maintain image quality and identification accuracy. Each circumisland survey was completed in approximately 30 minutes. UAV footage was used to record: (1) shark abundance (individual counts per survey location); (2) body size estimation (precaudal length, converted from pixel measurements calibrated to UAV altitude); (3) distance from shore (straight-line distance from each individual to the nearest shoreline); and (4) substrate type (seagrass, sand, or mud). Dongsha Island was divided into three zones based on human activity intensity. The High Human Impact Zone (HHIC) encompassed the airport, military bases, docking facilities, and research stations. The Low Human Impact Coast (LHIC) contained minimal military infrastructure. The Lagoon zone represented the least disturbed area with the lowest level of human presence. Generalized Linear Mixed Models (GLMMs) were used to analyze the relationships between shark abundance, body size, and distance from shore against seasonal, tidal, temperature, substrate, and human impact variables, with survey session and location included as random effects.

Innovation and Cross-Disciplinary Collaboration

The primary innovation of this study lies in the successful adaptation of UAV technology to a highly restricted and logistically challenging remote marine environment. Dongsha Atoll is located 450 km from Taiwan's main island, accessible only through military coordination, and subject to restrictions on specific drone brands due to national security concerns. These constraints demanded a purpose-built survey approach. The dual-UAV synchronized circumisland protocol developed in this study effectively resolves the critical problem of double-counting in surveys of mobile species, and establishes a standardized, repeatable UAV monitoring workflow for large marine fauna. The non-contact nature of the method minimizes disturbance to the study subjects, making it particularly suitable for long-term ecological monitoring in protected or sensitive marine areas. This work represents a genuine cross-disciplinary integration of engineering technology and marine ecology.

Expected Results and Contributions

This study establishes the first systematic spatiotemporal baseline dataset for the sicklefin lemon shark population at Dongsha Atoll, addressing a critical gap in Taiwan's elasmobranch ecology research. Contributions span three dimensions:

1. Conservation management evidence: Research findings provided a scientific basis for seasonal dynamic zoning, priority microhabitat protection, and human disturbance regulation.
2. Ｍonitoring methodology: The standardized dual-UAV circumisland protocol developed here can be directly replicated across other remote reef systems around Taiwan, laying the groundwork for long-term population monitoring.
3. International academic visibility: Publication in Scientific Reports elevates Taiwan's profile in South China Sea shark ecology research, helping shift the international association of Taiwan from fisheries catch data toward rigorous field ecology.