High-resolution photogrammetry reveals links between reef structural complexity, fish communities, and human influence in a remote Pacific atoll

Coral reef degradation is accelerating globally, yet monitoring efforts remain uneven in scale, accessibility, and resolution. Visual surveys of percent live coral cover offer low-cost but coarse assessments of reef health, while more advanced tools (e.g., autonomous underwater vehicles, airborne LiDAR) often remain inaccessible for many low-income, tropical nations. In this study, we used low-cost, high-resolution Structure-from-Motion (SfM) photogrammetry across 21 sites in Ulithi Atoll, Federated States of Micronesia, spanning a gradient of oceanic and lagoonal exposure, fishing pressure, and human population density, as a community-centered approach to assess reef habitat complexity and its relationships to benthic and fish communities. Habitat complexity was not strongly correlated with live coral cover or coral morphotype composition, two common indicators of reef health, but instead, was most closely associated with overall benthic community composition, including the relative abundance of sponges, macroalgae, and turf algae. Contrary to expectations, habitat complexity did not significantly correlate with total fish abundance but was a stronger predictor of fish and benthic assemblage structure across all reef types and exposure zones. Notably, several sites under active community-led management exhibited higher-than-average structural complexity despite their proximity to human settlements, suggesting that local management practices may help preserve reef architecture. SfM photogrammetry provides a scalable, community-accessible monitoring approach to inform adaptive management, particularly in remote or data-limited reef systems facing compounding climate and human pressures.