Mapping the relationship of reef structure and surfer spatial patterns at Cloudbreak, Fiji.

This study examines how three-dimensional reef structure relates to surfer activity at KuruKuru Mailani, Fiji (Cloudbreak). We combined GPS surfer track data with structure-from-motion (SfM) photogrammetry to map surfer behavior on high resolution, orthorectified reef models. Surfer track density and cumulative track length differed significantly among three zones, with the highest activity in the Middle zone. Live coral cover did not differ among zones and showed no association with surfing activity. In contrast, three-dimensional structure varied by zone: VRM was higher in the Left than the Right, slope was smaller in the Left than in the Middle and Right, and surface complexity was higher in the Middle than the Left. A multivariate matching approach identified a combination of surface ruggedness, curvature, planform curvature, and surface complexity that best aligned with surfer activity. A PCA of these variables explained 39.8% and 24.7% of variance on the first two axes and showed that the Middle zone spanned the widest range of 3D values and the highest amount of surfer activity. Boosted regression trees indicated that surfer activity increases most within specific ranges of surface complexity and slope, with additional contributions from curvature and ruggedness. The partial dependence plots showed threshold-like responses rather than simple linear trends. Collectively, these results indicate that specific aspects and ranges of reef structure are most closely associated with the spatial concentration of surfer activity at Cloudbreak. The workflow presented here provides a consistent approach that can be applied at other surf breaks, and the findings offer clear directions for future studies to link structural features with wave processes and to inform management of recreational reef environments.