Scaling Up Coral Demography: Measuring Coral Vital Rates Using Repeated Photomosaics

Developing models that document the changes in coral reef communities and trends in reef recovery is crucial in identifying resilient reefs. However, traditional approaches to generating the coral vital rates necessary for demographic modeling are time consuming and field intensive. To address the challenges associated with both monitoring corals over time and identifying which colonies are likely to persist, we developed a novel vital rates data pipeline using fixed site photomosaics from the Hawaiian Archipelago combined with 3D photogrammetry to rapidly characterize coral demography at large spatial scales. Our approach follows 5 main steps: 1) Collecting images on SCUBA, 2) Post-processing and Quality Control, 3) Generating 3D models in Agisoft Metashape, 4) Aligning multi-year 3D models and generating 2D orthoprojections in Viscore and 5) Extracting demographic data in ArcMap by delineating live patches and creating patch transition tables. Patch transition tables were created by randomly placing 0.5 m² circular plots on the 10-12 meter fixed-site photomosaics in ArcMap. All live patches of Pocillopora spp. and Porites spp. located inside of the circular plots were delineated and assigned a unique identification number. To measure changes in size and identify the transition type (growth, fission, fusion, mortality or recruitment) over time, coral patches were retraced for each subsequent sampling year. Our methods successfully show how accurate colony level vital rates can be generated for hundreds of coral colonies in a short amount of time. This new method resulted in error rates between repeated independent measurements of colony planar area less than 15 cm² (Pocillopora) and 8 cm² (Porites) and offers a robust alternative to measure patterns in the spatial variation in coral demography. The estimated vital rates data will be used to construct an Integral Projection Model to evaluate the drivers of reef resiliency and predict how colony-level demographic processes influence long-term population dynamics.