How Large Area Imagery Can Be Used to Quantify Growth of a Complex Branching Coral Species

Species of branching Acropora, once dominant, complex coral species, have experienced major decline over the last several decades due to physical and anthropogenic disturbances. Following this decline, species of Acropora have been a direct focus of coral monitoring and restoration efforts across the Caribbean, in hopes of recovering populations of these threatened species. Measuring growth in the field presents countless challenges, including inaccuracy and imprecision of measurements due to intricate branching morphologies, the amount of time a diver can spend underwater measuring aspects of coral health, and unpredictable diving conditions. Here we used large-scale 3D imagery derived using Structure from Motion photogrammetric techniques to quantify branching Acropora species across multiple reef terrace sites on Palmyra Atoll. We estimated branch density (number of branches per thicket) and various metrics of linear and areal size of colonies within imagery which facilitates a direct comparison of commonly used metrics to track growth. We were able to monitor growth based on initial size through larger thicket form for six Acropora thickets through multiple time points from 2012-2019. The use of large area imaging technology provides a permanent record and a powerful tool to quantify growth using a variety of metrics, not constrained by in situ logistics. Gaining access to and perfecting methods that account for accurate size and growth measurements for reef accretion can hold the key to long-term successful ecological monitoring and restoration efforts