Picture of Health: Evaluating an accessible method for quantifying coral thermal tolerance using photographic color analysis

Reef-building corals are increasingly threatened by the warming climate, necessitating the development of tools to monitor their responses to environmental stress. Furthermore, scalable methods of identifying resilient corals are a particular conservation priority. Techniques commonly used to quantify the severity of coral bleaching can be invasive or require specialized instruments, while more widely-accessible visual methods can be limited by subjectivity. This study evaluates a non-invasive, objective, color-based approach for evaluating the status of coral symbiosis and compares thermal tolerance results with traditional bleaching metrics. Using the staghorn coral Acropora cervicornis as a model, acute thermal tolerance was assessed across genets, sites, and seasons within an active coral restoration program in Bonaire, Caribbean Netherlands. Quantitative color scores were significantly correlated with both traditional bleaching metrics and also identified similar genetic and environmental effects on coral thermal tolerance. Most importantly, color-based metrics demonstrated a high level of agreement with both symbiont and chlorophyll retention in detecting significant pairwise differences in resistance. Variation in predictive accuracy and method agreement across seasons suggest that color-based assessments of thermal tolerance may be more effective in summer compared to winter. Overall the findings of this study provide support for color analysis as a reliable and cost-effective method for assessing coral resilience which can be readily incorporated into censuses of coral populations, especially for conservation and restoration efforts.