A novel biometric approach for non-invasive monitoring of octocoral forests using image analysis

Marine animal forests are ecosystems of high ecological and conservation value. Monitoring their status, through assessments of coral health and population structure, is essential for tracking changes in the marine environment. However, non-invasive image-based techniques are often limited by the absence of dedicated metric references, such as laser pointers. Here, we develop and validate the use of distal branches' width as an innovative metric to estimate colony heights of two octocoral species forming extensive coral forests in the Mediterranean Sea: the purple sea fan Paramuricea clavata and the yellow gorgonian Eunicella cavolini. Through underwater video transects, we gathered and analyzed distal branch width data to assess the size of different P. clavata and E. cavolini colonies using both a manipulative and a non-manipulative approach. We tested these models at the Tremiti Islands Marine Protected Area (Adriatic Sea). Population size-structures derived from the mean distal branch width showed strong agreement with standard laser-based measurements, confirming the accuracy and reliability of the method. This novel approach enhances monitoring flexibility, enabling its application in long-term surveys, historical footage, and citizen science datasets. The proposed model provides a robust and replicable framework for quantitative assessment of octocoral populations across diverse marine research contexts.