Detecting habitat preferences and monitoring population dynamics of sea cucumbers in coastal ecosystems through underwater photogrammetry.

Sea cucumbers constitute common benthic organisms in the subtidal zones capable of providing key ecosystem services. Due to the recent harvesting and increased commercial interest in Mediterranean species, fundamental ecological knowledge is required to promote adequate management measures. In this regard, a remotely sensed mapping method is proposed for deriving length-frequency distribution and defining habitat preferences of a common sea cucumber species. Image analyses and classification carried out on high spatial resolution cartographic products, including orthophoto mosaics and digital surface models derived by 'Structure-from-Motion' photogrammetric processing, allowed accurate estimation of sea cucumber densities over an area of 5500 m. The mean length of mapped specimens ranged from 8 to 50 cm, with a peak in abundance recorded in April. A distinct preference for sandy substrates was observed, characterized by low vector ruggedness measure values ranging from 0 to 0.15. A negative correlation was also identified in areas with slopes exceeding 15°, especially among larger size classes. Spatial patterns investigated with a Log-Gaussian Cox Process showed significant temporal trends with well-defined aggregation patterns during the summer months in specific areas covered by transplanted P. oceanica fragments with higher (up to 0.25) vector ruggedness measure values. This finding confirmed the essential role of this restored habitat in retaining fine organic particle used as a source of food for deposit-feeding holothurians. This approach is expected to enhance the creation of tailored management plans for sea cucumber populations in coastal regions, facilitating accurate assessments of the impacts of fishing activities on these species.