Detecting Common Seal Haul-Out Sites from Remote-Sensing Data in the Lower Saxon Wadden Sea National Park, Germany

Abstract

Common seals (Phoca vitulina) come ashore to rest, moult, and raise their pups. In the Lower Saxon Wadden Sea National Park, their haul-out sites are sandbanks in the intertidal mudflats of the Wadden Sea. For several years, a decline in the common seal population within the National Park has been observed. Rising sea levels and changing morphodynamics in the Wadden Sea potentially lead to changes in haul-out sites and habitat behaviour, thereby causing a decline in the common seal population. Although the remote-sensing-based detection of seal populations, coastline and sandbank mapping has long been a subject of scientific interest and research, the detection and mapping of seal haul-out sandbanks is less common. This thesis proposes a workflow for the semi-automated detection and mapping of potential common seal haul-out sandbanks using various remote sensing data. The study was conducted in three areas of interest within the National Park’s borders. Digital elevation models, PlanetScope and Sentinel-2 multispectral satellite imagery were tested alongside multispectral aerial imagery to determine their capability to detect sandbanks. This imagery was processed using machine learning and various threshold-based classification algorithms (local minima, Otsu and probability density) with Python, R and QGIS. The results indicate strong usability and high overall classification accuracy for the satellite imagery, limited usability and partly low accuracy for the aerial imagery, and no applicability for the digital elevation data. Potential haul-out sandbanks could be detected in all surveyed areas, partly aligning with seal sighting ground truth data for 2023. Therefore, mapping and detecting these habitats seems achievable, even exclusively using sustainable publicly available data and software. However, more research is necessary to further filter and define potential haul-out areas, and to increase automation in the processing.