| Summary: | When reporting under the EU Directives (Habitat Directive and Marine Strategy Framework Directive), species distribution and its trends are required. The research aimed to investigate the performance of three grid resolutions (5x5, 10x10 and 50x50 km) with two indexes (proportion of area occupied; shift) and an occupancy model to study the distribution of 4 species occurring in the Ligurian Sea (striped dolphin, fin whale, sperm whale and Cuvier’s beaked whale) along 3 periods (2008-2012; 2013-2018; 2019, 2021, 2022) corresponding to the HD reporting cycles. Data was collected onboard ferries as platform-of-opportunity. Detection/non-detection maps and the proportion of area occupied showed the striped dolphin as a species with generalized use of the study area, whereas the fin whale preferred the open sea and the sperm whale and Cuvier's beaked whale favoured slopes and seamounts. Changes in the area used were denoted for the sperm whale, which had a complete non-detection in the southwestern part in the last period, and for the Cuvier’s beaked whale, describing an expansion movement in the eastern part from the 1st to the subsequent periods. Encounter rate maps showed a decrease in detections for all the species from the 2nd to the 3rd period, being most prevalent in the western part, indicating a decrease in their presence. The occupancy model described similar use (%) of the study area, except for higher values for the sperm whale. Compared to coarser resolutions, the use of a finer resolution allowed, to a greater extent, the detection of changes in all methods. The model performed better when more surveying occasions (14) were given. This study supports the use of finer resolutions when reporting the distribution of cetaceans under the EU Directives, as well as the importance of long-term monitoring programmes for detecting species distribution changes. De modo a compreender o estado de conservação das espécies e atuar em direção à sua preservação, as Diretivas Europeias (Diretiva Habitat (DH) e Diretiva ...
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