Risso's Dolphin Site Fidelity in the Western Mediterranean Sea: The Importance of Submarine Canyons

Knowledge on the distribution of species provides baseline information towards their conservation and the management of human activities affecting them. For endangered species, such as the Risso's dolphin (Grampus griseus) in the Mediterranean Sea, highlighting the areas within their distribution range to which individuals regularly return can reveal patterns of site fidelity. For Risso's dolphin, this information is scarce in the Mediterranean Sea. Here, photo identification was used to assess Risso's dolphin site fidelity patterns in the Garraf submarine canyon system (NW Mediterranean Sea). Data collected along 237 boat surveys between 2017 and 2023 resulted in 56 sightings and 364 unique individuals. Eight Site Fidelity Indices (SFIs) were calculated for each individual based on three indicators: Occurrence, Permanence and Periodicity. A divisive hierarchical clustering analysis based on the eight SFIs was used to detect different degrees of residency. Individuals seen in more than 2 months were divided into three clusters and defined as seasonal residents, regular and occasional visitors, whereas individuals seen only in 1 month were considered to be transient. Results show the importance of the Garraf submarine canyon system for Risso's dolphin and highlight the need to expand the photo identification studies to further regions to better understand the distribution and movements of the species in a wider context in the western Mediterranean Sea.