Environmental DNA (eDNA) metabarcoding as a complementary monitoring tool in cetacean visual surveys: from broad to fine-scale spatial analysis in open waters

Monitoring cetacean populations is essential for understanding the status of marine communities. In this regard, environmental DNA (eDNA) metabarcoding is an increasingly popular innovative and non-invasive genetic tool for marine biodiversity monitoring, particularly useful for assessing the populations of highly mobile species with a wide distribution range, such as cetaceans. In this study, we compared cetacean species detection and distribution patterns obtained through eDNA metabarcoding with the sighting records obtained during simultaneous traditional visual monitoring. A total of 258 seawater samples was analysed, collected at 129 sampling stations during the SCANS-IV survey carried out in oceanic waters of the North Atlantic Ocean. Nine cetacean species were identified by eDNA metabarcoding and ten by visual monitoring; three species were detected solely by eDNA, while four were exclusively detected by visual observation, demonstrating the complementarity of the two methods. The four most frequently sighted species were also the most frequently detected species in eDNA samples, showing similar detection frequencies and distribution patterns at broad and intermediate spatial scales. At a finer scale, however, species occurrence did not overlap between methods. Additionally, eDNA provided new insights into striped dolphin (Stenella coeruleoalba), which may be under-represented in traditional monitoring data. Overall, the results support the use of eDNA metabarcoding as a valuable complementary monitoring tool for assessing cetacean diversity and distribution, providing additional information on species presence throughout the study area. This is particularly relevant for species that are rarely recorded visually and for which there is insufficient information to design and implement appropriate management strategies.