Surface-sourced eDNA to track deep-diver cetaceans: the study case of Cuvier's beaked whale (Ziphius cavirostris) at Caprera Canyon and surrounding areas (Western Mediterranean Sea)

Among cetaceans, the Cuviers beaked whale is considered an extreme diver, very challenging to be studied with standard monitoring methods due to its elusive behaviour and a preference for deep offshore waters. These limitations seem to be mitigated by the use of new molecular methodologies capable of intercepting small traces of DNA left in the environment (eDNA) by marine organisms. Moreover, the collection of water from the superficial layer of the sea represents a perfect case study for the targeting of marine mammals, as the constraints imposed by their nature implies periodic and frequent surfacing. Therefore, we designed and tested a taxon-specific primer set to infer the Cuviers beaked whale presence, with the aims of 1) examining the effectiveness of the eDNA technique to detect the presence of deep-diving cetacean in open waters, using the Cuviers beaked whale as case study; 2) providing data on the spatiotemporal occurrence of this species within the Canyon of Caprera; and 3) assessing the species occurrence in central northern Mediterranean Sea based on molecular traces. Results from this study demonstrated that collection of superficial waters is a valid approach to monitor deep-diving cetacean species, without the need for complementary visual survey monitoring. Specifically, this study provides evidence of the regular presence of the Cuviers beaked whale in the Canyon of Caprera, with a preference for bathymetry in the range of 700-1000 m. This study also showed a potential inshore movement of this species during fall, which is possibly related to migration of its cephalopod prey or a shift in prey preferences. Overall, the data presented here are particularly relevant in the optic of proposing the Caprera Canyon as an Important Marine Mammal Area (IMMA) in the Mediterranean Sea. At a wider level, this study also showed that the stronger positive signals were recorded in sampling stations located on submarine canyon systems, demonstrating the importance of these areas as elective habitats for the Cuviers beaked whale, thus the pivotal priority to their conservation.