Cuvier's beaked whale (Ziphius cavirostris) detection through surface-sourced eDNA: A promising approach for monitoring deep-diving cetaceans

Among cetaceans, the Cuvier's beaked whale is considered an extreme diver, thus not one of the easiest cetacean species to be studied due to its elusive behaviour and a preference for deep offshore waters. Although dedicated cetacean surveys have increased our knowledge on the distribution of Cuvier's beaked whales in the Mediterranean Sea, knowledge gaps still remain where the study effort is non-existent. In this context, the use of new molecular methodologies capable of intercepting small traces of DNA left in the environment (eDNA) by marine organisms represents a valid approach to infer species’ occurrence. Moreover, the collection of water from the superficial layer is suitable for targeting marine mammals, as the constraints imposed by their nature implies periodic and frequent surfacing in order to breathe, releasing exhalants rich in their epithelial cells. Therefore, we designed and tested a taxon-specific primer set to infer Cuvier's beaked whale presence, with the aims of 1) examining the effectiveness of the eDNA technique to detect the presence of a deep-diving cetacean in open waters, using the Cuvier's beaked whale as case study; 2) providing data on the occurrence of this species within the Canyon of Caprera over a six-months study period and 3) assessing the species presence in adjacent waters in the central northern Mediterranean Sea based on the analysis of samples collected in a Citizen Science campaign. Results from this study demonstrated that superficial waters may retain biological traces of this cetacean despite the fact that it mostly inhabits deep waters. Specifically, this study provides evidence of the regular presence of Cuvier's beaked whale in the Canyon of Caprera, with a preference for bathymetry in the range of 700–1000 m. Molecular traces’ distribution suggests a potential inshore movement of this species during Fall, which might be related to migration of its cephalopod prey or a shift in prey preferences, although this aspect requires further investigation. Overall, this study showed that the stronger positive signals were recorded in sampling stations located on surfaces above submarine canyon systems, demonstrating the importance of these areas as elective habitats for the Cuvier's beaked whale, thus the pivotal priority to their conservation.