Divergences and complementarities between eDNA versus passive acoustic marine mammal monitoring

The integration of eDNA with passive acoustics offers both divergence and complementarity in monitoring marine mammals. The 9-month “Sphyrna Odyssey 2019-2020” mission in the Mediterranean Sea assessed these methods for detecting cetacean presence, distribution, and diversity. Two drones ‘autonomous laboratory vessels’ equipped with 5 broad band frequency hydrophones sampled at 256 kHz, were complemented by an acoustic antenna with four additional hydrophones and deployed from an equipped accompanying vessel. Passive acoustics enabled detection of vocalizing species within a few nautic miles. The eDNA samples collected from surface waters, analyzed via qPCR, identified species across broader spatial scales, despite being affected by ocean currents and DNA degradation. Visual observations completed this multimodal protocol. Notable eDNA detections included Balaenoptera physalus , Ziphius cavirostris (22 % and 20 % respectively), and Physeter macrocephalus . Other detected marine mammals were Tursiops truncatus, Stenella coeruleoalba, Delphinus delphis, Globicephala melas, and Grampus griseus. Moreover, we also considered the pinniped Monachus monachus but without the acoustic protocol. Automatic classification of acoustic signals using convolutional neural networks achieved over 80 % accuracy for most target species. Pooling detections by spatial clusters of 40 nm × 20 nm, we estimate the agreement between the eDNA with the other types of detections to 58 %, (higher than random, which is 31 %). We then discuss the complementarities of the eDNA versus the acoustic plus visual detections. By combining the spatial and temporal precision of acoustics with the broad detection capacity of eDNA, this multimodal approach from drones offers new insights into cetacean distribution and abundance, supporting conservation in marine ecosystems.