Confirmed vocal activity of Mediterranean monk seals in the Inner Ionian Sea Archipelago, Greece

Mediterranean monk seals (Monachus monachus) are the world's rarest seal species, with fewer than 1000 individuals remaining across fragmented subpopulations. Monitoring this species is challenging due to their low densities and difficult-to-access habitats. Passive acoustic monitoring (PAM) data can be used to detect small numbers of animals over large areas, but for effective long-term PAM, it is essential to characterize the species' vocalizations throughout its range. This study provides a comprehensive description of the monk seal vocal repertoire in the Inner Ionian Sea Archipelago, Greece, and the first published documentation of the species vocalizing underwater. Acoustic recorders collected continuous data over 16 days during the breeding season in 2021. We identified 3442 calls, 1855 of which were determined to be high-quality and classified into ten call types within three broader categories. Video footage collected in 2019 and 2021 documented the seals producing five common call types, which comprised ∼87% of the recorded calls. Our findings offer insights into the structure of the monk seal vocal repertoire and lay the groundwork for long-term monitoring for this rare species using PAM, which could be an important tool for monitoring monk seals going forwards.