Building on the Mediterranean monk seal vocal repertoire: Foundations for long-term passive acoustic monitoring

With a population comprising <1000 individuals, the Mediterranean monk seal (Monachus monachus) is among the world’s rarest marine mammals. Monitoring this elusive species is challenging due to its population size, inaccessible haul-out sites, and wide historical range, which limit many traditional survey methods. Passive acoustic monitoring (PAM) offers a promising, low-impact approach to tracking range-wide population trends and assessing anthropogenic impacts. To support the development of long-term PAM for this species, we characterized the underwater vocal repertoire from recordings in the Inner Ionian Sea Archipelago, Greece. Ten call types were identified, five of which were also documented in underwater video of monk seals vocalizing. Eventual variety and social network analyses of vocal bouts revealed patterns of structure and complexity and identified three frequently co-occurring call types likely central to monk seal communication. Building on these findings, we are developing automated detection tools using PAMGuard and a novel multi-step framework to identify these calls in large datasets. Initial results show monk seal vocal activity year-round and reveal seasonal variation in calling rates. This work provides a foundation for scalable PAM of Mediterranean monk seals, offering tools to better understand vocal behavior, habitat use, and to support conservation of this rare species.