Underwater acoustic ecology of the Hawaiian monk seal

Abstract

Hawaiian monk seals (HMS; Neomonachus schauinslandi) are an endemic and endangered phocid species with a population of approximately 1,600 individuals. While extensive research has described HMS biology, movements, and population ecology, their underwater vocal behavior remains poorly understood. This dissertation advances knowledge of HMS acoustic ecology across their range by integrating passive acoustic monitoring (PAM), fine-scale behavioral observations from multi-sensor biologging tags, and opportunistic citizen-science videos. In Chapter 2, I characterized underwater soundscapes at four monk seal critical habitats across the Hawaiian Archipelago, quantifying biological, geophysical, and anthropogenic contributors. Broadband levels ranged from 107.8–123.4 dB re 1 μPa, and soundscapes were dominated by biological sources, with diel patterns driven by snapping shrimp, reef fishes, and occasional anthropogenic noise. Low-frequency anthropogenic noise overlapped temporally and spatially with HMS vocalizations at one site, indicating potential for acoustic masking. This study provides baseline measurements of soundscapes across the species’ expansive range. To describe the underwater vocal repertoire and diel patterns of sound production of free-ranging seals, in Chapter 3, I analyzed >4,500 hours of passive acoustic monitoring recordings from five sites across the Hawaiian Archipelago and manually detected >23,000 vocalizations. Twenty-five call types were identified, including five published elemental calls and 20 novel vocalizations, 19 of which were combinational calls—representing an undocumented communication strategy in pinnipeds. A novel elemental call type, the whine, was documented in foraging contexts via publicly available videos, representing only the second known example of a phocid producing underwater vocalizations during foraging. Vocalizations were predominantly low frequency (<1 kHz), often produced in bouts, and detected across all monitored sites, with calls occurring throughout the day at sites with greater seal abundance. In Chapter 4, I examined the behavioral contexts of calling using synchronized audio–video data from biologging tags and citizen-science videos. Vocal activity was significantly higher during social and foraging behaviors, and specific call types were strongly associated with particular contexts, including whoops during social interactions and whines during foraging. Female seals were also documented producing underwater vocalizations during social and nonsocial behaviors— representing a rarely documented behavior in a phocid species. Collectively, these findings provide the first quantitative, species-wide assessment of underwater sound production in free-ranging HMS, revealing a diverse and behaviorally meaningful vocal repertoire and demonstrating that seals are exposed to anthropogenic noise that may impact acoustic communication. This work establishes a foundation for incorporating HMS acoustics into population monitoring, behavioral research, and conservation strategies for this culturally significant and endangered species.