Underwater soundscapes at critical habitats of the endangered Hawaiian monk seal

Describing underwater soundscapes of critical habitats of marine mammals can provide valuable information on the acoustic environment utilized by sound-reliant animals. For the endangered Hawaiian monk seal (Neomonachus schauinslandi), whose underwater hearing abilities and vocal communication were recently described, the soundscapes of their aquatic habitats are poorly understood. We measured ambient noise levels and identified acoustic signals that contribute to the underwater soundscape at four critical habitats of the Hawaiian monk seal. We deployed SoundTrap 500HF acoustic recorders, at sites with varying habitat types, and recorded continuously for 179 days. We measured broadband (20–24 000 Hz) and octave-band (31.5, 250, 500, and 16 000 Hz center frequencies) sound pressure levels (SPLs) in hourly intervals at each site. Average hourly broadband SPLs ranged from 107.8–123.4 dB re 1 μPa. Octave-band SPLs confirmed diel patterns associated with biological and anthropogenic sources. We recorded two large-scale geophysical events: Hurricane Douglas (Category 4) and a 6.2 magnitude earthquake which increased the 31.5 Hz octave-band SPL. This study provides the first description of underwater soundscapes at critical habitats of the Hawaiian monk seal across its expansive range. These measurements serve as a baseline for future studies to understand the impact of human activity on underwater soundscapes.