Underwater acoustic surveys in the eastern Clarion Clipperton Zone

Abstract

A drifting, underwater sound recording system was developed and deployed for acoustic surveys of vocalizing marine mammals in the eastern Clarion Clipperton Zone (CCZ), Northern Tropical Pacific. The system was designed to position an Ocean Instruments Sound Trap 500HF recorder and hydrophone at a depth of 450 m. A depressor vane was inserted halfway along the drop line, with shock cord, to minimize hydrodynamic noise. The surface line included a Spotter V2 buoy that provided real-time tracking of the location of the surface line. The recording system was deployed ten times in the CCZ, between November 2020 and October 2021. Deployment length was between two and seven days. All deployments were set to continuously record underwater sound with a sample frequency of 288 kHz. Acoustic data were analyzed using the CHORUS software to manually review recordings, and an FM signal detector (in MATLAB) was used to detect potential biological sounds. A variety of marine mammals were detected, including Delphinidae species, Physeter macrocephalus (sperm whales); and clicks likely to be from Ziphiidae species (beaked whales). Beaked whales have been previously visually sighted in the CCZ, but to our knowledge these might be the first acoustic recordings of them in the CCZ.