2-1. Data collection
Passive acoustic monitoring was conducted on the east coast of Inexpressible Island, Ross Sea, Antarctica (Figure 1A, B), from 15:00 on December 9 to 21:00 on December 11, 2021 (54 hours), using local time (UTC+13). An autonomous passive acoustic recorder (µAURAL, Multi-Electronique Inc.) was deployed on the sea ice edge (74° 54.966’S, 163° 47.284’E) at a depth of 30 m, which is approximately 1.5 km from the coastline (red asterisk in Figure 1C). The acoustic recorder was scheduled to record continuously in 10-minute WAV files with 24-bit resolution and a 96,000 Hz sampling rate. Excluding data during deployment and recovery, the 324 WAV files had a time interval of 2~3 seconds between each file, resulting in a total missing data period of 14-minutes. The receiving voltage sensitivity of the hydrophone is −165 dB \(re\ 1V/\mu\text{Pa}\) and almost flat with a ± 2 dB error in the frequency range of 10 Hz to 10 kHz (De Robertis and Wilson, 2011). Therefore, the acoustic data were analyzed within the given frequency band, with frequencies above 10 kHz used only as a reference. Figure 1C shows aerial photographs taken by a UAV (Mavic 2 Pro, DJI) depicting the measurement area and the location of an autonomous passive acoustic recorder (red asterisk), including the spatial distribution of the wildlife and sea ice. The length of the sea ice edge in the photo was approximately 1.8 km, and a single leopard seal and Adélie penguins, Pygoscelis adeliae , were observed on the pack ice in the red square box of the expanded scale of the photo. The female seal was over 2 m long (Figure S1). The vocalizations of this individual were not included in the underwater acoustic data recorded at the time, and its contribution to the entire acoustic data cannot be confirmed. A monitoring camera (A6600, SONY) with a 20-200 mm lens was used to take videos and photos.