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.