Figure 7. Spectrograms of leopard seal vocalizations recorded in (A, B, C) underwater and in (D, E, F) the air. (a) triple ascending trill, (b) ascending trill, (c) hoot, (d) triple ascending trill, (e) low single trill, and (f) hoot (underwater data: 48,000 fast Fourier transform points and 4,800-point Hanning window, airborne data: 48,000 fast Fourier transform points and 2,400-point Hanning window).
According to Thomas and DeMaster (1982), the diurnal pattern of underwater vocalizations is negatively correlated with the haul-out pattern. In other words, the number of vocalizations was greater between 19:00 and 06:00 and lower between 09:00 and 18:00, as the species spends more times in the water at night. According to our results, the vocalization rate and sound pressure levels were relatively low from 05:00 to 17:00 on December 10, and were higher before and after, which was similar to the trend observed in the previous study but with a time shift. The persistence of elevated call rates and sound pressure levels from 05:00 to 17:00 on December 11 suggests that further validation of the diurnal pattern through long-term observation is necessary. However, our measurements were conducted at higher latitudes and later in the year when day and night conditions are indistinct than those of Thomas and DeMaster (1982), which may explain the differences compared to the trend. The results from the Perennial Acoustic Observatory in the Antarctic Ocean (PALAOA) conducted from January 2006 to January 2007 showed that the leopard seal was most vociferous on 16 December (Van Opzeeland et al., 2010). Although acoustic data collected over 54 hours in this study are insufficient for characterizing diurnal patterns and their correlations with environmental data, they were measured when vocalization rates might be near the peak so that leopard seal vocalizations could be the dominant sound source. The proportion of call types presented in Section 3-2 was subject to limited comparative analysis due to differences in call type classification from the previous study. The absolute call counts per minute were 3.7 (± 0.03 SE). By call type, they were 1.2 (± 0.02 SE) for HDT, 0.4 (± 0.01 SE) for MST, 1.3 (± 0.02 SE) for LDT, 0.5 (± 0.01 SE) for HST, 0.3 (± 0.01 SE) for DT, and 0.1 (± 0.01 SE) for triple ascending trill. These values were significantly lower than those reported by Shabangu and Rogers (2021), because we extracted only calls that the structure could be clearly identified, which likely excluded calls with low signal-to-noise ratios. Furthermore, we were unable to collect environmental data such as sound speed profiles and water depths in the field, and long-term PAM data are required to facilitate a more comprehensive analysis correlating these environmental variables with acoustic data.