3-2. Call types of underwater vocalizations and their acoustic characteristics
HDT and MST were observed in frequency bands between 2,300 Hz and 4,500 Hz, and between 1,300 Hz and 2,400 Hz, respectively, and they were relatively stereotyped and well time-resolved. Several vocalizations were clustered in relatively narrow bandwidths of approximately 100 and 500 Hz, and we detected and identified four call types: LDT, HST, DT, and AT. Over 54 hours, 12,006 calls were detected across six types, and representative spectrograms of these six call types are shown in Figures 4 and 5. Low-frequency vocalizations were the most frequent at 58%, followed by HDT (32%) and MST (10%). Among the low-frequency vocalizations, the combined LDT (35%) and HST (13%) accounted for the greatest proportion at 48%, followed by DT (7%), and AT (3%). As reported in previous studies, HDT consisted of two trills in series, and MST was a single trill characterized by beginning with a progressively accelerating pulse repetition, thus causing a gradual increase in amplitude modulation frequency (Rogers et al., 1996; Rogers, 2007; Kreiss et al., 2013).