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).