Steady inter and intra-annual decrease in the vocalization frequency of Antarctic blue whales
Number of individual calls from Antarctic blue whales detected daily in 2003 (a) and the mean value and standard deviation of call frequencies detected daily (b). The signal frequency was determined at the PSD maximum in the frequency band from 25 Hz to 30 Hz.
Variations of the power spectrum density of sea noise in a frequency band from 15 Hz to 30 Hz recorded at the Cape Leeuwin CTBT hydroacoustic station in 2002–2010.
Variations of the power spectrum density of sea noise during two weeks in 2003 (a) and a spectrogram of one of the calls recorded from an Antarctic blue whale on 22 May (b). The dashed rectangle indicates the time period when nearly 300 individual calls of high intensity were detected.
Power spectrum density of sea noise averaged over 7 days during week 21 (21 to 27 of May) in 2002, 2006, and 2010. The sharp spectral peaks correspond to the tonal component of Antarctic blue whale calls.
(a) Variations of the prevalent vocalization frequency of Antarctic blue whales observed in 2002–2010 (dots). The vocalization frequency was determined from the PSD of sea noise averaged for each week of observation. The small circles are the vocalization frequency measured in week 21 and the dashed line shows the best linear fit to their values. (b) Estimates of the linear decrease rate of the call frequency in each season and their 95% confidence intervals.
Variations in the maximum spectrum level of noise in a frequency band from 26 to 28 Hz containing acoustic energy from blue whale vocalization (solid line) corrected for the PSD of ambient noise averaged in a frequency band from 28 to 30 Hz (its level shown by the dashed line), showing seasonal changes in the noise intensity from blue whale calls. The noise spectrum levels were estimated from weekly average PSDs of ambient noise.
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