Fig 1.
A scheme of the sperm whale’s head and sound production.
(a) Bl: Blow hole; Di: Distal air sac; Fr: Frontal air sac; Jo: Junk organ; Ln: Left naris; Mo: Monkey lips; Rn: Right naris; So: Spermaceti organ. (b) According to the bent horn model, the production of a click generates multiple pulses (p0, p1, p2, p3 etc.).
Fig 2.
The OνDE cabled seafloor observatory.
(a) The site of installation of the OνDE acoustic array (NEMO Test Site—2,100 meters of depth, South-western Ionian Sea). ODV (2012) [45]. (b) Photo of the OνDE frame before the deployment. The hydrophones and electronics housing are labeled (H1, H2, H4, H3, Housing).
Fig 3.
Acoustic Presence of Sperm Whales from April 1st to December 31st 2005.
The days of data acquisition in 2005 are shown in gray (from April 1st to December 31st). The days with acoustic presence of sperm whales recorded in at least 1 file are shown in blue.
Fig 4.
The detection of clicks emitted by sperm whales.
(a) The detection of short pulses in the energy of the signal (Butterworth filter, bandpass 3–16 kHz). (b) A single click extracted; the pulses p1 and p2 are shown in the signal waveform. p0, p1/2 and p3 are not visible in the figure.
Fig 5.
The average of the cepstrum analysis for the clicks selected in a 5-min recording.
A Stable IPI of about 4.5 ms is shown as a time delay between the pulses. The cepstrum analysis is applied to the clicks identified (Fig 4), using both the amplitude (blue line) and the energy (red line) of the signal.
Fig 6.
The cepstral peaks identified in a 5-min recording.
The figure shows the cepstra values found for all the clicks detected (Figs 4–5). The value for each click varies as a function of the Inter Pulse Interval found, applying the cepstrum analysis to the energy of the signal extracted. A Stable IPI of about 4.5 ms is present and for some clicks the time interval p1–p3 (about 9 ms) is also visible.
Fig 7.
The Stable IPIs identified within the OνDE dataset from 24th January to December 2nd 2005.
The Stable IPI identified are shown by red dots. Highlighted in blue, the 93 days analysed with the automatic algorithm. The acoustic presence of sperm whales was documented by spectrogram analysis and listening for all the recordings where Stable IPIs were measured. The error on the single measurement is ± 0.05 ms, calculated as the square root of the sum in quadrature between the sampling time accuracy and the 3 σ of the cepstrum peak distribution, used to evaluate the Stable IPI.
Table 1.
A comparison between the Stable IPI obtained applying the manual and the automatic methods.
Fig 8.
The size of the sperm whales grouped in classes of 50 cm.
(a) The results obtained using Gordon’s formula. (b) The results obtained using Growcott’s formula.
Fig 9.
The size of the sperm whales detected from January 24th to December 2nd 2005.
The size of the sperm whales with a Stable IPI ≤ 4 ms are represented using the equation of Gordon (red dots) [15]; the equation of Growcott et al. (2011) [23] is instead used only for the animals with a Stable IPI > 4 ms (about 11 m in length) (blue dots). Knowing the size of the animals recorded, the sex and sexual maturity are hypothesized.