Sperm Whale Clicks Research Articles

Acoustic census of sperm whales in the eastern temperate North Pacific

An NOAA/ONR acoustic line-transect survey was conducted in spring 1997 to estimate the abundance of sperm whales in the eastern temperate North Pacific using a 52-m research vessel. An ITI hydrophone array was towed at 100-m depth as the vessel surveyed approximately 25 000 km of predetermined transect lines at 8 kn. Acoustic signals received by a pair of hydrophones were digitized and sent up to the ship via a coaxial tow cable. Bearing angles to the locations of sperm whales were calculated in real time based on differences in signal arrival times between the two hydrophones, and whale locations were determined by the convergence of bearing lines as the ship continued along its course. The ship was maneuvered to resolve the left/right ambiguity in whale locations and, during daylight hours, was directed to this estimated location to obtain visual estimates of group size when the whales surfaced. A total of 102 distinct groups of sperm whales were acoustically detected with sufficient amplitude to estimate bearing lines. ‘‘Slow clicks’’ (with a period of 4–8 s) were detected at ranges up to 20 nmi, whereas the more typical sperm whale clicks (with a period of 0.5–1.0 s) were typically heard at less than 5 nmi.

Time and frequency domain characteristics of sperm whale clicks.

Regular clicks from diving sperm whales, both large bull males and smaller females, were recorded in deep oceanic water off the Azores and subsequently sampled to computer disks for digital analysis. A total of 8540 clicks were marked and analyzed. Simple temporal analysis of the interclick intervals during feeding dives revealed mean click rates for male sperm whales of 1.1713 s-1 and 1.9455 s-1 for females. Fourier analysis showed distinctive peaks in the spectra of bull male sperm whales at 400 Hz and 2 kHz which were stable over extended periods of up to 20 mins. The clicks contained higher frequency components with energy ranging up to at least 12 kHz but not concentrated at any sharply defined frequency. The clicks of smaller female sperm whales showed similar spectral peaks, shifted to 1.2 and 3 kHz, respectively, but these peaks were less pronounced than those in the male click spectra and less stable with time. Higher frequencies were also present up to at least 15 kHz. The previously reported multiple pulse structure of sperm whale clicks is confirmed, but digital filtering reveals this structure to be frequency dependent. Analysis using the short-time Fourier transform confirms the complex time-frequency structure of individual clicks. The frequencies at which the multiples emerge in male and female clicks supports the idea of air cavities in the sperm whale head acting as sound reflectors, although the magnitude of the second pulse at high frequencies suggests some form of off axis distortion. It is also possible that air cavity resonance in the head of the sperm whale may act to reinforce the high-frequency components of the click, and that such components may have superior range and resolution performance in terms of echolocation.