The experience of using of mobile hydrophone stereosystem for acoustic localization of killer whales (Orcinus orca) in Avacha Gulf
1. Fedutin I.D., 2. Filatova O.A., 3,4. Burdin A.M., 3. Shevchenko I.N.
1. Central Forest State Nature Biosphere Reserve, Tver region, Russia
2. Moscow State University, Moscow, Russia
3. Kamchatka Branch of Pacific Institute of Geography, Far East Division of Russian Academy of Sciences, Petropavlovsk-Kamchatsky, Russia
4. Alaska SeaLife Center, Seward, USA
In order to effectively investigate a communication system, it is necessary to observe the signal production of identified animals. Terrestrial ethologists are able to localize sound in air to identifiable individuals or are able to observe motor actions indicative of sound production. The human ear cannot locate underwater sounds, and cetaceans show no visible movements related to sound production even when they can be observed underwater. While recording cetacean underwater sounds with an omnidirectional hydrophone, a researcher faces a serious problem of individual and group identification of signals, which strongly restricts the possibilities of investigating acoustic communication. The traditional solution for cetacean sound localization is the hydrophone array. Fixed and towed hydrophone arrays of different configurations have been successfully used by many researchers (Watkins and Schevill 1972; Miller and Tyack 1998; Lammers and Au 2003; Charif et al. 2001). But these methods have some restrictions while working from an inflatable boat when the situation requires quickness and maneuverability. Hydrophone arrays are usually rather unwieldy and need special software for the localization of acoustic signals, which makes it impossible for real-time identification of the signal. For this reason it becomes necessary to devise an easy and reliable device to localize underwater sounds by ear. A similar device was already used for the localization of sperm whale (Physeter macrocephalus) clicks (Miller, P.J.O. pers. comm.), and our goal was to adapt it for operating from an inflatable boat in the conditions of Eastern Kamchatka.
The working principle of this device is the use of the two hydrophones divided by a soundproof disc, which makes them directional. A signal from the each hydrophone is fed to one of the channels of stereo-input of recording system. Signal is listened through the earphones, and signal from the each hydrophone goes to the separate ear of the operator. Rotating of the device gives the illusion of turning of the operator’s head, which enables him to find bearing of underwater sound by changes of the sound coming from the hydrophones.
The base of the device is a metal (lead) disk 300 mm in diameter and 12 mm thick, which also functions as ballast (fig. 1, a)
On the disk is fastened an arm for attaching a rotating pole (fig. 1, b). On both sides of the base soundproof syntactic foam plates with hydrophone fastenings are arranged (fig. 1, c). The rotating pole consists of four plastic sections (each 1.6 meters in length) (fig. 1,e) fixed by split pins, and supplied with bibb with compass (fig. 1, f). For direction-finding and recording we used hydrophones with a frequency range from 300 Hz to 40 kHz and a Sony digital audio tape-recorder DAT TCD-D100.
Thus the mobile stereosystem provides an accessible alternative to traditional hydrophone arrays, enabling the ability to find the underwater sound direction by ear, with compactness, cheapness of materials and componentry, and comparative simplicity of construction. Recording the sound for subsequent analysis does not require expensive multichannel devices. The principle of working with the device is intuitively obvious, and the operator does not need a special qualification.
Localization consists of two stages. At the first stage, the operator determines which side the sound comes from by rotating the device until he achieves the audibility of signal in only one of the hydrophones.
Fig. 2. Arrangement of stereosystem and sound source for audibility of signal in only one of the hydrophones.
Then, for more accurate direction-finding, he rotates the device to achieve the audibility of signal in both hydrophones, which takes place when the edge of the disc is directed to the source of the underwater sound
The azimuth to the source of the signal is found by compass, which is built into the rotatory pole.
Stereosystem was successfully used for localization of killer whale groups within listening distance in situations when the visual localization was impossible. The distance of reliable listening of killer whale underwater calls usually exceeds the distance of visual detection from the boat. Observers often listen to killer whale calls by hydrophone, but cannot detect them visually. Stereosystem enables the observer to find the direction of the vocalizing animals in order to approach to a distance sufficient for visual detection. During the 2003 field season (end of June – beginning of July, end of August – beginning of September) in 5 cases of 17, killer whale groups were localized with the use of the stereosystem, including one case in which the group was found in the fog with visibility of 100-500 m. The stereosystem was also used for separating sounds of killer whale groups in acoustic contact. During the recording the boat was placed between the two groups, and the stereosystem was directed so that signal from each group went to a separate hydrophone. By the analyzing of recordings obtained by this method, we have found out that vocalizations of killer whale groups when they are in acoustic contact represent the interchange of discrete calls. This fact supports the hypothesis that one of the functions of discrete calls may be long-range communication between killer whale groups.
It seems likely that a mobile hydrophone stereosystem can also be useful for recording and localizing the sounds of other cetacean species.
Charif R.A., Clapham P..J., Clark C.W. 2001. Acoustic detections of singing humpback whales in deep waters off the British Isles. Marine Mammal Science 17(4): 751-768
Lammers M.A. and Au W.W.L. 2003. Directionality in the whistles of Hawaiian spinner dolphins (Stenella longirostris): a signal feature to cue direction of movement? Marine Mammal Science 19(2):249–264.
Miller P.J.O. and Tyack, P. L. 1998. A small towed beamforming array to identify vocalizing resident killer whales (Orcinus orca) concurrent with focal behavioral observations. Deep-Sea Research (45):1389-1405.
Watkins, W. A. and Schevill, W. E. 1972. Sound source location by arrival times on a non-rigid 3-dimensional hydrophone array. Deep-Sea Research (19): 691-706.
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