Concurrent recording of dolphin behaviours, frequency-modulated tones, and pulsed vocalisations (including echolocation clicks) underwater with a swimmer-propelled system [abstract]

A mobile video/acoustic system (MVA), previously developed (Dudzinski et al. 1995), was modified with new hydrophones and a new hydrophone mount to increase the recorded frequency bandwidth and to decrease low frequency noise due to wobble in the physical mount, respectively. The MVA permits real-time synchronous recording of vocal and behavioural activities of individual dolphins. The system is swimmer-propelled and facilitates localisation of dolphin sound sources by associating video data of animal distributions with audio data from two hydrophones spaced relative to the human interaural distance as scaled to sound speed in water. A recent addition to the original design was a small second housing containing a Sony TCD D8 digital recorder together with a pre-processor circuit which detects the highly directional part of a dolphin's echolocation 'clicks' and makes these audible and recordable. The pre-processor comprises a preamplifier with band pass filtering which selects only signals between 90 kHz and 130 kHz. The filter output is then rectified and the low frequency envelope extracted by a further filter. This technique produces a low frequency pulse accurately representing the original signal amplitude which can be recorded on the R- DAT sound track. As the directionality of the original signal, the inter-pulse intervals and the pulse amplitudes are retained this band-limited data carries significant information and the loss of the high frequency spectrum is acceptable in this application. This 'click detector' data is recorded onto one track of the recorder while unprocessed low frequency signals from the hydrophone are captured on the second. The R DAT stores real-time information (date and time) in the sub-code of recorded signals so that video image data, with in-picture timecode, can be correlated accurately with the echolocation behaviour. Data is captured when dolphins swim directly towards the MVA and click detector unit. The click detector housing mounts to the dorsal surface of the MVA housing, and externally measures 37cm by 9cm by 5cm. The modified MVA system (including click detector) has been used to document the behaviour and vocalisations of interacting Atlantic spotted dolphins Stenella frontalis in the Bahamas and bottlenose dolphins Tursiops truncatus in Japan. Example data sets will provide evidence of this system's utility for examine the social lives and signal exchange among interacting, free-ranging dolphins.