A system for automatically detecting the vocalisations of Harbour Porpoises Phocoena phocoena is described. The system consists of a high frequency hydrophone, an analogue electronics module and software running on a laptop computer. The analogue electronics module uses band-pass filters to split signals from the hydrophone into three frequency bands. Envelope tracing circuitry then converts the high frequency signals in these bands to lower frequencies, which are digitized and then analysed on the computer. Decisions as to which clicks are from porpoises and which are from some other source are made automatically, based on the relative amplitudes of the signal in the three frequency bands anti also on the shape of each detected pulse. If two hydrophones are used, it is possible to measure bearings (subject to a rotational ambiguity about the array axis) to detected clicks based on the difference in arrival time of the signal at each hydrophone.
The system has been implemented in a practical package that can be deployed easily from almost any type of vessel or mooring, giving it a wide range of applications. The software, which is designed to run on a PC compatible laptop computer, contains a number of displays to present the data and to facilitate analysis. Properties of individual detected clicks are stored on the computer's hard drive to give a permanent record of detections and to facilitate off-line analysis of data collected in the field.