In recent years research into the whistle structure of different species of odontocete calls has attempted to identify distinctive individual, pod, or simply species-specific, features, that might assist in acoustic identification and discrimination. So far most of these studies have concentrated on the frequency modulated signal by examining these for characteristic features in the frequency-time domain using FFT analysis. Such analysis has concentrated on the centre frequency, frequency deviation, number of inflection points (frequency reversals), and more significantly the general shape or contour of the call. The Source Level of echolocation signals has also been studied for other reasons but this parameter appears to reflect some body size dependency. This paper discusses the feasibility of using the maximum Source Level of the narrow band social calls (whistles) as an additional cue when attempting to distinguish between species while studying free-ranging animals. The data used for spectral and statistical analysis was recorded from a Dutch fishing research vessel (FRV Tridens) during the 1996 and 1997 CETASEL project sea trials. A single hydrophone was attached to a pelagic trawl net fishing in relatively deep water (100-200 m depth) along the edge of the continental shelf in the Eastern North Atlantic sea areas SW of Eire, through Biscay and towards Finnisterre. Maximum sound pressure levels of social calls were extracted and converted into Source Levels (SL re 1µPa at 1m) using a calibrated 13 kHz pulsed cw pinger (Dukane) to provide a known SL reference. The necessary range information (distance between hydrophone and vocalising animal) was obtained by means of a multi-path echo-ranging method (Kaschner et al 1997). Parallel visual observation in good weather conditions provided the species identification and a time series analysis established the probability of the association between these sighted animals and the acoustic recordings of species whistles. On one occasion a very vocal group of bowriding Common Dolphins Delphinus delphis were recorded simultaneously in air, using a gun microphone directly above them at the bow, and underwater using the hydrophone attached to the trawl at a distance of 518m behind them. The maximum underwater Source Levels of individual signals which were recognisable in both recordings were calculated and compared with the other results in order to test the validity of the method described above. This paper discusses the methodology and limitations of this relatively simple technique but the results achieved to date suggest considerable potential for SL estimation in open sea conditions. Alternative, quieter, platforms than a pelagic trawler are recommended.
Kaschner, K. et al. (1997). Analysis and interpretation of cetaceans sounds obtained by a hydrophone attached to a pelagic trawl. In European Research on Cetaceans - 11. Proc. 11th Ann. Conf. ECS, Stralsund Germany (Eds. P.G.H. Evans) European Cetacean Society, Cambridge