Source level estimation of dolphin communication calls as a potential method for species identification

Abstract

Research into the acoustic structure of communication calls of different odontocete species has attempted to identify distinctive features that might assist in discrimination of individuals, pods or species. Most studies have concentrated on frequency modulation characteristics, such as the general shape or contour of the call. Although the source level (SPL re: 1 μPa at 1 m) of echolocation signals has been studied and appears to reflect some body size dependency, signal intensities of ‘‘whistles’’ have not been investigated closely. In this study maximum source levels of communication calls were estimated in open sea conditions, using an acoustic multi-path ranging technique. Preliminary results suggest species-specific differences in maxium call intensities that may provide a useful additional cue to acoustically distinguish species in the wild. Data used for spectral and statistical analysis were recorded using a single hydrophone attached to a pelagic trawl fishing in continental shelf waters in the Bay of Biscay. Maximum SPLs of social calls were extracted and distances between hydrophone and vocalizing animals were obtained exploiting information inherent in recorded muli-path signals. Concurrent visual observation provided species identification. Source levels of calls were calculated based on a calibrated reference signal. Methodology and limitations of this technique are discussed.