The passive acoustic coding and tracking of cetaceans

Browne, Raymond Walter (2000). The passive acoustic coding and tracking of cetaceans. University of Birmingham. Ph.D.

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Abstract

The objective of the work presented in this thesis was to develop a means of passive acoustic monitoring of cetaceans, in particular the smaller dolphins and porpoises. These small cetaceans are very difficult to monitor using other techniques such as photo-identification or visual line transits, which have been successfully employed when monitoring larger cetaceans like the humpback whale. These techniques do not lend themselves to the smaller, faster cetaceans such as dolphins. This is due to the difficulty in spotting them and in counting them more than once due to their fast movements compared to the observation platform. Many of the larger whale species have been individually identified from these techniques allowing the social interactions of the whales to be monitored. It is unfortunately virtually impossible to use photo-identification techniques on the smaller cetaceans due to the difficulty in getting a clear enough picture and the sheer numbers of dolphins that can be present during an encounter.

These smaller cetaceans are however known to be highly vocal underwater where their sounds can travel for substantial distances. This thesis presents an underwater acoustic monitoring system which exploits this characteristic in order to gain individual information about smaller cetaceans from their vocal repertoire.

The Ligurian Sea was chosen for conducting the experimental trials presented in this thesis due to the abundance of striped dolphins known to inhabit the area. Surprisingly little is known about the vocal repertoire of the striped dolphin since most researchers tend to concentrate on the larger bottlenose dolphin. Therefore an initial trial was conducted to assess the vocal repertoire of this mammal.

The promising results obtained from this initial trial prompted the development of a more sophisticated acoustic monitoring system, which replaced the labour intensive method which was used to analyze the initial data set. Once perfected it was hoped that this system would be able to automatically detect, extract and compare cetacean sounds against a database of previously collected individual sounds. The sounds database could them be used to infer census, social and migratory paths of the smaller cetaceans.

The novel algorithms developed for detection, extraction and comparison of dolphin sounds have been compared with the current methods employed and a substantial improvement has been shown. The extension of these algorithms into a real time system is also a novel addition to the current methods employed for acoustically monitoring cetaceans. Also during analysis of the vocalizations of the striped dolphin sounds, a harmonic low frequency sound was discovered which has not been previously reported in the literature.

The author has hypothesized that this sound could be employed by the dolphin as a Doppler sensitive pulse and the ambiguity function is used to demonstrate the Doppler-range sensitivity of the pulse. This new sound is a harmonic tonal pulse of approximately 0.25 - 0.75 seconds duration. The sound tends to be emitted, before, after or in between echolocation tasks performed by the striped dolphins. The regularity of the pulse and its emission during these tasks suggest that it complements the echolocation information.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):