Behaviour and ecology of bottlenose dolphins in Doubtful Sound, Fiordland, New Zealand

Abstract

On 353 days during three years of fieldwork from 1994 to 1997 in Doubtful Sound, New Zealand I collected data on abundance, body proportions, seasonal movements, diving behaviour, surface behaviour and association patterns of a population of bottlenose dolphins (Tursiops truncatus). This population lives at the southern limits of the species' distribution.

Most of the dolphins could be found year-round in the fiord. Mark-recapture analysis estimated the population size to be around 67 (cv=1.9%). Photogrammetric measurements of body size and proportions showed that the dolphins in Doubtful Sound grow considerably larger than bottlenose dolphins in lower latitudes. This effect was interpreted as morphological adaptation to the cool climate. The dolphins also showed behavioural adaptations to the extreme environment. They showed seasonal differences in habitat usage and moved along a gradient of surface temperature. In summer the dolphins were found predominantly in the upper arms where they also gave birth. In winter the entrance areas of the fiord system were preferred. A variety of techniques was employed to study the diving behaviour of this population in order to infer on the feeding habits. It was found that these dolphins at least sometimes dive to great depth (222m) and that probably a mixture of individual and cooperative feeding takes place in this population. A sequence analysis of surface behaviour events showed a 'surface' cluster and an 'aerial' cluster of behaviours which may describe different motivational states. The analysis led to the development of a field key, which will help researchers to compare quantitatively behaviour studies in the field. A detailed analysis of data on associations amongst the resident dolphins of the population showed that the community is matrifocal; organised around old and probably post-reproductive females, together with mothers of young calves near the centre of interest. Males are either associated with each other in the periphery or associated to females in. the centre.