Group characteristics, site fidelity and seasonal abundance of bottlenose dolphins tursiops aduncus in Jervis Bay and port Stephens, south-eastern Australia

Abstract

Social organisation and abundance of bottlenose dolphins (Tursiops aduncus) in Jervis Bay (JB) and Port Stephens (PS), NSW, were investigated through behavioural/photo-identification surveys between May 1997 and April 2000. Mean group size was significantly larger at JB (12.3 ± 0.87, n =167) compared to PS (6.8 ± 0.37, n = 218). At both sites, groups were significantly larger when calves were present. Group size varied with activity, being smallest when feeding and largest when socialising. While mean group size of feeding dolphins did not vary between sites, travelling and socialising groups were significantly larger in JB. Site fidelity was assigned based on sighting rates and presence across seasons. Sighting rates varied significantly between areas, but the proportion of dolphins categorised as residents, occasional visitors and transients did not. Minimum abundance by season, based on mark-resighting of recognisable individuals, ranged from 61 ± 3.2 to 108 ± 7.1 in JB and 143 ± 8.1 to 160 ± 8.1 in PS. Differences in group size at the two sites may relate to social factors and/or human impacts, while differences in abundance may be associated with habitat size and complexity. The lack of dolphin matches between areas suggests that they represent distinct populations.