Daylight behaviour of humpback dolphins Sousa chinensis in Algoa Bay, South Africa

Abstract

Data on the daylight behaviour of humpback dolphins Sousa chinensis were collected during sea- and land-based surveys undertaken in Algoa Bay, Eastern Cape, South Africa, throughout a three year period. Dolphin activities/behaviour were categorised as: 'feeding', 'travelling', 'opportunistic feeding', 'socialising and playing', 'resting' and 'other'. It seems apparent that behaviour determines the spatial geometry of the dolphin group, but not the group size. The surfacing-breathing interval is similar for 'feeding', 'opportunistic feeding', and slow 'travelling', but differs considerably from the pattern displayed during fast 'travelling'. Daylight behaviour of humpback dolphins is dominated by 'feeding' and shows a regular pattern which is probably governed by the diurnal cycles of their prey. Generally, 'feeding' peaks in the morning and gradually decreases through the rest of the day. As 'feeding' decreases, 'travelling' and 'opportunistic feeding' increase, both peaking in the afternoon. 'Resting' and 'socialising and playing' occur with similar frequency throughout the day. This pattern varies little between summer and winter, as does the overall proportion of daylight behaviours. The only significant seasonal difference is in the frequency of 'social/sexual' behaviour which peaks in summer. Although tidal cycle influences to some extent the behaviour of humpback dolphins, in Algoa Bay their daylight activity/behaviour is predominantly governed by time of day.