Group size and seasonal pattern of occurrence of humpback dolphins Sousa chinensis in Algoa Bay, South Africa

The occurrence of humpback dolphins Sousa chinensis and their group size in Algoa Bay, South Africa, are described for the period May 1991-May 1994. The mean group size of dolphins was seven (ranging between 3 and 24). Group size was not affected by diurnal, tidal or lunar variability. Solitary, large individuals were commonly observed and they occasionally joined groups of other humpback dolphins. The number of sightings of humpback dolphins per unit effort and their group size varied seasonally, increasing in summer and again in late winter. These observations follow a regular seasonal fluctuation in water temperature, coincide with the dolphins' reproductive seasonal cycle and could be related to seasonal changes in the abundance and distribution of their prey.