Ecological studies of bottlenose and humpback dolphins in Maputo Bay, southern Mozambique.

Abstract

The ecology and population biology of bottlenose and humpback dolphins inhabiting the Maputo Bay, Southern Mozambique were studied through boat based photoidentification surveys and behavioural methods between December 1995 and December 1997. Data from preliminary surveys carried out in 1992 are also included. Bottlenose dolphins occurred throughout the year in Maputo Bay, but were sighted infrequently (36% of surveys). Their occurrence and group size were significantly larger during winter. Group size of bottlenose dolphins not differ significantly between months, daylight hours, semi-lunar tidal cycles and depth. Most identifiable bottlenose dolphin individuals had low site fidelity, but nursing females had relatively high site fidelity, implying that Maputo Bay is a nursing area. The group dynamics of bottlenose dolphins suggests a fluid (fission-fusion) social organisation. Age and sex appears to influence the degree of association between individuals. An influx of bottlenose dolphins occurred during winter and influenced group size and occurrence. Bottlenose dolphins occurred in restricted areas of the Bay, preferring the north eastern area (the pass between the Bay and the Ocean), and along the 3 km strip from the east coast of Inhaca Island. Their distribution here did not vary with depth, although they did not occur in intertidal areas. Feeding dominated both frequency and proportion of time of bottlenose dolphin behaviour. Social behaviour accounted for a small proportion of time (10%) and was greater in open unsheltered areas. Neither season, nor depth, nor daylight, nor tides influenced the proportion of time allocated to the types of behaviours. Non directional movement occurred on most of the sightings of bottlenose dolphins, but smaller groups of dolphins moved inshore at high tide late in the afternoon. Few bottlenose dolphin births were observed and the numbers of animals born varied between years. Recruitment rates at six months and one year were low and mortality rates of calves appeared high, but were probably inaccurate because the fate of mother-calf pairs that left the area could not be established. Population estimates of the bottlenose dolphins varied between 170 and 526 individuals. The precision of these estimates was poor, because of high proportion of transient individuals which almost certainly violated some model assumptions. Humpback dolphins occurred throughout the year in Maputo Bay, but were sighted infrequently (21% of surveys). The occurrence and group size of humpback dolphins were not influenced by season, months, daylight, semi-lunar tidal cycles and depth. Groups of humpback dolphins in Maputo Bay were the largest observed along the east coast of Southern Africa. Most identified humpback dolphin individuals showed low site fidelity, but a relatively high proportion of individuals (including nursing females) had high site fidelity. Humpback dolphin groups had a fluid (fission- fusion) social organisation, but there was a large proportion of stable associations between resident individuals. Age and sex appears to influence the degree of association between individuals. An influx of humpback dolphin individuals occurred during summer but did not change group size or occurrence, suggesting the occurrence of an outflux of other individuals. Humpback dolphins in the Bay ranged between the deep north eastern Maputo Bay to the southern intertidal areas. Most sightings occurred within 1 km from shore along the eastern coast of Inhaca Island, at variable depth. Humpback dolphins spent more time feeding (57%) and travelling (30%) than socializing and resting, because of their movements between extensive intertidal areas and deep channels. Social behaviour contributed a small proportion of time (10%), but increased when humpback dolphins moved towards and within shallow sheltered areas. Neither season, nor depth, nor daylight, nor tides influenced the proportion of time allocated to the types of behaviours of this species. Non directional movement occurred on most of the sightings of humpback dolphins, thought this was significantly influenced by diurnal tides. Few humpback dolphin births were observed and their numbers varied between years. Recruitment rates at six months and one year were low and mortality rates appeared high, but were probably inaccurate because the fate of mother-calf pairs that left the area could not be established. Population estimates of humpback dolphins varied between 105 and 308 individuals, but their precision was poor, because of a high proportion of transient individuals which almost certainly violated some model assumptions. Maputo Bay is a feeding, breeding and nursing area for bottlenose and humpback dolphins. These species are threatened by intense fishing activity, habitat degradation, coastal development and disturbance by powerboats and other activities (tourism, port) carried out in and around Maputo Bay. Their low estimated growth rates imply the implementation of precautionary conservation measures. The actual distribution of these species may reflect the deterioration of the habitats in the western part of the Bay. A coastal zone management plan to address environmental problems affecting the dolphin species need to be formulated and implemented.