Abstract:
Heaviside‟s dolphins, Cephalorhynchus heavisidii, are among the least known
cetaceans. They are endemic to the coastal waters along the west coast of southern
Africa. The occurrence, behaviour and group dynamics were investigated for these
dolphins in Table Bay, South Africa during the summer of 2008 and 2009. Markrecapture
photographic identification techniques, modelling techniques and
Geographical Information System (GIS) were used to analyse the data.
The dolphins were observed primarily in a well-defined 7 km2 area on the south side
of Table Bay along a 3 km east-west stretch of coast known as Granger Bay, and
within 2 km from shore. They used this area heterogeneously; for the array of
behaviours observed, all core areas (50 % kernels) were located in Granger Bay with
very little overlap of areas used for milling, socialising, resting and travelling.
Milling and socialising were exhibited more often than travelling and resting.
Foraging was never observed in areas close to shore, which supports earlier studies
suggesting that these dolphins forage primarily offshore.
Of the 95 identified individuals, 24.2 % of the individuals were seen only once and
73.7 % of the individuals were sighted less than five times while two individuals were
seen 11 times. Groups varied in size from one to 26 animals with a median of five
animals. Many of the potential dyads were never seen and those observed had
generally weak measure of association. The group structure appears highly dynamic,
with great lability in group sizes and membership, suggesting a fluid social system
with mostly weak inter-individual bonds. Individuals in Table Bay display low site
fidelity, at least in the short-term.
The observed pattern of area use and behavioural preferences indicate that coastal
management strategies should consider the ecological and behavioural needs of
Heaviside's dolphins to appropriately safeguard this species as a valuable component
of South African natural treasures and legally protected taxa. Further continuing
research is needed to build on this initial study to better understand the processes that
shape the population structure and group dynamics of Heaviside's dolphins, and to
provide further detail on habitat preferences and areas used, both in Table Bay and
environs.
