Abstract:
Photo-identification surveys over three years along 390 km of coastline north of Cape Town,
revealed that Heaviside’s dolphin distribution was consistent between years and higher in
areas more exposed to swells and with greater long-term availability of small hake Merluccius
capensis (their principal prey). Dusky dolphin sighting rates varied considerably between
years, but were generally higher in areas with lower hake availability and sandier shores
(mostly straighter coastline). Large groups of 50–200 dusky dolphins were only seen in
St Helena Bay, the site of a wind-driven upwelling zone. Heaviside’s dolphins were found in
shallower, cooler water than dusky dolphins and were more likely to be seen during brighter
phases of the moon (when nocturnal light conditions may influence the vertical migration
patterns of prey) and in areas of high hake abundance. Near-shore fishing activity was higher
in the northern half of the study area and clustered around harbours. Set netting occurred only at Yzerfontein and St Helena Bay, but due to changes in the industry is currently thought to be
a low threat to the population. Interactions between Heaviside’s and dusky dolphins were
usually neutral and sympatry appears to be mediated by differences in overall range and the
type and size of prey species taken.
