Movement patterns of coastal bottlenose dolphins in the presence of a fast-flowing, prevailing current: shore-based observations at Cape Vidal, South Africa

Abstract

The presence and movement of Indo-Pacific bottlenose dolphins Tursiops aduncus were investigated using shore-based observations made during a humpback whale Megaptera novaeangliae migration survey at Cape Vidal, South Africa, undertaken between June and October 1988–1991. Occurrence was analysed as counts of dolphin sightings per hour within a generalised estimating equation framework. Bottlenose dolphin sightings decreased throughout the daily survey period, and there was a small seasonal peak in sightings during August. Data on movement trajectory were collected using a theodolite for 60% of the groups of dolphins sighted. Most groups were  observed travelling in a northerly direction, with few groups recorded moving south. A generalised linear model indicated that northwardtravelling dolphins  were located closer to shore, travelled at higher speeds, and occurred in bigger groups under low-tide conditions, than those seen moving in a southerly direction. The southwardflowing Agulhas Current is close to shore at Cape Vidal, where the continental shelf is particularly narrow. We suggest that bottlenose dolphins in this area have adopted a specific movement regime to cope with the consistent fast-flowing currents that dominate their environment at Cape Vidal.

Keywords: Agulhas Current, generalised estimating equation, generalised linear model, sightings, theodolite, Tursiops aduncus

African Journal of Marine Science 2011, 33(3): 393–401