Mapping cetacean distribution in the Western Cape to explore potential range shifts in light of climate change

Abstract

The southern tip of South Africa is characterised by two major current systems, each of which is associated with its own species. The southwest coastal waters represent the limit of the local or global distribution ranges of four of the eight cetacean species that most frequently occur in the area. The goal of the present study was to determine the fine scale distribution of these species and subsequently investigate which environmental factors influence and possibly limit their current distribution ranges. The current study was focused around the Cape Peninsula, the south-western tip of South Africa, where one of the strongest thermal gradients in the southern Benguela occurs. The waters surrounding the Cape Peninsula, including False Bay which lies on the eastern side of the Peninsula, fall within the transition zone between the Benguela Current and Agulhas Current and are subject to high seasonal variety in mean sea surface temperature. Surveys with conducted in the waters ranging from Table Bay on the west coast to Cape Hangklip, the eastern tip of False Bay. Sightings data were collected from dedicated boat-based surveys conducted over a two-year period. Dedicated surveys resulted in over 3 000 kilometres of searching for animals and in the detections of all eight species. The effective southern range end of Heaviside’s dolphins (Cephalorhynchus heavisidii) was redefined as Hout Bay on the western side of the Peninsula, as encounters were absent further south than this. Dusky dolphins (Lagenorhynchus obscurus) are likely restricted from moving farther east of Cape Point, the southern tip of the Peninsula, by a combination of the warmer waters present in False Bay with changes in prey type and availability. Humpback dolphins (Sousa plumbea) were frequently encountered in False Bay showing a clear westward range expansion since reports in the early 1990s, possibly a result of tracking warmer sea surface temperatures within the bay. A further westward expansion is possibly limited by cooler temperatures and the presence of steep rocky shores in the south-western corner of False Bay as humpback dolphins prefer habitats with sandy bottom types. Common dolphins (Delphinus delphis) were encountered throughout the study area. Their distribution range is most likely determined by the movements of their prey and less influenced by water temperature as they are recorded in both warm and cooler waters. Two migratory whale species, southern right whales (Eubalaena australis) and humpback whales (Megaptera novaeangliae), were recorded in shallow, coastal waters throughout the study area, whereas the non-migratory Bryde’s whale (Balaenoptera brydei) was seen predominantly in False Bay in slightly deeper waters. Species distribution models were built using the occurrence data to investigate relationships between the spatial occurrence of a species and its surrounding environment. The models predicted a strong influence of temperature and chlorophyll concentration on the distribution of Heaviside’s and dusky dolphins on the west coast. All models indicated depth as the most influential factor driving humpback dolphin distribution, while suitable habitat for common dolphins and bottlenose dolphins was primarily influenced by water temperatures slightly offshore. Changing oceanographic conditions have direct impacts on the distribution and availability of prey species, which in turn affects the movement patterns of cetaceans. Increases in water temperature are of particular concern for cool water limited species such as those in the Benguela Ecosystem, especially for the endemic Heaviside’s dolphin which are subject to range reduction in response to changing oceanographic conditions.