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Chapter 2 Near-shore distribution of Heaviside’s (Cephalorhynchus heavisidii) and dusky dolphins (Lagenorhynchus obscurus) at the southern limit of their range in South Africa, interspecific interactions and potential conflicts with fisheries.
Abstract
The range of Heaviside’s dolphins off South Africa overlaps with several fisheries and the dusky dolphin. Using data collected during small boat photo-ID surveys of Heaviside’s dolphins from 1999-2001 along 390km of coastline north of Cape Town (340 S), we investigate the distribution of Heaviside’s and dusky dolphins with respect to environmental conditions and inshore fishing effort. Although not feeding diurnally near-shore where data were collected, longshore concentrations of Heaviside’s dolphins were consistent between years and tended to be higher adjacent to areas which over the long term had higher availability of small hake Merluccius capensis (their principal prey). Preference was also shown for areas of higher swell and wave activity and to some extent areas with sandy rather than rocky shores. Heaviside’s dolphins were found in significantly shallower water than dusky dolphins and both species were sighted in significantly cooler waters than in the environment generally. Dusky dolphin sighting rates varied considerably between years, but were generally higher in areas with sandy shores (mostly straighter coastline). No evidence of a nursery area was found as both mixed groups and exclusively mother-calf groups were seen throughout the study area. Very large groups of 50-500 dusky dolphins were only seen in St Helena Bay, which is the site of a wind-driven upwelling zone. Very large groups were possibly aggregations of several smaller feeding groups, suggesting variability in feeding strategy throughout their range. Near-shore fishing activity (line fishing, crayfishing and setnetting) was higher in the northern half of the study area and clustered around harbours. Set netting, the most threatening fishery type for dolphins, occurred in two main areas only (Yzerfontein and St Helena Bay) but due to an industry collapse is currently thought to be a low threat to the population. The opportunity for interactions between Heaviside’s and dusky dolphins was high but those observed were usually neutral and sympatry appears to be mediated by differences in overall range and the type and size of prey species taken by the two species.
Introduction
Within the range of most species there is considerable variation in the number and density of animals at a variety of scales (Begon et al. 2005). Marine mammals are no exception and variation in their abundance has been associated with both environmental and biological influences including depth, sea floor gradient, oceanographic features, predator avoidance and competitive exclusion (Goodall et al. 1995; Tynan 1997; Davis et al. 1998; Elwen & Best 2003; Hastie et al. 2005; Heithaus & Dill 2006). The relationships between abundance and covariates may change with the scale of the study, and it is these changes which can illustrate the ecological relationships involved (Benoit-Bird & Au 2003; Johnston et al. 2005). Along the west coast of southern Africa, Heaviside’s and dusky dolphins are known to be sympatric
within the coastal environment but knowledge of their distribution is currently limited to broad scale descriptors. Findlay et al. (1992) describe the general range of Heaviside’s dolphins as “west of Cape Point (18.50 E)… possibly into southern Angola…all sightings in waters shallower than 200m, the highest densities being inshore of the 100m isobath”, and dusky dolphins as entirely sympatric but with wider, although not entirely known limits extending to 190 E (east of Cape Point into False Bay), northwards into Angola to at least 120S and offshore to at least 500m depth and possibly as much as 2000m of water. Such a broad scale overlap of ranges between potentially competing predators may be more clearly differentiated by habitat selection at finer spatial (Parra 2006; Heinrich 2006; Goodall et al. 1995) or temporal (Thompson et al. 2004) scales. To date, no studies have investigated either the environmental factors influencing niche or habitat selection or the sympatry of Heaviside’s and dusky dolphins at finer spatial scales.
Chapter 1 Near-shore diurnal movements and behaviour of Heaviside’s dolphins (Cephalorhynchus heavisidii), with some comparative data for dusky dolphins (Lagenorhynchus obscurus).
Abstract
Introduction
Methods
Results
Discussion
Literature Cited
Figures
Chapter 2 Near-shore Distribution of Heaviside’s (Cephalorhynchus heavisidii) and dusky dolphins (Lagenorhynchus obscurus) at the southern limit of their
range in South Africa, interspecific interactions and potential conflicts with fisheries.
Abstract
Introduction
Methods
Results
Discussion
Literature Cited
Chapter 3 Site fidelity in a dynamic environment: Range, dispersal and social structure of Heaviside’s dolphin Cephalorhynchus heavisidii.
Abstract
Introduction
Methods
Results
Discussion
Literature Cited
Figures
Chapter 4 A population estimate of Heaviside’s dolphins in the southern end of their range.
Abstract
Introduction
Methods
Results
Discussion
Literature Cited
Figures
Appendix 1 – Example images of marks used to identify Heaviside’s dolphins
Chapter 5 Range and movements of female Heaviside’s dolphins Cephalorhynchus heavisidii, as determined by satellite-linked telemetry.
Abstract
Introduction
Methods
Results
Discussion
Literature Cited
Figures
Chapter 6 Synthesis
Literature Cited
