Victor G. Cockcroft

The distribution of small odontocete cetaceans off the coasts of South Africa and Namibia

A total of 2 077 records of approximately 49 000 small cetaceans, including dedicated and incidental sightings and specimens, was analysed to define distribution patterns of the 28 species found within southern African waters. Distribution analyses reveal distinct component patterns, including cosmopolitan (found in all waters) and pelagic cosmopolitan (found in all pelagic waters) components, tropical, subtropical and warm temperate components of the Agulhas Current system, an Agulhas Bank component, a South and East Coast inshore component, and West Coast neritic and pelagic components. While the offshore distribution appears to be determined by water depth, possibly through distribution of the principal prey, longshore distribution appears to be determined by water temperature. The high diversity of small cetacean species found within the relatively small study region results from the wide range of zoogeographic components present. These components arise from the wide range of water temperature resulti...

Phylogenetic analysis of mtDNA sequences suggests revision of humpback dolphin (Sousa spp.) taxonomy is needed

Humpback dolphins (Sousa spp.) have a wide distribution in the tropical Atlantic and Indo-Pacific Oceans and a confused taxonomy. Morphological assessments suggest three species groupings - Sousa teuszii (eastern Atlantic), Sousa plumbea (western Indo-Pacific), and Sousa chinensis (eastern Indo-Pacific) - but most taxonomies recognise only two species - S. chinensis (Indo-Pacific), and S. teuszii (Atlantic). To investigate phylogenetic relationships, mitochondrial DNA control region sequences (338 base pairs) from 72 Sousa representing three populations in the Indo-Pacific (South Africa: S. plumbea, n = 23; China: S. chinensis, n = 19; and Australia: S. chinensis, n = 28), and S. teuszii in the Atlantic (Mauritania, n = 2) were generated. All three Indo-Pacific populations formed robust, monophyletic clades with high bootstrap (BS) and Bayesian posterior probability (BPP) scores. Surprisingly, humpback dolphins from South Africa and China formed a strongly-supported clade with the Atlantic S. teuszii (BS 63%, BPP 0.92) to the exclusion of animals from Australia. Genetic divergence between animals from China and Australia (DA = 8.4% ± 2.47%) was greater than between China and South Africa (DA = 5.1% ± 1.80%). These results strongly suggest that Australian humpback dolphins are not S. chinensis but may represent a distinct species in their own right.

Group size and seasonal pattern of occurrence of humpback dolphins Sousa chinensis in Algoa Bay, South Africa

The occurrence of humpback dolphins Sousa chinensis and their group size in Algoa Bay, South Africa, are described for the period May 1991–May 1994. The mean group size of dolphins was seven (ranging between 3 and 24). Group size was not affected by diurnal, tidal or lunar variability. Solitary, large individuals were commonly observed and they occasionally joined groups of other humpback dolphins. The number of sightings of humpback dolphins per unit effort and their group size varied seasonally, increasing in summer and again in late winter. These observations follow a regular seasonal fluctuation in water temperature, coincide with the dolphins’ reproductive seasonal cycle and could be related to seasonal changes in the abundance and distribution of their prey.

Seasonal fluctuations in occurrence of inshore Bryde's whales in Plettenberg Bay, South Africa, with notes on feeding and multispecies associations

Seasonal fluctuations in the occurrence of inshore South African Brydes whales Balaenoptera edeni were investigated between November 2005 and June 2008. Sighting data were collected in Plettenberg Bay on the south-east coast of South Africa. Brydes whale occurrence was modelled in relation to the following environmental covariates: sea surface temperature, chlorophyll a concentrations and wind speed. Seasonal increases in encounter rates (sightings per day) were observed during summer and autumn, with a peak in April that corresponded to increased feeding activity and above average aggregation sizes. All three environmental covariates were significant factors in terms of explaining variability in the occurrence of whales. Multispecies associations with common dolphins Delphinus capensis and Cape gannets Morus capensis were most common in summer and autumn, when feeding activity was highest.

Whistle vocalizations of Indo-Pacific bottlenose dolphins (Tursiops aduncus) inhabiting the south-west Indian Ocean

Populations of Indo-Pacific bottlenose dolphin (Tursiops aduncus) are distributed along coastal regions of the south-west Indian Ocean (SWIO), from South Africa to Kenya. An account of whistles from wild T. aduncus inhabiting the SWIO is provided here. Recordings were made at Plettenberg Bay (South Africa) and Zanzibar Island (Tanzania) and the frequency trace of whistle contours (n = 1677) was extracted. Multiple parameters were measured from each whistle and compared between regions and encounters. Regional variation was significant in all parameters assessed except for start and middle frequency (frequency at half the duration). Whistles from Zanzibar Island ended on average 4 kHz higher than those from Plettenberg Bay, and had a steeper frequency gradient. However, mean frequencies differed by <1 kHz and population averages for the adopted frequency distribution showed similar patterns, with a peak between 5 and 7 kHz. Whistle parameters were strongly influenced by recording encounter, likely reflecting the presence of different individuals, group compositions and behavioral contexts during recording occasions. Comparisons within the genus showed that T. aduncus from the SWIO have amongst the lowest start and minimum frequency of whistles within Tursiops.

Recolonisation of the Robberg Peninsula (Plettenberg Bay, South Africa) by Cape fur seals

The Cape fur seal Arctocephalus pusillus pusillus colony at Robberg Peninsula, Plettenberg Bay, on the south-east coast of South Africa, was driven to extinction by indiscriminate harvesting by the late 1800s. Seals only began to recolonise this site in the 1990s. This study describes the recolonisation process from 2000 to 2009, exploring both within- and between-year count data of seals using the site. Counts increased over the study period from <300 animals to >3 100. Generalised linear models indicated the importance of year and month in explaining variability in the counts. Withinyear variability in the counts decreased over the study period, which may be related to an increasing proportion of resident (as opposed to transient) seals in the colony. However, the colony is currently still in a transition phase with a low ratio of breeding to non-breeding animals, based on the low numbers of pups born in the colony (currently still <100 per year). The influx of seals to the Robberg area may be associated with shifts in prey availability at the ecosystem level. The colony benefits from the protection afforded by the reserve status of the Robberg Peninsula and the existence of a marine protected area adjacent to it. However, human interference associated with fishing and/or ecotourism on the peninsula may inhibit development into a substantial breeding colony. Potential interventions for the conservation and management of this colony are discussed.

The Natural History and Conservation of Indian Ocean Humpback Dolphins (Sousa plumbea) in South African Waters.

Although most knowledge on the biology of Sousa plumbea has primarily come from South African waters, a number of research gaps remain on the natural history and status of the species in the region. Research on two populations in South African waters for which some historical data exist may aid in highlighting long-term changes in the biology and natural history of this little known coastal delphinid. Recent studies on the age, growth and reproduction of animals incidentally caught in shark nets in Richards Bay, KwaZulu-Natal, yielded a lower maximum age estimate of 24 (previously 46) growth-layer-groups (GLGs), sexual maturity of 7.5 and 8 GLGs in males and females (previously 12-13 and 10 GLGs, respectively), an ovulation rate of 0.2 and a 5-year calving interval (previously 0.3 and 3-year calving interval) than previously reported. These differences may be due to a difference in the interpretation of GLGs between observers or a predominance of young males being caught in the shark nets. Stomach content analysis revealed a change in the relative proportions of the main prey items over the past 25 years, but no difference in species richness or diversity was found between the sexes. No change in trophic level was recorded between 1972 and 2009. Field studies in Algoa Bay, Eastern Cape, conducted 16 years apart indicated a decline in the mean group size (from 7 to 3 animals), a decline in the maximum group size (from 24 to 13 animals), an increase in solitary individuals (15.4-36%), and a change in behaviour from predominantly foraging (64-18%) to mainly travelling (24-49%). The observed changes are suggestive of a change in food availability, resulting in a range shift or a potential decline in numbers. These studies indicate the importance of long-term studies to monitor population changes and their possible causes. A number of threats, such as shark nets, pollution (noise and chemical), and coastal development and disturbance, to the humpback dolphin populations in South Africa have been identified. Urgent action is required to ensure continued existence of the species in South African waters.

Dugong abundance and distribution in the Bazaruto Archipelago, Mozambique

Despite the distribution of dugongs Dugong dugon ranging across nearshore waters of the tropical and subtropical regions of the Indian Ocean and western Pacific Ocean, their distribution in the western Indian Ocean is highly fragmented and appears to be declining. The population of the Bazaruto Archipelago is believed to comprise the only viable population in the region. In all, 27 surveys were flown over the Bazaruto Bay area to define the distribution and estimate the abundance of the species in the area. A total of 9 052 nautical miles of survey effort was flown during the surveys, from which there were 355 sightings of 760 dugongs. Two core areas of distribution were apparent within the surveyed area; a northern core area spread within the 10 m isobath between the Save River mouth and Ponta Bartolomeu Dias (21°24′S), and a southern core area aligned with the shallow sandbanks to the north and south of Santa Carolina Island. Group sizes recorded in the Bazaruto Archipelago were comparable to group sizes recorded in other regions where dugongs occur, although few large (>20) groups of dugongs were seen in this study. Line transect analyses of each survey showed dugong densities were considerably lower than densities recorded in surveys in Australian waters or in the Arabian Gulf, with a population estimate of 247 dugongs (CV = 34.1) when all surveys were considered, and 359 dugongs (CV = 38.2) when only the surveys that were carried out under adequate sighting conditions were included.

Spinner dolphins Stenella longirostris off south-west Mauritius: abundance and residency

Spinner dolphins Stenella longirostris longirostris off the south-west coast of Mauritius are subject to ongoing anthropogenic disturbance in the form of daily dolphin tourism, which has intensified since 1998. Abundance of this species was estimated using photo-identification data and mark-recapture analysis. Between April 2008 and June 2010, identification photographs were collected from dolphins occurring along a 30 km length of the coast of south-west Mauritius. A total of 250 groups were encountered over 229 survey days. Mark-recapture analyses were performed on a photographic dataset of more than 8 000 good- and excellent-quality images and 83 animals were identified as distinctively marked individuals. The majority (85.5%) were seen more than once and resightings indicated a resident population. The compiled version of SOCPROG 2.4 was used to investigate the lagged identification rate. The fitted model supported a mostly resident population with additional animals moving in and out of the study area. The estimated abundance of the total population in the study area ranged between 138 and 399 individuals. Our results can be used for monitoring the population for fluctuations and for encouraging both the enforcement of laws regarding dolphin watching and the development of further means of management needed to ensure the long-term presence of this population.

Abundance of the Indo-Pacific bottlenose dolphin Tursiops aduncus off south-west Mauritius

The abundance of Indo-Pacific bottlenose dolphins Tursiops aduncus off the south-west coast of Mauritius was estimated using capture-mark-recapture modelling. Over the past two decades this population has been subjected to ongoing anthropogenic disturbance in the form of extensive coastal development. Furthermore, daily dolphin tourism, which started in 1998, has rapidly increased in intensity. Identification photographs were collected between April 2008 and June 2010 from dolphins occurring along a 30 km length of coast where a dolphin tourism industry is concentrated. A total of 137 groups were encountered over 229 survey days. Over 5 000 photographs were taken, from which 35 individuals were considered to be sufficiently distinctively marked to use in mark-recapture analyses. The majority (85.7%) were seen more than once and resighting frequencies indicated a resident population. Three newborn calves were recorded during the study. Open population models produced abundance estimates of <100 individuals in the population. These results will be used to make recommendations for the conservation and management of this small, resident population, which is a valuable economic resource for the island but is currently under threat from high levels of human activity.