Browsing by Author "Cockcroft, Victor Gavin."

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Biological studies of bottlenose dolphins from Natal coastal waters.
(1989) Cockcroft, Victor Gavin.; Ross, Graham.
The biology of bottlenose dolphins from the east coast of southern Africa is investigated. Births occur predominantly in summer, after a one year gestation period. Calves are born at a length of approximately 103 cm and a mass of nearly 14 kg. Neonates are closely attended by the mother, she determines the calf's respiration rate and behaviour as it swims in a negative pressure vortex high on her flank. The initial weeks are marked by the calf's acquisition of swimming skills and its ability to regulate its own respiration rate. Although the calf takes solid food between six months and one year, suckling may continue for as much as three years. Initial growth is rapid but slows with the approach to puberty. Females reach sexual maturity at about ten years of age and may undergo a series of rapid ovulations before fertilisation occurs. Subsequently, the mean ovulation rate indicates a three year calving interval. Males show a mass growth spurt at the onset of puberty, between 10 and 12 years, and reach sexual maturity between 12 and 15 years of age. In both sexes, physical maturity and asymptotic size are reached at about 15 years and maximum life-span is in excess of 40 years. Although a wide variety of fish and cephalopods are taken, the fishes Pomadasys olivaceum, Scomber japonicus, Pagellus bellotti, Trachurus delagoae, and the cephalopods Sepia officinalis and Loligo sp. contribute some 60 % by mass of all prey taken. Different sex and maturity classes of dolphins consume differing sizes and species of prey, implying a partitioning of food resources within the group. Comparisons of stomach volumes with estimates of prey mass and nutritional requirements suggest that calves and lactating females may need to feed more often than other sex and maturity classes. Examination of shark stomachs indicates that shark predation may be an important component of dolphin natural mortality. Four species of shark, the Zambesi (Carcharhinus leucas), the tiger (Galeocerdo cuvien), the great white (Carcharodon carcharias) and the dusky shark (Carcharhinus obscurus) are implicated as dolphin predators. Estimates from the number of these four species caught annually and the frequency of occurrence of dolphin flukes and vertebrae in their stomachs suggest that a minimumof 20 bottle nosed dolphins or 2.2 % of the estimated population in southern Natal coastal waters are killed each year by sharks. Some 32 bottlenose dolphins, about 4 % of the estimated population, are captured in shark nets annually. Calves of two years or less constitute nearly 45 % of the catch, while lactating females make up a further 15 %. An analysis of biological, environmental and physiographic factors associated with each capture, suggest that feeding and probably prey movement and distribution are directly associated with capture. PCB, t-DDT and dieldrin concentrations in the blubber of male bottlenose dolphins increase with age and reach levels that may impair testosterone production. In females, there is an 80 % decline in residue concentrations after the first or second ovulation. Evidence presented suggests that first born calves receive possibly lethal doses of residues within two months of birth. Residue concentrations in dolphins from different geographical areas varied significantly, implying a degree of isolation of sections of the population. Investigation of reproductive parameters indicates that mortality of bottlenose dolphins off Natal probably equals or exceeds the replacement rate. The necessity for and requirements of further research on the natural history of bottlenose dolphins off the east coast of southern Africa are summarised.
The distribution and abundance of the humpback dolphin (Sousa chinensis) along the Natal coast, South Africa.
(1994) Durham, Ben.; Cockcroft, Victor Gavin.; Forbes, Anthony Tonks.
Populations of the humpback dolphin in Natal, South Africa, are subject to increasing pressures including capture in the shark nets and habitat degradation, and concern has been raised about the status of the population. A minimum of 95 humpback dolphins were caught in the shark nets during the period from 1980 to 1992. Capture and sighting records of the Natal Sharks Board revealed a relatively high occurrence of humpback dolphins at Richards Bay. Elsewhere, in southern Natal, the infrequent sightings and captures were attributed to a seasonal occurrence of dolphins, possibly due to temporary movements away from resident areas. Sighting rates reported by the Natal Sharks Board has decreased by 55%from 1984-86 to 1990-92 and may reflect a decrease in the population. In a photo-identification study, searches took place in ten search areas in Natal. The sighting rates in the different areas revealed a relatively high density of humpback dolphins occurring in north central Natal, from the Tugela River to the St. Lucia estuary (including Richards Bay). This distribution correlated significantly with the turbidity of the water and the width of the inshore continental shelf, and was inversely related to the density of bottlenose dolphins. Within the northern Tugela Bank region, higher densities of dolphins were found surrounding the five river mouths and estuaries. The Natal population was estimated to be between 161 to 166 animals (95% confidence limits 134 to 229). The annual mortality due to shark net captures approximates 4,5%of the population. Various evidence, including a high mortality rate and a decrease in the annual sighting per unit effort reported by the Natal Sharks Board suggest that the humpback dolphin population in Natal is vulnerable and may be decreasing in size. A proposal is made to reduce the capture rate by relocating shark nets away from the Richards Bay harbour.
Ecological studies of bottlenose and humpback dolphins in Maputo Bay, southern Mozambique.
(2008) Guissamulo, Almeida Tomas.; Cockcroft, Victor Gavin.; Perrin, Michael Richard.
The ecology and population biology of bottlenose and humpback dolphins inhabiting the Maputo Bay, Southern Mozambique were studied through boat based photoidentification surveys and behavioural methods between December 1995 and December 1997. Data from preliminary surveys carried out in 1992 are also included. Bottlenose dolphins occurred throughout the year in Maputo Bay, but were sighted infrequently (36% of surveys). Their occurrence and group size were significantly larger during winter. Group size of bottlenose dolphins not differ significantly between months, daylight hours, semi-lunar tidal cycles and depth. Most identifiable bottlenose dolphin individuals had low site fidelity, but nursing females had relatively high site fidelity, implying that Maputo Bay is a nursing area. The group dynamics of bottlenose dolphins suggests a fluid (fission-fusion) social organisation. Age and sex appears to influence the degree of association between individuals. An influx of bottlenose dolphins occurred during winter and influenced group size and occurrence. Bottlenose dolphins occurred in restricted areas of the Bay, preferring the north eastern area (the pass between the Bay and the Ocean), and along the 3 km strip from the east coast of Inhaca Island. Their distribution here did not vary with depth, although they did not occur in intertidal areas. Feeding dominated both frequency and proportion of time of bottlenose dolphin behaviour. Social behaviour accounted for a small proportion of time (10%) and was greater in open unsheltered areas. Neither season, nor depth, nor daylight, nor tides influenced the proportion of time allocated to the types of behaviours. Non directional movement occurred on most of the sightings of bottlenose dolphins, but smaller groups of dolphins moved inshore at high tide late in the afternoon. Few bottlenose dolphin births were observed and the numbers of animals born varied between years. Recruitment rates at six months and one year were low and mortality rates of calves appeared high, but were probably inaccurate because the fate of mother-calf pairs that left the area could not be established. Population estimates of the bottlenose dolphins varied between 170 and 526 individuals. The precision of these estimates was poor, because of high proportion of transient individuals which almost certainly violated some model assumptions. Humpback dolphins occurred throughout the year in Maputo Bay, but were sighted infrequently (21% of surveys). The occurrence and group size of humpback dolphins were not influenced by season, months, daylight, semi-lunar tidal cycles and depth. Groups of humpback dolphins in Maputo Bay were the largest observed along the east coast of Southern Africa. Most identified humpback dolphin individuals showed low site fidelity, but a relatively high proportion of individuals (including nursing females) had high site fidelity. Humpback dolphin groups had a fluid (fission- fusion) social organisation, but there was a large proportion of stable associations between resident individuals. Age and sex appears to influence the degree of association between individuals. An influx of humpback dolphin individuals occurred during summer but did not change group size or occurrence, suggesting the occurrence of an outflux of other individuals. Humpback dolphins in the Bay ranged between the deep north eastern Maputo Bay to the southern intertidal areas. Most sightings occurred within 1 km from shore along the eastern coast of Inhaca Island, at variable depth. Humpback dolphins spent more time feeding (57%) and travelling (30%) than socializing and resting, because of their movements between extensive intertidal areas and deep channels. Social behaviour contributed a small proportion of time (10%), but increased when humpback dolphins moved towards and within shallow sheltered areas. Neither season, nor depth, nor daylight, nor tides influenced the proportion of time allocated to the types of behaviours of this species. Non directional movement occurred on most of the sightings of humpback dolphins, thought this was significantly influenced by diurnal tides. Few humpback dolphin births were observed and their numbers varied between years. Recruitment rates at six months and one year were low and mortality rates appeared high, but were probably inaccurate because the fate of mother-calf pairs that left the area could not be established. Population estimates of humpback dolphins varied between 105 and 308 individuals, but their precision was poor, because of a high proportion of transient individuals which almost certainly violated some model assumptions. Maputo Bay is a feeding, breeding and nursing area for bottlenose and humpback dolphins. These species are threatened by intense fishing activity, habitat degradation, coastal development and disturbance by powerboats and other activities (tourism, port) carried out in and around Maputo Bay. Their low estimated growth rates imply the implementation of precautionary conservation measures. The actual distribution of these species may reflect the deterioration of the habitats in the western part of the Bay. A coastal zone management plan to address environmental problems affecting the dolphin species need to be formulated and implemented.