Biological studies of bottlenose dolphins from Natal coastal waters.
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Date
1989
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Abstract
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.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1989.
Keywords
Dolphins., Bottlenosed dolphins., Theses--Zoology.