The Okavango Delta, Botswana, is a unique ecosystem and this is reflected in its
extraordinary biodiversity. The Nile crocodile (Crocodylus niloticus Laurenti) is the
apex predator, and performs a number of vital functions in this system, making it a
keystone species. The panhandle crocodile population has declined significantly over
the last 80 years and is now threatened as a result of past over-exploitation and
present human disturbance. In order to effectively conserve this species and in turn
the health of this important region it is imperative to gain an understanding of their
ecology and population dynamics.
The population status of the Nile crocodile in the panhandle region of the Okavango
Delta, Botswana, was assessed using a combination of capture-mark-recapture
surveys, spotlight surveys and aerial surveys. The capture-mark-recapture
experiment was conducted continuously from 2002 - 2006. A total of 1717
individuals, ranging in size from 136 mm – 2780 mm SVL, were captured, of which
224 animals were recaptured. Using a Bayesian technique, the total annual
population in the panhandle region of the Okavango Delta was estimated to be 2 570
± 151.06 individuals, with an adult population of 649.2 individuals with the number
of breeding females estimated to be 364 individuals. It was concluded that this
population cannot sustain the further harvest of breeding animals prior to the increase
and stabilization of the population.
Spotlight counts revealed a decline in the encounter-rate of crocodiles on the
Okavango River with time, although more long-term data needs to be collected to
confirm this trend. During the low-water season (September - February), 22.34 % of
all crocodiles were observed, while during the flood-season only 13.34 % were
observed, yielding correction factors for spotlight surveys of 4.46 (low-water) and
7.49 (high-water) for all animals in the panhandle.
Two aerial surveys, conducted at the low-water and high-water peaks yielded total
estimates of 588 (77.7 % of adults) during the low-water period and 350 (56.7 % of
adults) during the high-water period. Correction factors of 1.28 (low-water) and 1.77
(high-water) were calculated for aerial surveys.
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An annual average of 50 nests was located in the panhandle, representing a 50 - 60 %
decrease over the last 20 years. In regions with high human disturbance, breeding
females situated their nests in hidden locations, away from accessible channels.
Hatchlings exhibited elongation of the jaw in order to capture smaller prey items and
morphometric shifts in jaw shape coincided with a dietary change at 400 mm SVL.
The jaw became broader and deeper as animals matured, presumably in preparation
for larger mammalian prey. The average growth rate of recaptured yearlings was
0.198 ± 0.116 mm.d-1 SVL and was closely correlated to the amount of time an
individual spent in above-average water temperatures. Body condition (RCF) was
significantly and positively correlated with a rise in water-level and negatively
correlated to time spent in above-average water and air temperatures. Average RCF
values were intersected when animals had spent 50 % of their time in above-average
temperatures and water level.
Generally crocodiles in the panhandle showed no significant sex-related differences
in their sizes or the distances they travelled. The majority of recaptures (62.5 %)
moved less than 500 m from the initial capture site. Adults in the panhandle occupied
definite ranges, within which were preferred core areas where the majority of their
time was spent.
The panhandle crocodile population has declined significantly over the last 80 years,
and is now threatened as a result of past over-exploitation and present human
disturbance. The management of this population, including both its conservation and
sustainable commercial utilisation, requires an adaptive strategy based on accurate
monitoring procedures.