The ecology of Nile Crocodile (Crocodylus niloticus) in Pongolapoort Dam, Northern KwaZulu-Natal, South Africa.
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Date
2010
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Abstract
In general Nile Crocodile Crocodylus niloticus numbers in South Africa appeared to have
recovered after persecution and eradication attempts during the last century. Within the last
decade, however, the future of South Africa’s Nile Crocodiles seems ominous yet again, as they
are faced with renewed threats including habitat destruction and/or degradation. The primary
Nile Crocodile populations in South Africa, Kruger National Park, Lake St Lucia and Ndumo
Game Reserve are all currently threatened as a result of anthropogenic actions. The vulnerability
of South Africa’s major Nile Crocodile populations has highlighted the need for further studies
on and assessment of other populations in the country.
This study was conducted from April 2009 to July 2010 on the Nile Crocodile population
found in Pongolapoort Dam. The aim of the study was to obtain baseline data on the ecology of
this previously unstudied population, which included obtaining an estimate of population size
and structure, the reproductive dynamics and success of the population, general distribution of
the population in the dam and seasonal changes in their distribution. The impact of the
impoundment on this population was also discussed.
Initial surveys from 1981 and 1989 described few crocodiles in the system. Currently
Pongolapoort Dam contains a significant Nile Crocodile population that was previously not
considered as substantial. A conservative estimate of 273 Nile Crocodiles was determined for
Pongolapoort Dam in 2009-2010. A combination of survey methods allowed for a population
structure to be gauged and identified as having 116 juveniles (< 1.2 m), 75 sub-adults (1.2 - 2.5
m), and 82 adults (> 2.5 m). Currently the population has a high percentage of juveniles (42 %),
suggesting a growing population, with the proportion of adults (30 %) able to sustain a viable
population into the future.
From the construction of the Pongolapoort impoundment in 1972 the water level has
fluctuated and the surrounding landscape has been altered. As a result the Nile Crocodiles
residing in the area had to adapt to the ever changing environment. Their general distribution
changed after dam wall completion, when the dam began to fill. First distributional change was a
movement out of the gorge section into the newly flooded areas. After the Domoina floods
(1983) the dam level rose by over 70 % and the crocodiles moved into the current inlet section.
The majority of the crocodile population is now found in the inlet section of the Pongolapoort
Dam, utilizing the Phongola River in summer months and residing in the inlet section as
historical basking sites during the winter months.
Investigating reproductive ecology is essential in order to access the population dynamics
of an unstudied population, as reproductive output can be a measure of population health.
Reproduction and nesting of Nile Crocodiles in Pongolapoort Dam, and in particular determining
the effects of the impoundment on these were investigated. No previous reproductive effort had
been documented prior to this study. Crocodiles congregated at a major basking site, where the
Phongola River entered the dam, during August 2009 with a 576 % increase in numbers. This
signalled the commencement of the breeding season. Females with transmitters made short trips
upstream during this time. In November, with the first rains, the river rose and the majority of
crocodiles moved up the inlet, and females established nests. Three major nesting areas were
identified, two of which were located in the river inlet to the dam. Approximately 30 nesting
females were identified during the 2009/2010 nesting season. All nesting areas identified had
been used in prior nesting seasons. Nests were located on a variety of substrate types, from clay
formed through culluvial and fluvial deposits to course river sand. Several of the nests were
predated by Water Monitor (Varanus niloticus). Although the number of nesting females was
greater than expected, during the study period there was a total recruitment failure of nests along
the river due to a flash flood of the Phongola River in January 2010, destroying all nests prior to
hatching. As several juvenile crocodiles were found during surveys, this preliminary study
suggests that the Pongolapoort Dam Nile Crocodile population has a relatively high potential
reproductive out-put, although their annual successes may vary greatly because of loss of nesting
sites because of water level fluctuations and predation. It appears that the impoundment has
generally had a positive impact on this Nile Crocodile population recruitment although suitable
nesting sites may become limited.
There appear to be no current threats to the Nile Crocodile Pongolapoort Dam population,
however illegal gill-netting and poaching on the dam and surrounding reserves is on the rise and
if not prohibited can result in future problems. A second concern is the high abundance of alien
invasive plants that dominate the area, most notably in the river inlet section, the Nile Crocodiles
main nesting area. The water quality entering the system is unknown at present and should be
tested in future studies to assess whether there may be any reason for concern.
In general the Nile Crocodile population in Pongolapoort Dam appears to be one of the
least vulnerable and most reproductively successful in South Africa at present. The population
has increased dramatically as a result of successful reproductive output even with the ecosystem
changes as a result of the impoundment of the Phongola River. It is unlikely that the population
increase was as a result of immigration from surrounding areas as the dam wall is a substantial
barrier between the dam and the lower crocodile population of Ndumo Game Reserve some 70
km downstream. The high number of crocodiles found through all size classes, juveniles to large
adults, also suggests that this population has been stably increasing for a number of years and has
a sustainable breeding population.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.
Keywords
Nile crocodile--Ecology--KwaZulu-Natal--Pongolapoort Dam., Nile crocodile--Reproduction., Nile crocodile--Effect of dams on., Population biology--KwaZulu-Natal--Pongolapoort Dam., Spatial behaviour in animals--KwaZulu-Natal--Pongolapoort Dam., Theses--Zoology.