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The conservation ecology of the Nile crocodile (Crocodylus niloticus) at Ndumo Game Reserve in North Eastern KwaZulu-Natal and the Rio Maputo floodplain in South Eastern Mozambique.Calverley, Peter. 09 September 2014 (has links)
Up until 1969 Nile Crocodiles (Crocodylus niloticus) were considered as vermin in South Africa and were actively persecuted throughout the country. In an effort to re-establish viable populations within protected areas in Zululand a restocking program was initiated in the late 1960‟s and early 1970‟s. Ndumo Game Reserve (NGR) in north eastern KwaZulu-Natal was one of the reserves involved in the restocking program and the Nile Crocodile population in the reserve increased from a minimum absolute abundance of 348 (SE ± 3.39; N = 3) in the early 1970‟s to a minimum absolute abundance of 992 (SE ± 58.70; N = 4) in the 1990‟s. However, in recent years there has been some concern that the NGR Nile Crocodile population may be on the decline, initiating the current investigation into the ecology and conservation of the NGR population.
We examined changes in relative abundance using aerial survey data from 1971 – 2009. The precision and accuracy of population estimates was affected by water level, season, aircraft type and the use of different observers. A correction factor was applied to survey data and the current NGR Nile Crocodile population is estimated at an absolute abundance of 846 (± 263).
Distribution data from the aerial surveys were also used to examine habitat use over the last 40 years and revealed that Nile Crocodiles were not evenly distributed in NGR and that crocodiles favoured the Phongola over the Usuthu floodplain systems. NGR is characterised by a floodplain mosaic landscape and crocodile distributions between the various habitat patches were influenced by landscape physiognomy and composition as well as connectivity and corridor quality. Anthropogenic disturbances influenced the functionality of the floodplain landscape negatively with impacts on habitat use and connectivity. To quantify the effects of environmental conditions on crocodile habitat use we conducted 40 diurnal counts at Lake Nyamithi between 2009 and 2012 and related changes in crocodile numbers here to temperature, rainfall and water level. Crocodile density in Lake Nyamithi was significantly and negatively related to average maximum ambient temperature and numbers increased in the lake over the cool, dry winter season. Water level and rainfall had strong but not significant (p >0.05) negative influence on crocodile density in Lake Nyamithi. Environmental variables influenced different size class of Nile Crocodiles differently and the density of crocodiles in the 1.5 – 2.5 m Total Length (TL) size class were significantly influenced by rainfall and average minimum monthly temperature.
Movement patterns of 49 Nile Crocodiles between 202 – 472 cm total length (TL) were followed over 18 months using mark-resight (n = 36), radio (n = 10) and satellite (n = 3) telemetry. The duration of radio transmitter attachment (131 days, SE ± 11.35) was significantly related to TL and reproductive status. Satellite transmitters stopped functioning after 15 (SE ± 12.53) days and home range was calculated for 7 crocodiles ranging in size from 202 cm TL – 358 cm TL. Sub-adults (1.5 - 2.5 m TL) occupied smaller, more localized home ranges than adults (> 2.5 m TL). Home ranges overlapped extensively suggesting that territoriality, if present, did not cause Nile Crocodiles to maintain spatially discrete home ranges in NGR during the dry season. A single large scale migration event occurs every year between October and November whereby the majority of the NGR crocodile population leaves the reserve and enters the Rio Maputo floodplain in adjacent Mozambique and only return in April/May.
Nesting effort (19 – 21 %) in NGR was comparable to other populations of Nile Crocodile in southern Africa. Nests are completely destroyed by floods once every 10 years and predation rates may range from 20 – 86 % per year. In addition to aerial surveys, nesting surveys and movement studies crocodiles (n = 103) were caught opportunistically to collect demographic data on population structure. The population structure of Nile Crocodiles in NGR is currently skewed towards sub-adults and adults suggesting an aging population that may decline naturally in the future. This could be due to low recruitment levels in NGR that are not able to sustain the artificially high population size created by the restocking program. Sex ratios were skewed towards females in the juvenile and sub-adult size classes and towards males in the adult size class while the overall sex ratio was even between males and females.
It is predicted that the NGR Nile Crocodile population will decline in the future and that this decline should be considered as a natural process. However, the rate of decline will be accelerated at an unnatural speed and to an unnatural extent due to poaching, uncontrolled harvesting and destruction of nesting habitat within NGR. Based on the findings of the current study, management recommendations for the conservation of the combined NGR – Rio Maputo Nile Crocodile population were made. It is important that further research takes place in the Rio Maputo floodplain in Mozambique to better quantify the nesting ecology of the NGR Nile Crocodile population and to identify possible threats facing Nile Crocodiles in this region. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.
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The ecology of Nile Crocodile (Crocodylus niloticus) in Pongolapoort Dam, Northern KwaZulu-Natal, South Africa.Champion, Gareth. January 2010 (has links)
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. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.
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