Browsing by Author "Combrink, Alexander Stanislou."

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Population survey of Crocodylus niloticus (Nile crocodile) at Lake Sibaya, Republic of South Africa.
(2004) Combrink, Alexander Stanislou.; Korrubel, Jan L.; Ross, James Perran.
The Greater St Lucia Wetland Park (GSLWP) World Heritage Site, is one of the most important remaining protected areas for the conservation of Crocodylus niloticus (Nile crocodile) in the Republic of South Africa Although crocodiles are present at low densities in some of the wetlands throughout the GSLWP, at Kosi Bay and Lake Sibaya, the majority is found at Lake St Lucia Concern about the status of the crocodile population at Lake Sibaya prompted a quantitative assessment of the present situation to direct appropriate management action to secure the future viability of this population Due to the complex nature of crocodile surveys, and the paucity of available scientific literature in South Africa, a literature review of global crocodilian survey techniques provided the basis for a survey strategy to determine an index of relative abundance of the crocodile population at Lake Sibaya The population was estimated in 2003 through aerial and spotlight surveys and nest surveys were conducted in 2003 and 2004. The highest count dwing the 2003 aerial surveys was 36 crocodiles, suggesting a decline of 66% during the past 13 years in the population index based on earlier surveys. Sixty five crocodiles were cowlted during the spotlight surveys, 72% more than the highest aerial count (excluding hatchlings), which indicates the importance of using a combined survey approach. A correction factor of 1.72 was calculated for future aerial surveys and the population is estimated at 112 crocodiles, \\-ith a variance of22.49 and standard error of 4.47. Three nests were found during the 2003 nest survey, but none during 2004. Crocodiles rarely produce every year in the wild, and the Lake Sibaya population might display a low reproductive frequency, similar to the nearby Lake St Lucia population Sixty three potential nesting areas were identified and evaluated in terms of their relative suitability for nesting. These sites could play an important role in increasing the population to support a sustainable use progralllffie at Lake Sibaya. Despite legal protection, the population is clearly under threat as a result of direct and secondary pressures. To secure the future viability of this population, I recommend that Ezemvelo KZN Wildlife develop an integrated crocodile management plan through collaboration with The Greater St Lucia Wetland Park Authority and the local communities adjacent to Lake Sibaya, where the conservation and increase of crocodiles will benefit the communities that are dependent on the lake for their daily livelihoods. The likely alternative might be extinction of this important predator from the largest freshwater ecosystem in South Africa's first World Heritage Site.
Spatial and reproductive ecology and population status of the Nile crocodile (Crocodylus niloticus) in the Lake St Lucia estuarine system, South Africa.
(2014) Combrink, Alexander Stanislou.; Downs, Colleen Thelma.
The Lake St Lucia estuarine system, Africa’s largest and oldest protected estuary, also contains the largest Nile Crocodile (Crocodylus niloticus) population in a single waterbody in South Africa. We investigated the species’ spatial and reproductive ecology as well as population status in order to make management recommendations. During the first decade of the 21st century, the St Lucia estuarine lake experienced a prolonged drought, streams ceased flowing and in 2006 more than 90% of the total water area evaporated. We conducted 10 aerial surveys from 2009 - 2013 and recorded the majority of crocodiles in the Narrows, a ~27 km low salinity channel south of the lake. Above average rainfall at the end of 2010 resulted in the refilling of the lake, and most crocodiles moved north to the lake. We estimated the sub-adult and adult population at 1005 ± 137 individuals. We investigated detailed movements and activity for 18 Nile Crocodiles using GPS-satellite transmitters. The overall activity level was 41.0 %, and it differed significantly throughout the day. There was a significant seasonal effect on activity, peaking during autumn (52.0 %), while crocodiles were most inactive in winter (30.5 %). Crocodile size and mobility were positively correlated with mean daily movement (1244 m). Adults moved more at night, but sub-adults were significantly more mobile during the day. There was a considerable seasonal variation in mobility, with the longest movements during autumn and the shortest in winter. About 60 % of total daily movements were < 1 km per day, but for sub-adults this calculation was 96 %. We recorded complex and varied home range patterns for 14 Nile Crocodiles, resulting from differences in size, sex, reproductive status and habitat. The median home range and core-use area of adults were significantly greater than sub-adults. Three size- related patterns of home range behaviour emerged for adult males; transient, (< 3.0 m TL), topographically confined (3.5 - 4.0 m TL) and “territorial” (> 4.0 m TL). Adult males revealed an inverse correlation between home range size and crocodile size, while the home range sizes of adult females were generally more homogeneous. All nesting females displayed an explosive increase in mobility and space-use subsequent to the nesting period, and all adults, except one female in the central lake, moved during winter in the drought period to large crocodile congregations south of the lake. Sub-adults occupied significantly smaller home ranges than adults, which were habitat specific with strict spatial partitioning. They remained in shallow vegetated areas adjacent to deep water, avoiding open deep water altogether. Nile Crocodile nests have been monitored since 1982, with mean nest abundance = 76.19 ± 6.42, range: 29 - 141. The macro-level heterogeneity of nesting habitats reflects the spatio-temporal diversity of the Lake St Lucia system, and is possibly unique within a single Nile Crocodile population. Changes in nest abundance and distribution were seemingly related to increased human disturbance and habitat transformation in the northern and southern parts of the lake. Hydrological variability, especially during droughts, combined with the state of the estuary mouth (i.e. open or closed), affected prey abundance/availability contributing to large variation (6.9 % - 56.4 %) in nest effort from 1982 - 2013. All nests were located close to freshwater streams or seepage areas. We confirmed the re-use of the identical nest-site by a female, while other females oviposited in nest-sites occupied by different females during previous years. Despite variable nest effort, the St Lucia nesting population remains the largest recorded nesting population in South Africa, and least vulnerable to flooding. The mean home range of nesting Nile Crocodile females (0.85 ha) was significantly smaller than non-nesting females (108.41 ha) during the nesting season. Activity levels and mean daily movements on the nest were 8.1 ± 2.5 % and 213 ± 64 m, respectively, and increased to 47.9 ± 11.7 % and 2176 ± 708 m during the post-nesting period. Overall levels of nest fidelity were 82.8 ± 11.7 %, which increased to 87.3 ± 7.8 % at night. The highest nest fidelity recorded during incubation was 99.7 % over a 96 day period. We investigated nest predation, hatchling liberation and nest-guarding activities of nesting Nile Crocodile females using remote camera traps. We captured 4305 photographs of 19 nest-guarding females over four years. Seven nests (36.8 %) were raided by the egg predators Water Monitors (Varanus niloticus) and Marsh Mongooses (Atilax paludinosus), on average 12.1 days ± 6.2 subsequent to trap camera employment. All females settled back on the nest following the first predation event and on average, females returned to their nests three times ± 0.8 between nest raids before finally abandoning the nest. Nest raids continued on average 5.9 days ± 1.6 while on average 18.8 ± 4.0 raids per nest were recorded. Five females were captured by trap cameras liberating hatchlings. During the day females were almost never photographed on the nest, but during the late afternoon or early evening females moved onto the nest and continued to stay there during the night. Females always defended their nests aggressively against non-human intruders. We investigated homing behaviour and specific movements using a GPS-satellite transmitter by translocating an adult female (2.7 m), with a known home range, ~50 km north (straight line distance) to the False Bay area of Lake St Lucia. Following release, the individual moved a total distance of 178.3 km over 136 days (mean daily movement = 1311 ± 207 m), compared with 60.4 km (mean daily movement = 444 ± 32 m) for the identical time period the previous year. Homing movement was not continuous, but characterised by periods of extensive and directed mobility followed by prolonged periods of inactivity associated with freshwater or low salinity habitats. The translocated crocodile displayed remarkable navigational abilities, even though this required negotiating complex habitat challenges including extensive areas of the lake that were either hypersaline or completely dry, resulting in frequent and extensive overland movements. On 14 September 2012, the individual returned to the same freshwater pool where it was captured 136 days previous. This is the first study to confirm homing behaviour for Nile Crocodiles, and supports growing evidence that crocodilians and other ectothermic taxa possess complex navigational abilities. Our study revealed numerous novel insights into the ecology and behaviour of Nile Crocodiles and some of the findings may be applicable to other crocodilian taxa. We hope the results will guide the management and conservation of this threatened species.