Spatial and reproductive ecology and population status of the Nile crocodile (Crocodylus niloticus) in the Lake St Lucia estuarine system, South Africa.
Combrink, Alexander Stanislou.
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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.