Movements, nesting and the effects of pollution on the Nile crocodile (Crocodylus niloticus) in the Olifants River, Kruger National Park.
Swanepoel, D. G. J.
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A pilot study was launched in 1993 to determine the movements of crocodiles in the Olifants River in Kruger National Park (KNP). This turned into a research project in 1994 and lasted for five years until 1998. The objectives of the study were to determine movement, study nesting preferences and to investigate the possible effects of metal pollution in crocodiles. The movements of the total population were studied by doing regular foot counts, four times per annum. The basis for movement was found on the principle that a change in numbers signified movement. Foot counts were supported by spot counts at three different locations in the study area at 14 day intervals. The foot counts were done along a pre-determined route, followed with every count, and this resulted in data indicating a definite change in numbers at certain times of the year. A comparison was made between the movements of crocodiles in large bodies of open water (lakes & estuaries), a so called "closed" system and the Olifants River or "open" system. There was a movement of the population during August of every year, followed by another one in December/Jan and yet another in May. The largest movement, in August, co-coincided with the prelude to mating and ultimately nesting. The spot counts indicated that there was a smaller and probably localized movement during May and December of each year. This corresponds to nesting behaviour and the flood status of the river. All indications are that major movements took place during periods of high flow and thus during the rain season (Sept - Feb). A major flood was experienced during February 1996 which changed the geomorphology of the river. In addition to the foot and spot counts, eleven large adult crocodiles were fitted with radio transmitters to monitor their movements for a period of one year (1997 -1998). The results indicated that movements over large distances occurred. The longest distance covered was in access of 36 km. Some individuals returned to the location of origin, thus completing a round trip from their point of departure. The majority of crocodiles fitted with transmitters left the protection of KNP, spent time in neighbouring Mozambique and some returned to Kruger after an average period of three months. Indications are that there is a lack of territorial dominance amongst large individuals at certain times of the year, allowing for less restricted movement up and down the river. A difference was found between the movements of crocodiles in the Olifants River as opposed to a large body of water eg. a lake or estuary. The second objective was to collect data on nesting preference, nest content and egg dimensions. A total of seven parameters were measured including slope, distance to water, height above water, soil type, vegetation, exposure to direct sunlight and the size of the female. Besides these parameters, a comparison was made between egg size, mass and length and the correlation with female size. The results indicated that egg mass and length increased with an increase in female size. The average clutch size was 34 eggs. A difference was found between two nesting years (1997 and 1998) with the larger females nesting in 1998 (egg mass 122.7g). A higher rainfall was measured for 1998, leading to the conclusion that smaller females nested during dryer years. The larger females, because of physical abilities, dug deeper nests ,36 cm in 1998 as opposed to 25 cm in 1997. No difference was found between nesting habits elsewhere in Africa and that in the Olifants River. Twelve individuals of varying sizes ranging from 1.4 m to 4.1 m, were collected from three locations and various tissue samples were analysed for metal content. Tissue from muscle, liver, kidney and fat were analysed for Al, Cu, Cr, Fe, Mn, Ni, Pb, Section ranger and Zn. As this was the first data of its kind to be collected, no meaningful conclusions could be reached, except to offer the data as base line data and for future comparisons. There was however a correlation between Fe concentration in muscle tissue and body size, total length (TL). An increase in TL resulted in an increase in Fe concentration. The opposite were found with Fe in fat tissue. Smaller crocodiles (TL) had higher concentrations of Fe in their fat tissues. Besides the large amount of base line data that resulted from this study, it also highlighted the shortage of knowledge on this species, particularly in Kruger, to enable us to manage and protect them successfully.