Abstract:

Resilience of a depressed Nile Crocodile population at Lake Sibaya

Xander Combrink1, S Kyle1 , RH Taylor2

1 Ezemvelo KwaZulu-Natal Wildlife
2 School of Life Sciences, University of KwaZulu Natal

Formerly widespread throughout the waterbodies of eastern South Africa, viable Nile crocodile (Crocodylus niloticus) populations are now restricted to three disjunct protected areas in KwaZulu-Natal (KZN), Mpumalanga and Limpopo. Growing evidence suggests that protected populations are declining, including the C. niloticus population at Lake Sibaya in northeastern KZN. Historically, Lake Sibaya supported a much larger crocodile population than present. In 1958 Ken Tinley noted large numbers of crocodiles together with several nesting sites and in 1970, 30 crocodile nests were counted. Crocodile monitoring at Lake Sibaya started in 1985 and aerial, nesting and spotlight surveys have been conducted since then, although not annually. In 1990, 107 crocodiles were counted from the air. Since then, aerial surveys have revealed a consistent decrease in the index of relative abundance and the seven individuals observed during the 2009 aerial survey, represented a 93.5% decrease compared with the 1990 count. Conversely, crocodile breeding declined sharply from the 30 nests recorded in 1970 to an average of 1.5 known crocodile nests recorded during the last decade. During January 2011 we conducted a spotlight count and recorded 24 juveniles (<1.5m). Spotlight counts have been shown to be more effective in counting the juvenile component of a crocodile population. These juveniles, estimated between 4 to 5 years old, must have survived from the 2005/06 and 2006/07 breeding season, despite relative few adults and no nesting activities reported for those years. Crocodile nests are easily overlooked, and some females must have managed to nest successfully with ensuing high levels of survival for those hatchlings. High hatchling survival rates have been reported in other studies where few hatchlings enters a system, but as their numbers increase the survival rate decreased dramatically, possibly reflecting predation by non-crocodilian predators (fish, birds, monitor lizards etc.). The high recruitment rate of crocodiles at Lake Sibaya revealed during the spotlight count is very encouraging for this depressed population. It also emphasise the value of conducting both aerial and spotlight counts, where possible, for a more complete insight into the relative abundance and estimated size structure of the population.

Presentation Topic

Resilience of a depressed Nile Crocodile population at Lake Sibaya

 

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