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The Population Ecology of the Nile Crocodile (Crocodylus Niloticus) in the Panhandle Region of the Okavango Delta, Botswana

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The Population Ecology of the Nile Crocodile (Crocodylus Niloticus) in the Panhandle Region of the Okavango Delta, Botswana The Population Ecology of the Nile crocodile (Crocodylus niloticus) in the Panhandle Region of the Okavango Delta, Botswana. by Sven Leon Bourquin A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy Department of Conservation Ecology and Entomology Faculty of Agrisciences University of Stellenbosch Supervisor: Dr. A. J. Leslie November 2007 ii DECLARATION I, the undersigned, hereby declare that the work contained in this thesis is my own original work and that I have not previously, in its entirety or in part, submitted it at any other university for a degree. Signature: .……………………….. Date: ………………………... Copyright ©2008 Stellenbosch University All rights reserved iii ABSTRACT The Okavango Delta, Botswana, is a unique ecosystem and this is reflected in its extraordinary biodiversity. The Nile crocodile (Crocodylus niloticus Laurenti) is the apex predator, and performs a number of vital functions in this system, making it a keystone species. The panhandle crocodile population has declined significantly over the last 80 years and is now threatened as a result of past over-exploitation and present human disturbance. In order to effectively conserve this species and in turn the health of this important region it is imperative to gain an understanding of their ecology and population dynamics. The population status of the Nile crocodile in the panhandle region of the Okavango Delta, Botswana, was assessed using a combination of capture-mark-recapture surveys, spotlight surveys and aerial surveys. The capture-mark-recapture experiment was conducted continuously from 2002 - 2006. A total of 1717 individuals, ranging in size from 136 mm – 2780 mm SVL, were captured, of which 224 animals were recaptured. Using a Bayesian technique, the total annual population in the panhandle region of the Okavango Delta was estimated to be 2 570 ± 151.06 individuals, with an adult population of 649.2 individuals with the number of breeding females estimated to be 364 individuals. It was concluded that this population cannot sustain the further harvest of breeding animals prior to the increase and stabilization of the population. Spotlight counts revealed a decline in the encounter-rate of crocodiles on the Okavango River with time, although more long-term data needs to be collected to confirm this trend. During the low-water season (September - February), 22.34 % of all crocodiles were observed, while during the flood-season only 13.34 % were observed, yielding correction factors for spotlight surveys of 4.46 (low-water) and 7.49 (high-water) for all animals in the panhandle. Two aerial surveys, conducted at the low-water and high-water peaks yielded total estimates of 588 (77.7 % of adults) during the low-water period and 350 (56.7 % of adults) during the high-water period. Correction factors of 1.28 (low-water) and 1.77 (high-water) were calculated for aerial surveys. iv An annual average of 50 nests was located in the panhandle, representing a 50 - 60 % decrease over the last 20 years. In regions with high human disturbance, breeding females situated their nests in hidden locations, away from accessible channels. Hatchlings exhibited elongation of the jaw in order to capture smaller prey items and morphometric shifts in jaw shape coincided with a dietary change at 400 mm SVL. The jaw became broader and deeper as animals matured, presumably in preparation for larger mammalian prey. The average growth rate of recaptured yearlings was 0.198 ± 0.116 mm.d -1 SVL and was closely correlated to the amount of time an individual spent in above-average water temperatures. Body condition (RCF) was significantly and positively correlated with a rise in water-level and negatively correlated to time spent in above-average water and air temperatures. Average RCF values were intersected when animals had spent 50 % of their time in above-average temperatures and water level. Generally crocodiles in the panhandle showed no significant sex-related differences in their sizes or the distances they travelled. The majority of recaptures (62.5 %) moved less than 500 m from the initial capture site. Adults in the panhandle occupied definite ranges, within which were preferred core areas where the majority of their time was spent. The panhandle crocodile population has declined significantly over the last 80 years, and is now threatened as a result of past over-exploitation and present human disturbance. The management of this population, including both its conservation and sustainable commercial utilisation, requires an adaptive strategy based on accurate monitoring procedures. v OPSOMMING Die Okavango Delta in Botswana is ‘n unike ekosisteem en word gereflekteer in dié sisteem se buitengewone biodiversiteit. Die Nyl krokodil ( Crocodylus niloticus Laurenti) is ‘n hoeksteen spesie in hierdie sisteem, maar dié populasies het betekenisvol afgedaal die afgelope 80 jaar. Om hierdie spesie en die gesondheid van hierdie belangrike streek te bewaar is dit noodsaaklik om hul populasie dinamika te verstaan. Die populasie status van die Nyl krokodil in die pansteel streek van die Okavango Delta, Botswana, was geasseseer deur gebruik te maak van ‘n kombinasie van vang- merk-hervang opnames, kolig opnames en opnames van die lug. Die vang-merk- hervang experiment was op ‘n konstante basis gemonster gedurende 2002 - 2006. ‘n Totaal van 1717 individue wat opeengevolg het in grootte van 136 mm – 2780 mm SVL, was gevang, waarvan 224 diere hervang was. Deur gebruik te maak van die Bayesian tegniek word die totale jaarlikse populasie van die pansteel streek van die Okavango Delta geskat by 2570 ± 151.06 individue. Hierdie sluit ‘n volwasse populasie van 649.2 individue in, met 364 telende wyfies. Die gevolgtrekking is dat die populasie nie verdere oes- of teel-diere kan steun vroër as die toename en stabilisering van die populasie nie. Kollig tellings wys ‘n afname in die ontmoetings-skaal van krokodille van die Okavango Rivier met tyd. Meer lang-termyn data word egter benodig om hierdie koers te bevestig. Gedurende die laag-water seisoen (September - Februarie), was 22.34 % van alle krokodille opgemerk, terwyl gedurende die vloed-seison was net 13.34 % opgemerk, wat korreksie faktore van 4.46 (laag-water) en 7.49 (hoog-water) vir al die diere in die pansteel streek toegee. Twee opnames van die lug, gemonster by laag-water en hoog-water pieke het ‘n totale skatting van 588 (77.7 % van volwasse diere) gedurende die laag-water tydperk en 350 (56.7 % van volwasse diere) gedurende die hoogwater tydperk toegegee. Korreksie faktore van 1.28 (laag-water) en 1.77 (hoog-water) was gereken vir opnames van die lug. ‘n Jaarlikse gemiddeld van 50 neste was bepaal in die pansteel, wat ‘n 50 - 60 % afname gedurende die afgelope 20 jaar verteenwoordig. In streke van hoë mense vi versteuring, het telende wyfies hul neste in versteekte lokasies weg van bekombare kanale geplaas. Die brooisels het verlengde kake ontwikkel sodoende om kleiner prooi items te vang, en morfometriese verskuiwing in kaak vorm het gepaartgegaan met ‘n dieets verandering van 400 mm SVL. Die kaak het breër en dieper geraak soos die dier groter geword het, klaarblyklik in voorbereiding vir groot soogdier prooi. Die gemidelde grooi tempo van hervange jaarlinge was 0.198 ± 0.116 mm.d-1 SVL en was nouliks gekoreleer met die hoeveelheid tyd wat ‘n individu spandeer het in bo- gemidelde water temperature. Liggaams kondisie (RCF) was betekinisvol en positief gekoreleer met ‘n toename in watervlak en negatief gekorelleer met tyd gespandeer in bo-gemidelde water en lug temperature. Gemidelde ligaams kondisie waardes het gekruis toe die diere 50 % van hulle tyd in bo-gemidelde water temperature en watervlakke spandeer het. Oor die algemeen het krokodille in die pansteel geen betekenisvolle sex verwante verskille gewys in hulle grootte of die afstande beweeg. Die meerderheid van hervangde krokodille (62.5 %) het minder as 500 m beweeg van die oorspronklikke vang area. Volwasse krokodille in die pansteel het bepaalde gebiede bewoon, waarin daar sekere kern streke was waar die meerderheid hul tyd spandeer het. Die pansteel krokodil populasie het afgeneem oor die afgelope 80 jaar, en word nou bedryg as gevolg van uitbuiting in die verlede en huidige mense versteurings. Die bestuur van hierdie populasie, insluitend sy bewaring en volgehoue komersiële gebruik benodig ‚n aanpasbare strategie gebaseer op akurate moniterings stelsels. vii The ultimate value of a crocodile lies not in his bellyhide, nor his value as a tourist attraction, nor even in his ecological significance, but simply in the fact that he is a crocodile: big and ancient and monstrously magnificent. James Powell: IUCN Crocodile Specialist Group, 1971. viii ACKNOWLEDGEMENTS I would like to thank my colleagues and friends, Audrey Detoeuf-Boulade, Kristi Maciejewski, Georgina Thomas, Kevin Wallace and Vince Shacks for their friendship, assistance and support during our time in the Okavango. Aliki and Rene, who began the project, collected the first year’s worth of data and gave advice and encouragement. Thanks go to Thoralf Meyer for his GIS expertise and friendship, and the Earthwatch Institute, USA for providing the financial support and to all the volunteers who helped both financially and physically by supporting our project. To Dr. Alison Leslie, my supervisor, for the opportunity to undertake this project and for her patience and understanding throughout its duration. I wish to thank Prof. Daan Nel (Center for statistical consultation, University of Stellenbosch) for the numerous and invaluable statistical consultations as well as Dr. Ken Pringle (Department of Entomology) and for statistical advice. Dr. Res Altwegg (Avian Demography Unit, University of Cape Town) assisted with the modeling aspects of this project. Prof. Dave Ward for his early advice on population estimation and Ms. Cara Niewoudt for her tireless assistance with many departmental tasks. My thanks to the Office of the President and the Department of Wildlife and National Parks, Botswana for providing our research permits and logistic support.
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