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78
Population Mod€l for The Nile Crocodile witb
Analysis of Sustrinabl€ Harvesting Strategies
G.C. Craig
P. Bag 8R165 Gaborone
Botswana
INTRODUCTION
In the late 1980's x model was requned by CI IES to help make more wide nnging recommcnuduons tor
conservation xnd management of the nile crocodile . The snme need had been idenfned by Zimbabwe's
Department of Nadonal Park and Wildlife Managemcnl where an inirirl studv (Craig 9l!3!' 1989)' $ew
int; a report to CITES (Craig q!4. 1992). This paper is a repelilion of the most impormnt conclusions
feached as a resuli of that work,
The main need was for predictons about likoly levels of sDstained yields of various harvesting stmtegies
and theil impacts on crocodile populations. It was recognised lhnt existing infomation mighl be
inadequate to give accumte predictions, but these were nevenhelcss urgenlly requi€d' based on such
informxtion as wa-s available, wilh some stalement of lhe reliability of the original information and about
the effecl of that on lhe confidencc in the prediclions- This approach, it was hoped, would provide
provisional options for cromdile man:tgement and at the same tirnc highlight those aspeds whcr€
;dditional information was urgently needed. In the event. exisling infornation was found to be so
inadequate in some respecls that novel infonnation was also incorporated.
It was felt lhat the lack of accurate informalion did not permit the construciion of a model incor?orating
the degrec of complexity ofsome previoos crocodilian models (c.g Nichols 9!!L (1976). This resulted
in an approach where, for tho most part, calculalion replaced computcr simulation, and where the results
and their underlying ceuses were intuitively simple to interpret.
ME"THODS
The model was based on the discrete Lotki-Volterm equation:
E ]x n* e''* = r (1)
No attempt to simulate slochsticxlly determined outcomes was made' as lhis was considered unimporhni
under the envisaged circumstances of the populalion. Density dependen( effects were not allowed for
a) there is inadequate infotmation to allow thcse to be simulated realistically and
b) the conclusions can usefstly be linited to a strge of populntion devolopenent where densny
dependent effects are unimporlant.