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Sustainablc harvcsls were calculaled by rsking what level of change in I, or m, would convert an
ircreasing populaiion to a slaiionary one, i.e. would mnkc c'= l Equation I is thcn solved for lhe new
parameter value, for example, fof egg harvests, wherc q is lhe proporiion ofeggs collected, m. in equation
I becomes (l 'q)m. at equilibrium, reananging thc equalion then gives:
\-r
n
(craig 9! 4!. 1992).
RT]SUI,TS AND DISCUSSION
Given lhe pammeler values of Table 1, the polential rate of incrci$e for Nile crocodiles was estimatcd lo
be e'= 1.08, i.e. an 87, annual rate of incre.se. Unceilxirly in the parametea used. however. result in
npproximate conficence linits of e' = 1.03 lo e' = 1,13.
The vast majority of uncert:inty (Column 4 of Table 1) in lhe value of c' appears to derive from pmr
cslimatcs of three piuamctcn, namcly survivorship Io age l. survivorship from 1 lo maturily and ag€ at
maturily. ClcaJly, if lhere is to bc ar improvement in otu ability lo make prediclions, future res€lrch must
concentrale on obhining beller estimates of lhcse.
Susmined yields of a variety of harvesling slralegieshow that egg collcclion xnd reffing to a size of 12
melres is superior to any other sIrxtegy. e.g. Txble 2 compares the yie,d o[ egg collection with crcpping
animals lrom lhc wild direclly for skin.
Table 2. Comparison of egg collection wilh cropping animals >1,2 metres
STRATDCY
SUSTAINABLE
% TAKFN
9t%
SKIN YIELD
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There may be less differential economically between the slrategies of Table 2, because of the cost of
hxrhing and rearing crocodiles. but in cons€rvalion terms. it is the absolute compxrison which is valid.
The robustness ofegg collcction as a slralegy is also obvious from lhc pcrccnlagc of eggs it is permissible
to collect. Such a high proportion collected would be unlikely to be achieved evcn if lhere were no
restrictions on egg collection. This robushess bsromes even mo.e apparent when options involving
replacement of juvcniles are investigated. Here the relum of 0,5% (of lhe number of collected eggs) as
crocodiles rearcd to the length of l.2m resulls in reslonlion of sustainabilily even when all eggs are
collecred (Gaig 9! 4., 1992).
The simple model of crocodllc population dynamics describcd here enables somo useful conclusions to
be drawn about safe harvesting slrategies. That these also seem inluilively acceptcbl€ is additionally
encouraging.