size: 5036KB - Crocodile Specialist Group

1,9
rejecting nufl hypotheses that reduced-parameter rnodels fit as
vell as the full Richards nodel.
c. Crocodilian populations often have lots nore
young anirnals than 01d ones. 17
Furtherrnore, practicable
schemes (especially
:lTpli"g
those tnat secure known-age
anrnals) sornetihes exacerbate the problen of disproportion;I
representation by age. For these rJasons, individual' observa_
t1ons of o1der, larger ani-mals often have extrerne statistical
leverage on the shape of fitted growth curves. Thus one neeats
to be careful- when fitting nathematlcal curves to age_stze
data. For.exanple, one or tr^ro observations (perhlps of
aberrant individuals) can lar.gely define the asyhpt;tic ienqth
estimated for a popufation. 10 ihus .""o..6.rd trrit peopre
renain cautious about describing crocodilian ""
growth palrerns
until- their sarnple includes a substantial nirnber of large
an rnals.
d. Given (1) the typicat structure of wild
crocodr l lan-populations, (2) the nature of sampling strategies
conventionally applied to their study, and (3) the behaviol of
our sanple statistics under sirnulation, vre would reconnenal
caution in the use of inferential procedures involving confi_
dence intervals or fornal hypothesis tests. A1I too often our
sample statistics did not fie vithin purported ,,95? confidence
intervalsrr of the known population piraieters.
3. Things one rnight cons.ider doing.
a. If one is interested i; the overall qro,rth
pattern of a crocodilian population, then it,s probably worth
the extra trouble to ensure that older, larqer animals are
represented aq least proportionat to their abunaahce in sanples
to be analyzed.
b. I,Ie believe that either the von Bertalanffy
model or the full Richards hodel Inay often be used for sunnarizing
the grovrth of crocodilians. The forner is rnore appro_
priate if one \./ishes to cornpare results with other published
data; the latter is probably better if one is concer;ed about
the possibitity of technical 'rspecification error.,, ot course
" In a sense, even lf one could examine every rnenber of
a.living crocodilian population, the older, larger individuals
l19l: pl""id..in inadequare ',sampten of the groinrlh trajecEorres
ava IIaD.L e to the sDecies-
18
For exarnple, in our 35 sub-satnples of the C. acutus
growth data, our best estirnation method produced e=titnaE""=ot
asynptotic length ranging froh 263.4cn t-o szz.scrn. inese tvo
extreme values were the exceptions to a general pattern lrith a
sonewhat nore reasonable standard devi;tioh (g3cm), but one
should note that they irere generated from randotn sarn6ies of the
sane population of data boints