Cappo et al.: Causes and consequences of a latitudinal cline in the demography Lut/anus johnii 
311 
- 8 ° 
- 10 °— | 
- 12 ° 
6 -14° 
cn 
<D 
3 -16°— 
-1 8 ° — 
- 20 ° — 
- 22 ° 
A 500 km 
N 1 
Arafura Sea 
Timor Sea 
Cape Voltaire 
Hail Point 
• Kimberley 
Darwin 
Cape York 
** 
i % 
Gulf of 
Carpentaria 
Broome A 
Cairns b 
North Queensland ^ 
Townsville *'&J* 
120 ° 
125° 
130° 
135° 
Longitude (East) 
140° 
145° 
Figure 1 
Map of sampling locations (circles) for an examination of a latitudinal cline in the demography of 
John’s Snapper ( Lutjanus johnii) collected over the period of February 1989-April 2002 in 4 re- 
gions in northern Australia: Kimberley, Arafura Sea, Cape York, and north Queensland. Triangles 
indicate major centers of human inhabitation. 
estimate birth months with measurements of gonad 
weight available for 176 eastern fish. First, W\y esti- 
mates were calculated from FL (Lp) measurements us- 
ing this equation: 
IFw = a x Lp b , (1), 
with a nonlinear regression: a=7.701xl0 -5 ± 2.046x10' 
5 , b=2.741 + 0.0401, coefficient of multiple determina- 
tion (i? 2 =0.987, n-11). Equation 1 was applied to pre- 
dict whole weight for all fish in the pooled subsample 
( W w ( prec j)), and GSI values were calculated with the 
following equation: 
GSI = (W G / (W w(pred ) - W G )) x 100. (2) 
Equation 2 showed higher GSI values in the aus- 
tral summer after October; therefore, the nominal birth 
date for John’s Snapper in this study was chosen to be 
Oct. 1. Individual fish ages (in years) were the number 
of opaque increments (years) plus fractions of a year 
elapsed between sampling and Oct. 1. 
Estimation of mortality 
The instantaneous rate of total mortality (Z) was de- 
rived with the maximum age in years (f ma x) from the 
equation of Hoenig (1983): 
log e Z — 1-46 — 1.01 logg f ma x- (3) 
This estimate of Z is from a lightly exploited popula- 
tion; therefore, the estimate of natural mortality (M) 
should be similar to Z. It has been applied as a reason- 
able approximation for unfished or lightly fished tropi- 
cal demersal fishes in the absence of enough samples 
for catch curve analysis (Newman et al., 2000). 
Growth parameters 
The von Bertalanffy growth function (VBGF) was fitted 
to estimates of length at age through the use of non- 
linear least squares estimation. The VBGF is defined 
by the equation 
Lf = L„ |i _ e -mt - < 0 >} } (4) 
where Lf = mean FL (in millimeters) of fish of age t (in 
years); 
L„ = asymptotic mean length; 
K = is a rate constant that determines the rate 
at which Lf approaches L TC ; 
t = age of a fish; and 
ty = the hypothetical age at which the mean 
length is zero. 
The fit of the VBGF to different data sets was com- 
pared by using the likelihood ratio test for coincident 
curves (Cerrato, 1990) across comparable age ranges 
(Haddon, 2001) and analysis of covariance (ANCOVA) 
with type-III sums of squares and with log e -trans- 
formed age (year) as the linear covariate. This ANCO- 
VA allowed for testing of an interaction of sexxregion 
and accounted for type-I errors. 
