309 
Abstract— Demographic parameters 
were derived from sectioned otoliths 
of John’s Snapper (. Lutjanus johnii ) 
from 4 regions across 9° of latitude 
and 23° of longitude in northern 
Australia. Latitudinal variation in 
size and growth rates of this species 
greatly exceeded longitudinal varia- 
tion. Populations of John’s Snap- 
per farthest from the equator had 
the largest body sizes, in line with 
James’s rule, and the fastest growth 
rates, contrary to the temperature- 
size rule for ectotherms. A maximum 
age of 28.6 years, nearly 3 times 
previous estimates, was recorded 
and the largest individual was 990 
mm in fork length. Females grew to 
a larger mean asymptotic fork length 
(Loo) than did males, a finding consis- 
tent with functional gonochorism. Oto- 
lith weight at age and gonad weight 
at length followed the same latitu- 
dinal trends seen in length at age. 
Length at maturity was -72-87% of 
Loo and varied by -23% across the 
full latitudinal gradient, but age at 
first maturity was consistently in 
the range of 6-10 years, indicating 
that basic growth trajectories were 
similar across vastly different envi- 
ronments. We discuss both the need 
for complementary reproductive 
data in age-based studies and the 
insights gained from experiments 
where the concept of oxygen- and 
capacity-limited thermal tolerance 
is applied to explain the mechanis- 
tic causes of James’s rule in tropical 
fish species. 
Manuscript submitted 29 June 2012. 
Manuscript accepted 10 July 2013. 
Fish Bull. 111:309-324. 
doi: 10. 7755/FB. 111.4.2 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not neeesarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
James's rule and causes and consequences of 
a latitudinal cline in the demography of John's 
Snapper (Lutjanus johnii ) in coastal waters 
of Australia 
Mike Cappo (contact author ) 1 
Ross J. Marriott 2 
Stephen J. Newman 2 
Email address for contact author: mcappo@aims.gov.au 
1 Australian Institute of Marine Science 
PMB 3, Townsville MC 
Townsville 4810 
Queensland, Australia 
2 Western Australian Fisheries and Marine Research Laboratories 
Department of Fisheries 
Government of Western Australia 
P.Q. Box 20 
North Beach 6920 
Western Australia, Australia 
Body sizes generally increase across 
a range of taxa with increasing dis- 
tance from the equator, a principle 
known as Bergmann’s rule; another 
theory about variation of size by lati- 
tude, James’s rule, applies this phe- 
nomenon within species (for review, 
see Blackburn et al., 1999). The poten- 
tial existence of such an underlying 
latitudinal cline raises major ecologi- 
cal questions about 1) the mortality 
and lifetime reproductive contribu- 
tion of adults throughout their geo- 
graphic range, 2) the implications 
for recruitment rates at marginal 
latitudes, and 3) the implications 
for fishery harvests. James’s rule 
has been well demonstrated in both 
cold temperate and tropical fish spe- 
cies (e.g., Choat and Robertson, 2002; 
Choat et al., 2003; Robertson et al., 
2005a; Portner et al., 2008; Trip et 
al., 2008; Berumen et al., 2012), and 
the advent of warming oceans has 
caused an upsurge in interest in the 
underlying physiological mechanisms 
of this rule (Portner and Peck, 2010; 
Clark et ah, 2012). 
However, major equatorial gaps 
still remain in such descriptive 
knowledge of demographic processes 
for the large, economically important 
lutjanid snappers, lethrinid emper- 
ors, and serranid groupers, espe- 
cially in the Indo-West Pacific, where 
most studies have been done south of 
15°S. Ecosystem “mass balance” mod- 
els (sensu Pauly et ah, 1993) and life 
history classifications used for esti- 
mating resilience (King and MacFar- 
lane, 2003) are sensitive to estimates 
of maximum size, growth rate, and 
longevity. In data-poor tropical fish- 
eries, these estimates are often de- 
rived from only one part of a species’ 
distribution (e.g., Binohlan and Fro- 
ese, 2009) or from a mix of param- 
eters derived from different studies 
in separate regions (e.g., Pauly et al., 
1993). Consequently, there is a need to 
investigate regional differences to de- 
termine whether growth trajectories 
and reaction norms (i.e., the pattern of 
phenotypic expression of a single geno- 
type across a range of environments) 
such as age at maturity are conserved 
in populations along latitudinal gradi- 
ents (see Arendt, 2011). 
John’s Snapper ( Lutjanus johnii), 
is widely distributed in the Indo- 
