Cappo et al. : Causes and consequences of a latitudinal cline in the demography Lut/anus johnii 
321 
known along a latitudinal cline (Portner et al., 2008). 
Permanent physiological differences induced by tem- 
perature and climate have been identified in Atlantic 
Cod populations along that cline, resulting in popu- 
lation-specific patterns of OCLT. The Hb-I(l/1) allele 
displays an increasing frequency toward the (warmer) 
south, leading to a higher oxygen affinity at higher 
temperatures, and this feature is considered to be a 
microevolutionary adaptation to optimize oxygen trans- 
port (Portner et al., 2008). 
The OCLT concept does not imply strict positive or 
negative correlations between longevity, maximum size, 
or growth rate along latitudinal dines. It offers a prom- 
ising new way forward to use physiological challenges 
under controlled conditions (see Clark et al., 2012) to 
disentangle true mechanistic causes (and contradic- 
tions) of the temperature-size rule from effects of fish- 
ing and unknown environmental differences between 
regions. This approach may explain why responses in 
growth rate and maximum size along long latitudinal 
gradients are inconsistent in statistical correlations 
used in intensive field studies of tropical fishes. For 
example, Robertson et al. (2005a) concluded that varia- 
tion in growth and terminal size is related strongly to 
both habitat and temperature, yet Trip et al. (2008) 
proposed that growth and adult size are most respon- 
sive to local environmental features unrelated to lati- 
tudinal (temperature) effects. 
Growth trajectories and length at maturity 
Despite vast differences in the local environments 
sampled, the basic patterns in the growth curves of 
John’s Snapper are conserved. This snapper species 
has a relatively gradual growth trajectory through 
early life, maturing at 6-10 years and at 70-80% of 
and reaching an asymptotic length at -18-20 years. 
The consistent, sex-specific differences in growth rates 
are consistent with functional gonochorism for John’s 
Snapper, for which there is a higher selective pressure 
for females to grow to a larger size and have a higher 
fecundity (Roff, 1983). Longevities >20 years are known 
for many small and large lutjanids (e.g., Heupel et al., 
2010; Martinez-Andrade, 2003) and are considered to 
be beneficial by ensuring a long reproductive life. This 
life history minimizes the risk that unfavorable events 
at large scales will result in the loss of a metapopula- 
tion. In life history terms, John’s Snapper is an “inter- 
mediate strategist” falling in the center of a continuum 
between large species that mature at later ages and 
have large eggs and those that are long-lived, slow- 
growing, and highly fecund species (King and MacFar- 
lane, 2003). 
Our demonstration of a longevity that is nearly 3 
times that reported from early studies is not surpris- 
ing, or novel, but it is nonetheless very important to 
improve meta-analyses, such as analyses with Ecopath 
and stock reduction models. Compared with the pa- 
rameters derived by Khan (1986), which appear in the 
online database FishBase (Froese, 2011), the param- 
eters we have shown for John’s Snapper give evidence 
of a higher longevity (28.6 versus 10 years derived by 
Khan), lower K (0.16-0.21 versus 0.28), about 4% of the 
total instantaneous mortality Z (0.146 versus 2.700), 
and, consequently, about 25% of the rate of natural 
mortality M (<0.146 versus <0.590). 
The estimates of length at maturity of John’s Snap- 
per are coarse and preliminary but indicate differences 
as high as 24% between the latitudinal limits sampled. 
Therefore, it is interesting to note that the minimum esti- 
mates exceeded predictions from regressions on the basis 
of L„ and L max from published meta-analyses. With our 
L ma x °f 990 min FL and L OT of 843.5 mm FL for the north 
Queensland region, we calculated an estimated L m of 495 
±10 mm FL using the Binohlan and Froese (2009) method 
and estimated L m of 448 ±10 mm FL in the Froese and 
Binohlan (2000) equation. Martinez-Andrade (2003) gen- 
eralized that L m occurred at a length about half (0.52) of 
the L„ for lutjanids, producing an even smaller estimate 
(L m =438 mm FL) for John’s Snapper. Our estimates were 
considerably larger at 590 mm FL for males and 690 
mm FL for females in north Queensland, representing 
from 59.6% (males) to 69.7% (females) of L max and from 
69.9% to 81.8% of L tc . The legal limits to fish size at first 
capture of John’s Snapper in Western Australia (300 mm 
total length) and Queensland (350 mm total length) do 
not approach any of the estimates discussed above. The 
Northern Territory has no size limit. 
The northernmost (Arafura Sea) samples were at 
the smallest extremes of length and otolith weight at 
age and of gonad weight at length, when compared with 
samples from the other regions. However, the fishery on 
the coastal reefs of the Northern Territory, inshore of the 
Arafura Sea trawl grounds, recorded John’s Snapper up 
to 820 mm FL and 23 years of age (Hay et al. 3 ). Of these 
coastal females, 50% reached sexual maturity (L m 5 o) at 
a much larger size of 630 mm FL (8-10 years) than did 
Arafura Sea females, although males reached maturity at 
a similar size (L m 5 q= 470 mm FL). There is clearly a need 
to accurately measure regional length at maturity and 
establish fecundity-size curves to fully understand the 
nested hierarchy within growth curves. In general terms, 
larger adults of tropical fish populations farther from the 
equator might be expected to have much larger ovaries 
(and hence batch fecundity) and a longer spawning life 
in comparison with their smaller counterparts close to 
the equator. However, there is no evidence that recruit- 
ment rates are higher for populations at these margins. 
Instead, Portner et al. (2008) proposed that recruitment 
rates should show a dome-shaped distribution about an 
optimal temperature range. 
3 Hay T., I. Knuckey, C. Calogeras, and C. Errity. 2005. NT 
coastal reef fish: population and biology of the golden snap- 
per. Fishnote No: 21 Department of Primary Industry, Fish- 
eries and Mines, Darwin, Northern Territory, Australia, 4 
p. [Available from http://www.nt. gov.au/d/Content/File/p/ 
Fishnote/FN21.pdf.[ 
