Hurst et al. : Growth rates of Gadus macrocephalus 
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Figure 4 
Size- and temperature-dependent growth rates of early life stages of Pacific cod (Gadus macrocephalus) from the central 
Gulf of Alaska in (A) standard length and (B) dry mass. Points are the tank mean growth rates measured in laboratory 
experiments. Surfaces are best-fit descriptions of growth rates. Parameter values are presented in Equations 3 and 4. 
This reduction in growth during the egg-larva tran- 
sition appears to be the result of increased metabolic 
expenditures associated with swimming in posthatch 
larvae and possibly a reduction in energy available for 
growth associated with the transition from reliance on 
endogenous energy stores to exogenous feeding (Torres 
et al., 1996; Yufera and Darias, 2007). In Pacific cod, 
yolk reserves are depleted 3-12 dah, depending on 
water temperature (Laurel et al., 2008) and stomach 
fullness increased through the first 28 dah (B. J. Lau- 
rel, unpubl. data). The increase in growth rates after 
the preflexion transitional feeding stage coincides 
with the onset of diel vertical migrations (Hurst et 
al., 2009) and increased responsiveness to prey (Colton 
and Hurst, 2010) among postflexion Pacific cod larvae. 
The negative departure from an allometrically defined 
growth pattern after hatching indicates that the first- 
feeding stage represents a “critical period” in the early 
life history of Pacific cod and that the consequences 
for recruitment of this low growth may be greater at 
low temperatures (Kamler, 1992; Houde, 1996). In 
addition to inclusion of embryo measurements into 
larval studies, future studies with other species should 
encompass other major life history and habitat transi- 
tions, such as metamorphosis and settlement in flat- 
fishes (Christensen and Korsgaard, 1999; Neuman et 
al., 2001) in order to clarify the physiological basis 
of growth patterns and to determine parameters for 
growth models. 
Based on exploration of model structures for a unified 
STDG for early life stages of Pacific cod, a two-stage 
model was developed. The first stage described growth 
in the egg stage as a direct function of water tempera- 
ture. The second stage described growth in posthatch 
fish as a function of water temperature and fish size. 
In addition to providing the best fit to experimental 
data, this formulation is logically consistent with the 
life history. Explicit discrimination between life stages 
coincides with hatching, whereas the function provides 
a continuous growth surface for all free-swimming stag- 
es. This stage-independent model for posthatch fish 
provides more realistic growth trajectories in modeling 
applications where fish are tracked over multiple stages. 
Applications of growth models 
By quantitatively accounting for the influence of tem- 
perature variation, laboratory-determined growth rates 
are being increasingly used to evaluate factors regulat- 
ing growth rates of fishes in the wild (Folkvord, 2005; 
Rakocinski et al., 2006; Hurst et al., 2009). In these 
analyses, “realized growth” expresses observed growth 
in the field as a fraction of the potential growth at the 
encountered field temperatures (Hurst and Abookire, 
2006), with field growth rates estimated from changes 
in mean size, otolith increment measures, or biochemi- 
cal measures (RNA:DNA). In these studies it is impor- 
tant to recognize that growth rates of individuals are 
determined by both genetic and environmental factors. 
Growth models from laboratory experiments generally 
describe mean growth of a representative population 
under optimal foraging conditions, which should not be 
mistaken for the maximum growth rates that would be 
observed for the fastest growing individual. Therefore, 
field growth rates should be similarly expressed as a 
population mean rather than at the individual level. 
Realized growth rates near 100% indicate that growth 
rates in the population are directly limited by ambient 
