Hurst et al.: Growth rates of Gadus macrocepha/us 
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Table 1 
Summary of experiments conducted at the Alaska Fisheries Science Center laboratory in Newport, Oregon, where growth rates 
were determined for early life stages of Pacific cod (Gadus macrocephalus ) collected from the central Gulf of Alaska. 
Stage 
Temperatures 
(°C) 
Year 
Experiment duration 
(dah=days after hatching) 
Sample type 
No of tanks 
n fish 
Eggs 
0, 2, 4, 6, 8 
2006 
Fertilization to hatching 
Measured at hatching 
15 
30/sample 
Preflexion larvae 
3, 8 
2007 
Hatching to 35 dah 
Tank sample 
6 
10/sample 
2, 5, 8 
2008 
Hatching to 36 dah 
Tank sample 
9 
10/sample 
Postflexion larvae 
2, 4, 8, 11 
2007 
50 to 105 dah 
Tank sample 
12 
10/sample 
Juveniles 
2, 5, 8, 11 
2008 
approx. 132 to 150 dah 
Serial measures 
12 
10/tank 
cific cod averaged 28 times those of Atlantic cod ( Gadus 
morhua ). 
Despite the pervasive influence of temperature on all 
aspects of biology and its potential linkage to recruit- 
ment patterns, there has been little examination of 
the thermal ecology of Pacific cod. The effects of tem- 
perature on the vertical distribution of larvae (Hurst 
et al., 2009) and juveniles (Davis and Ottmar, 2009) 
have been examined. Thermal effects on growth of Pa- 
cific cod has been examined in only the very early life 
stages: development rates of eggs and prefeeding larvae 
(Alderdice and Forrester, 1971; Laurel et al., 2008) have 
been examined across a wide range of temperatures. 
In addition, B. J. Laurel (unpubl. data) compared the 
effects of prey density on growth of preflexion larvae at 
two temperatures. However, these studies are insuffi- 
cient to describe the functional response to temperature 
for larvae, and as of yet, no data exist for later larval 
stages or juveniles. 
In this article we describe the growth of early life 
stages of Pacific cod as a function of temperature and 
body size. Separate experiments were conducted with 
preflexion larvae, postflexion larvae, and postsettlement 
juveniles. From these experiments and published data 
on embryos, we determined the parameters for models 
of stage-specific growth and for an integrated model of 
size- and temperature-dependent growth. These func- 
tions will be used to evaluate the relative contribu- 
tions of temperature and feeding conditions to observed 
variation in growth among wild Pacific cod (Folkvord, 
2005; Hurst et al., 2010). 
Materials and methods 
We determined the thermal sensitivity of growth rates 
at three life stages: preflexion larvae, post-flexion larvae, 
and postsettlement juveniles (Table 1). Growth rates at 
each ontogenetic stage were measured in three replicate 
tanks at four to five temperatures, encompassing the 
range likely to be encountered by fish in the Gulf of 
Alaska and Bering Sea. Nonlinear regression was used 
to describe the relationship between growth rate and 
temperature at each developmental stage and to describe 
the combined effects of temperature and body size on 
growth rates of early life stages. 
Preflexion larvae 
Two experiments were conducted to describe the growth 
of Pacific cod larvae after hatching. In 2008, fish were 
reared at 2°, 5°, and 8°C to 36 days after hatching (dah). 
These data were combined with data on fish reared 
under identical conditions to 35 dah at 3°C and 8°C in 
2007 (B. J. Laurel, unpubl. data). The 8°C treatment 
was conducted in both experiments to evaluate potential 
differences in overall growth rates between years. 
Fish for the larval growth experiments were reared 
in the laboratory from eggs collected from spawning 
adults. Female and male Pacific cod were caught by 
commercial jigging gear from spawning grounds in 
Chiniak Bay, Kodiak Island, Alaska. The gametes were 
mixed and placed into 4-L incubation trays at 4°C. At 
24 hours after fertilization, fertilized eggs were shipped 
in insulated containers to the Alaska Fisheries Science 
Center’s (AFSC) laboratory facilities in Newport, Or- 
egon. Eggs were transferred to flow-through 4-L plastic 
trays and incubated at 4°C. Hatching occurred 19-22 
days after fertilization, after which larvae were trans- 
ferred to larval rearing tanks. 
Experimental rearing tanks were 100-L cylinders 
with conical bottoms and dark green walls. Water was 
supplied to the tanks at a rate of 250 mL/min. Weak 
upwelling circulation was maintained in the tank by 
positioning the in-flow at the bottom center of the tank 
and with light aeration. Light regime during larval 
rearing was maintained at 12:12 h light:dark; light was 
provided by overhead fluorescent bulbs at a level of 6.7 
pE/m 2 s at the water surface. 
Larval growth experiments were initiated by stocking 
rearing tanks (maintained at the egg incubation tem- 
perature of 4°C) with 400 larvae which hatched over a 
4-day period in the middle of the hatch cycle. The last 
day that newly hatched fish were stocked into rearing 
tanks is nominally referred to as experimental day 0. 
After the tanks were stocked with fish, tank tempera- 
tures were adjusted to treatment temperatures over 2 
days. Larvae were reared on a combination of rotifers 
