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Fishery Bulletin 109(3) 
Table 3 
Growth rate for juvenile walleye pollock ( Theragra chalcogramma ) in the western Gulf of Alaska by season and region (see Fig. 1, 
Table 1) was estimated with a bioenergetics model. Model inputs included mean water temperature at 40-m depth, predator 
body weight, predator energy density, and daily food ration. Daily ration (%BW/d) was estimated with a simple evacuation model 
(see text). Diet energy density and specific growth rate (grams of growth per gram of body weight per day, [g/g/d] ) are included. 
LSum00=late summer 2000, Win01=winter 2001, and LSum01=late summer 2001. 
Season 
Region 
Temp (°C) 
Body wt (g) 
Energy density (J/g) 
Daily ration 
Growth rate 
Predator 
Diet 
mm SL/d 
g/g/d 
LSumOO 
Semidi 
9.0 
3.6 
3999 
5094 
4.99 
0.26 
0.011 
LSumOO 
Shumagin 
8.3 
2.0 
3441 
4070 
6.62 
0.27 
0.013 
WinOl 
Kodiak 
5.0 
15.2 
4284 
5261 
3.46 
0.33 
0.008 
WinOl 
Shumagin 
4.5 
9.3 
4187 
3530 
1.30 
-0.38 
-0.011 
LSumOl 
Semidi 
9.8 
81.4 
4782 
5578 
2.43 
0.39 
0.006 
LSumOl 
Shumagin 
9.3 
55.0 
4576 
4725 
3.45 
0.56 
0.010 
(Wilson, 2000) of age-0 juveniles in the Kodiak region 
during late summer. Fish body size and water tem- 
perature are other relevant factors because they affect 
respiration, which in the bioenergetics model was the 
largest use of input energy. Respiration decreases with 
fish size, but increases with water temperature. Thus, 
the wintertime size-related reduction in respiration of 
Kodiak fish allowed more energy for growth than that 
for Shumagin fish although water in the Shumagin 
region was 0.5° C cooler. In addition to physiological 
conditions, body size directly affects the acquisition of 
large euphausiids (Wilson et al., 2009) and consequent 
energy input (Mazur et al., 2007). For summer 2001 
and late summer 2001, too few samples were available 
to estimate the growth rates of Kodiak yearlings for 
comparison with the Shumagin region and therefore we 
were unable to explore seasonal shifts in the region of 
most-favorable growth. Nevertheless, the direct implica- 
tion of seasonal and regional variation in food habits 
on growth rate leads us to speculate that the region 
most favoring rapid growth of juvenile walleye pollock 
varies with year in response to ecological determinants 
of euphausiid availability and oceanographic effects on 
local euphausiid abundance. 
Opportunistic sampling enabled us to obtain the 
sample set necessary to formulate hypotheses, but rig- 
orous control over sampling effort and site location was 
lacking. This was least a concern during winter when 
sampling effort was relatively high and most sites were 
located in close proximity to sea valleys where yearlings 
likely concentrate (e.g., Hollowed et al., 2007; Wilson, 
2009). However, the number of stomachs available dur- 
ing winter from the Semidi region was <60; there- 
fore the apparent number of prey types consumed (i.e., 
dietary breadth) was probably biased low (Ferry and 
Cailliet, 1996). Similarly insufficient stomach numbers 
were available in each region sampled during summer 
2001, but the negative bias was probably most extreme 
for Kodiak during summer and late summer 2001 when 
only 20 and 7 fish were examined, respectively. It was 
encouraging, however, that the late summer 2001 resur- 
gence in the dietary proportion of euphausiids among 
Kodiak fish was mirrored in the diet of Semidi fish, 
which was represented by ample stomachs. Site location 
in the Kodiak region during summer and late summer 
(2000 and 2001) was generally farther offshore than in 
winter. Therefore, the available samples did not rep- 
resent the nearshore where juvenile walleye pollock 
population density can be high (Wilson et al., 2005). 
Previously, at least for age-0 juveniles, no significant 
difference was found between nearshore and shelf sites 
regarding stomach content weight, and dietary dif- 
ferences were attributed to nearshore prey (e.g., crab 
larvae) rather than euphausiids (Wilson et al., 2005), 
which in the present study were primarily responsible 
for diet variation. Another problem with using oppor- 
tunistic sampling was the variety of cruise objectives 
and collection methods (e.g., gear), which created un- 
certainty about how to quantify the population fraction 
represented by each sample. Clearly, there are many 
drawbacks to using opportunistically collected samples; 
however, those samples provided the empirical informa- 
tion necessary to formulate our hypotheses. 
Conclusions 
In summary, an annual cycle in juvenile walleye pol- 
lock food habits was primarily evident as a postwin- 
ter rebound in stomach content weight that was not 
explained by a body-size effect. We hypothesize that 
seasonal changes in stomach content weight were driven 
by the combined effects of juvenile growth, predator-prey 
size constraints, and the cycle of euphausiid abundance 
and growth. Further, we hypothesize that the most- 
favorable feeding area cycles annually from the Kodiak 
region during winter to the Shumagin region during 
summer in response to euphausiid availability. The simi- 
larity in growth rate between food-based model output 
and independent otolith-based estimates imply that the 
