Boldt et al.: Factors influencing cannibalism and abundance of Theragra chalcogramma on the eastern Bering Sea shelf 
301 
overlap variable in this study indicated stations 
where age-1 pollock and adult pollock were caught 
in the same bottom trawl haul, and hence, both 
age groups of pollock would have occupied similar 
depths at these stations. Spatial overlap between 
age 0 and other pollock age classes could not be 
estimated in this study because the bottom trawl 
net does not catch small individuals efficiently 
and age-0 pollock are distributed higher in the 
water column. It is worth noting, that we also 
tested the effect of an indicator of water column 
stratification (and its interaction with depth) on 
the occurrence and amount of cannibalism. The 
water column stratification indicator was calculat- 
ed as residuals from a linear regression between 
the day of year and the temperature difference 
between surface and bottom waters (an indicator 
of the level of water column stratification). The 
results were similar to those of the GAM models 
presented here that included bottom temperature 
instead of water column stratification, and in 
fact the stratification variable was significantly 
correlated with bottom temperature. The most 
parsimonious model, therefore, was based on bot- 
tom temperature and is the only result presented 
in this study. 
A confounding factor in our models was the day 
of year that samples were collected. The dates of 
the NMFS EBS shelf bottom trawl surveys varied 
annually and the sampling started in the south- 
east EBS and generally proceeded northwestward. 
Start dates for the surveys usually occurred in the 
first week of June of each year, but ranged from 
24 May in 1999 to 19 June in 1986. End dates for 
the survey were typically in the last week of July 
in each year, but ranged from 11 July in 1982 
to 14 August in 1985. Day of year is, therefore, 
confounded with the factors year and location to 
some degree in our models, and as such, the effect 
of sample day cannot be completely separated from 
these factors. 
Water temperatures and water column strati- 
fication can affect multiple biological processes 
on the EBS shelf and have implications for the 
zooplankton community and predation on pollock. 
In warm years, stratification of the water column 
tends to occur earlier and result in stronger sum- 
mer stratification (Coyle et al., 2008; McKinnell 
and Dagg, 2010). During warm years, both the 
zooplankton community and diet of age-0 pol- 
lock tend to be dominated by smaller copepods, 
whereas, in cold years, diets are dominated by 
larger copepods and euphausiids (Baier and Napp, 
2003; Coyle et al., 2008, 2011; Hunt et al., 2011). 
Accompanying the reduction in prey availability 
in warm years, more cannibalism on age-0 pollock 
was observed in warm than in cold years (Moss et al., 
2009; Coyle et al., 2011). Warm years also bring more 
potential pollock predators, such as arrowtooth floun- 
der (Atheresthes stomias), northward and onto the EBS 
12% 
io%- 
8% 
6 %“ 
□ Prey pollock no length 
D Prey pollock >200 mm SL 
□ Prey pollock <60 mm SL 
■ Prey pollock 60-200 mm SL 
<=19 20-29 30-39 40-49 50-59 >60 
<=19 20-29 30-39 40-49 50-59 >60 
£ 0 . 12 - 
3 
1 o.io 
tr 
0.081 
CD 
O) 
2 0-061 
CD 
> 
< 0.04- 
0 . 02 - 
0.00 
<=19 20-29 30-39 40-49 50-59 >60 
Predator fork length (cm) 
Figure 5 
Walleye pollock (Theragra chalcogramma ) cannibalism by 
predator pollock fork length (cm) in the eastern Bering Sea 
during 1982-2006 (excluding 1983 and 1984). (A) Average 
percent frequency of occurrence ( %FO ), (B) average percent 
body weight (%BW), and (C) average partial fullness ( PF ) of 
prey pollock measuring less than 60 mm, 60-200 mm, and 
greater than 200 mm standard length, and those prey pollock 
that were too digested to measure (no length). 
shelf (Mueter and Litzow, 2008; Ianelli, et al., 2011). 
We found that the cold pool affected the distribution 
of large pollock and, therefore, in years with a large 
cold pool, a reduced overlap in distribution of large 
and small pollock may reduce the occurrence of can- 
