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Fishery Bulletin 1 10(3) 
are largely determined by sea-ice conditions during the 
preceding winter and mixing forces during the summer 
(Kachel et ah, 2002; Stabeno et ah, 2010). As a result, 
the EBS shelf comprises 3 depth domains (generally, 
coastal <50 m, middle 50-100 m, and outer 100-180 m), 
each with its own hydrographic features: a mixed water 
column in the coastal domain, a 2-layered water column 
in the middle domain, and a 3-layered water column in 
the outer domain (Stabeno et ah, 2001). Water tempera- 
ture and stratification can affect the amount of primary 
and secondary production, the growth and timing of 
zooplankton, and zooplankton species composition (Sta- 
beno et ah, 2001; Napp et ah, 2002; Rho et ah, 2005), 
thereby affecting food availability for EBS fish and their 
subsequent survival. Mueter et ah 1 found evidence for 
environmental effects on age-1 pollock survival and 
recruitment. During November to March, ice forms 
and extends from the Bering Strait in the north to the 
Alaska Peninsula in the south (Napp et ah, 2000). The 
southerly extent and timing of ice (arrival and persis- 
tence) in winter and early spring have a strong effect 
on the size and southerly extent of the cold pool (bottom 
water <2°C) (Stabeno et ah, 1998, 2001; Kachel et ah, 
2002). The cold pool can directly affect the distribution 
of some upper trophic level species, including adult pol- 
lock (Napp et ah, 2000). 
The vertical and horizontal distribution of pollock 
varies ontogenetically, seasonally, and temporally 
(Hinckley, 1987; Kotwicki et ah, 2005; Bacheler et ah, 
2010). Adults generally spawn at depths between 100 
and 250 m from January to August, depending on the 
spawning area (Hinckley, 1987; Bacheler et ah, 2010). 
As part of a feeding migration in summer, most pollock 
on the shelf migrate towards the northwest and pollock 
on the southeast shelf migrate towards the northeast 
(Kotwicki et ah, 2005). Adult pollock are demersal and 
distributed on the outer and middle depth domains in 
summer. Adults prefer cooler and stable temperatures 
(Duffy-Anderson et ah, 2003) but avoid the cold pool 
(Chen, 1983; Bakkala and Alton, 1986). Upon hatching, 
young pollock larvae are distributed in the upper water 
column and are subject to currents and wind-driven 
advection (Nishiyama et ah, 1986). Larvae undertake 
small diel vertical migrations within the upper 30 m of 
the water column (Pritchett and Haldorson, 1988). Bai- 
ley (1989) observed that, in the fall months, small-size 
age-0 pollock (<60 mm fork length) remained in surface 
EBS waters above the thermocline, but larger age-0 pol- 
lock (70-89 mm fork length) moved to deeper depths, 
below the thermocline in the daytime, thereby making 
them vulnerable to cannibalistic adults at that time of 
year. In the summer months, older pollock (age-1 and 
age-2) have been found within and inshore of the cold 
1 Mueter, F. J., M. C. Palmer, and B. L. Norcross. 2004. Envi- 
ronmental predictors of walleye pollock recruitment on the 
Eastern Bering Sea shelf. Pollock Conservation Coopera- 
tive Research Center, Univ. Alaska, Fairbanks. [Available 
from http://www.sfos.uaf.edu/pcc/projects/03/norcross/Mueter 
Norcorss Final Report 2003.pdf, accessed April 2012.] 
pool (Francis and Bailey, 1983), as well as throughout 
the water column (Bakkala and Alton, 1986) because 
they are able to tolerate more variable temperatures 
than adults (Chen, 1983; Duffy-Anderson et al., 2003). 
There is a tendency for age-1 and age-2 pollock to be 
vertically separated, with the age-1 pollock near the 
bottom and age-2 pollock distributed higher in the wa- 
ter column (Duffy-Anderson et al., 2003). In the sum- 
mer months, therefore, age-1 pollock are vulnerable to 
cannibalistic adults. In fact, in the EBS during summer, 
age-1 pollock comprised the majority of cannibalized 
prey pollock (Dwyer et al., 1987). The factors that de- 
termine the occurrence and amount of cannibalism on 
age-1 pollock are not well understood. 
Our goal was to determine the factors that affect the 
occurrence and amount of cannibalism on vulnerable 
pollock in the summer. Age-1 pollock often occupy the 
same depths as adult pollock in the summer, thereby 
making them vulnerable to cannibalism (Dwyer et al., 
1987; Duffy-Anderson et al., 2003). Our specific ob- 
jectives were to determine which environmental fac- 
tors influence the occurrence and amount of pollock 
cannibalism, as well as the factors that influence the 
abundance of predator and prey pollock and their co- 
occurrence. One working hypothesis was that the abun- 
dance of large predatory pollock and small prey pollock 
and their co-occurrence are determined by temperature, 
depth, location, the presence of the cold pool, and year. 
These covariates were also examined for their influence 
on the occurrence of cannibalism, along with other co- 
variates: the size of predators, and the co-occurrence of 
both large predatory pollock (>200 mm standard length 
[ SL] ) and small prey pollock. Owing to their vertical 
distribution in the water column, small prey pollock 
that may be vulnerable to cannibalism measure 60 to 
200 mm SL, a size that corresponds to the size of age-1 
pollock in the summer. A final hypothesis tested was 
that, where cannibalism occurs, the amount of canni- 
balism is determined by the size of predators, bottom 
temperature, bottom depth, location, the presence of the 
cold pool, year, and the abundance of vulnerable prey 
pollock (60-200 mm SL). 
Materials and methods 
Pollock abundance 
Abundance, co-occurrence, and diet data on predator and 
prey pollock were collected during the annual National 
Marine Fisheries Service (NFMS) bottom trawl surveys 
of the EBS shelf (<200 m depth) from 1982 to 2006 
(excluding 1983 and 1984, when pollock diet data were 
not collected). Groundfish species were captured at sta- 
tions on a regular grid of the EBS shelf (Fig. 1). The 
details of the survey design and sampling methods are 
described in Lauth (2010) and Stauffer (2004). Pollock 
captured in the bottom trawl were weighed and measured 
for length. A subset of pollock was selected for stomach 
content analyses. Pollock catch per unit of effort (CPUE, 
