293 
Factors influencing cannibalism and abundance 
of walleye pollock ( Theragra chalcogramma ) 
on the eastern Bering Sea shelf, 1982-2006 
Jennifer L. Boldt (contact author)' 
Troy W. Buckley 2 
Christopher N. Rooper 2 
Kerim Aydin 2 
Email address for contact author Jennifer Boldt@dfo-mpo.gc ca 
1 Pacific Biological Station 
Fisheries and Oceans Canada 
3190 Hammond Bay Road 
Nanaimo, BC, Canada V9T 6N7 
2 Alaska Fisheries Science Center 
National Marine Fisheries Service 
National Oceanic and Atmospheric Administration 
7600 Sand Point Way N E 
Seattle, Washington 98115 
Abstract — Cannibalism is thought 
to be an influential top-down pro- 
cess affecting walleye pollock (Ther- 
agra chalcogramma) recruitment in 
the eastern Bering Sea (EBS). In 
summer, many age-1 pollock occupy 
the same depths as those of adult 
walleye pollock, making them vul- 
nerable to cannibalism. We examine 
factors that influence the occurrence 
and amount of cannibalism, as well 
as the abundance and co-occurrence 
of predator and prey walleye pollock. 
Large walleye pollock were generally 
found in deeper waters and avoided 
cold temperatures; whereas, age-1 
walleye pollock were found in broader 
bottom depth and temperature ranges. 
The occurrence of cannibalism was 
highest in the area where predator 
and prey walleye pollock co-occurred 
and the amount of cannibalism was 
highest on the middle and outer EBS 
shelf. Both the occurrence and amount 
of cannibalism were influenced by 
location, bottom temperature and 
bottom depth, and the abundance of 
prey walleye pollock. The abundance 
of both large and small walleye pol- 
lock decreased during the 1982-2006 
survey period in the EBS and, hence, 
the occurrence and amount of canni- 
balism also decreased. The occurrence 
and amount of cannibalism observed 
in the diet samples from the summer 
survey were good indicators of year- 
class strength, as estimated by the 
stock assessment model. There was 
more cannibalism of age-1 walleye 
pollock when predicted recruit abun- 
dance was highest, indicating that 
summer cannibalism on age-1 walleye 
pollock, a top-down process, does not 
control walleye pollock recruitment 
in the EBS. 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Manuscript submitted 26 April 2011. 
Manuscript accepted 17 April 2012. 
Fish. Bull. 110:293-306 (2012). 
Recruitment of fish to fisheries 
depends on survival during the early 
life history (Hjort, 1914; Parker, 1966; 
Bailey and Spring, 1992; Bradford, 
1992). Factors that can affect survival 
during early life stages and subse- 
quent recruitment to fisheries may be 
related to environmental or bottom-up 
factors (Hollowed and Wooster, 1992; 
Hunt et ah, 2002) and predatory or 
top-down factors (Wespestad et ah, 
2000; Hunt et ah, 2002; Mueter et 
ah, 2006). Both top-down and bottom- 
up forces have been hypothesized as 
contributing to recruitment of one of 
the largest fishery resources in the 
eastern Bering Sea, walleye pollock 
( Theragra chalcogramma ; hereafter 
referred to as pollock; Mueter et ah, 
2006) 
Pollock in the eastern Bering Sea 
(EBS) comprise the highest-volume 
commercial fishery in the United 
States (NMFS, 2010). Their recruit- 
ment is widely believed to be influ- 
enced by a combination of environ- 
mental and density-dependent effects 
(Hollowed and Wooster, 1992; Mueter 
et ah, 2006). In the EBS, for example, 
bottom-up forces that may affect pol- 
lock recruitment include the effect of 
the extent, timing, and duration of 
ice, ocean currents, and temperature 
on primary and secondary productiv- 
ity (Wespestad et ah, 2000; Mueter et 
ah 2006; Coyle et ah, 2011; Hunt et 
ah, 2011). Top-down processes, such 
as predation, including cannibalism, 
are also important influences on EBS 
pollock survival (Dwyer et ah, 1987; 
Mueter et ah, 2006, 2011). 
Cannibalism is recognized as an 
important factor occurring year- 
round and affecting the survival and 
recruitment of pollock in the EBS 
(Bailey and Houde, 1989; Mueter et 
ah, 2006, 2011). In the fall, age-0 
pollock are heavily cannibalized by 
older pollock (Dwyer et ah, 1987), 
a density-dependent process that is 
hypothesized to affect the year-class 
strength and recruitment to the adult 
population (Wespestad et ah, 2000). 
Additionally, in the summer, canni- 
balism on age-1 pollock occurs (Bailey 
and Dunn, 1979; Francis and Bai- 
ley, 1983; Dwyer et ah, 1986, 1987) 
and may also affect subsequent year- 
class strength. Factors hypothesized 
to affect pollock cannibalism include: 
water column stratification and tem- 
perature and their effects on food 
availability; and the horizontal and 
vertical overlap of prey and preda- 
tor pollock (Francis and Bailey, 1983; 
Bailey, 1989; Wespestad et ah, 2000; 
Duffy-Anderson et ah, 2003; Mueter 
et ah, 2006; Hunt et ah, 2011). 
Summer temperatures and water 
column stratification on the EBS shelf 
