369 
Food habits of Steller sea lions 
( Eumetopias jubatus ) off Oregon 
and northern California, 1986-2007 
Email address for contact author: susan. d.riemer@state.or.us 
1 Oregon Department of Fish and Wildlife 
Marine Mammal Research Program 
1495 E Gregory Road 
Central Point, Oregon 97502 
2 Oregon Department of Fish and Wildlife 
Marine Mammal Research Program 
7118 NE Vandenberg Avenue 
Corvallis, Oregon 97330 
Abstract — We described the diet of 
the eastern stock of Steller sea lions 
(Eumetopias jubatus ) from 1416 scat 
samples collected from five sites 
in Oregon and northern California 
from 1986 through 2007. A total of 
47 prey types from 30 families were 
identified. The most common prey was 
Pacific hake ( Merluccius productus), 
followed by salmonids ( Oncorhynchus 
spp.), skates (Rajidae), Pacific lam- 
prey (Lampetra tridentata ), herrings 
(Clupeidae), rockfish (Sebastes spp.), 
and northern anchovy (Engraulis 
mordax). Steller sea lion diet composi- 
tion varied seasonally, annually, and 
spatially. Hake and salmonids were 
the most commonly identified prey 
in scats collected during the summer 
(breeding season), whereas hake and 
skate were most common in the non- 
breeding season. Continued research 
on Steller sea lion diet and foraging 
behavior in the southern extent of 
their range is necessary to address 
issues such as climate change, inter- 
action with competing California sea 
lions, and predation impacts on valu- 
able or sensitive fish stocks. 
Manuscript submitted 27 October 2010. 
Manuscript accepted 18 May 2011. 
Fish. Bull. 109:369-381 (2011). 
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. 
Susan D. Riemer' (contact author) 
Bryan E. Wright 2 
Robin F. Brown 2 
Knowledge of an animal’s diet is 
important for understanding its for- 
aging behavior, habitat use, and popu- 
lation dynamics, and this knowledge 
is of particular importance when con- 
sidering threatened and endangered 
species. Steller sea lions (Eumetopias 
jubatus ) are a case in point. Rang- 
ing throughout the North Pacific Rim 
from California to Japan (Loughlin 
et al., 1984; Pitcher et al., 2007), 
Steller sea lion populations in western 
Alaska underwent dramatic declines 
from the late 1970s to early 1990s 
(Braham et al., 1980; Merrick et al., 
1987; Loughlin et al., 1992; Trites 
and Larkin, 1996). This population 
was listed as “threatened” under 
the Endangered Species Act (ESA) 
in 1990, and later the western stock 
was listed as “endangered” (Lough- 
lin, 1997; NMFS, 2008). The primary 
hypothesis for the decline has been 
chronic nutritional stress related to 
changes in diet (Springer, 1992; Mer- 
rick and Loughlin, 1997; Trites and 
Donnelly, 2003; NMFS, 2008). 
With the nutritional stress hypoth- 
esis (Springer, 1992; Merrick et al., 
1997; Trites and Donnelly, 2003), 
and its successor, the ocean climate 
hypothesis (Trites et al., 2007a), de- 
clines in the Steller sea lion western 
distinct population segment (WDPS) 
were proposed to be the result of 
changes in the quantity, quality, and 
availability of prey, brought about 
by an ocean climate regime shift in 
1976-77 (but see Fritz and Hinckley, 
[2005]). This shift is hypothesized 
to have forced Steller sea lions to 
change their diet and to have re- 
sulted in chronic nutritional stress 
manifested by reductions in body 
size, productivity, and juvenile and 
pup survival (York, 1994; Trites and 
Donnelly, 2003). Other explanations 
for the decline of the WDPS that 
were considered but rejected included 
population redistribution, commercial 
and subsistence harvest, predation, 
pollution, and entanglement in ma- 
rine debris (Merrick et al., 1987). 
Although the Steller sea lion 
WDPS experienced annual declines 
in abundance ranging between 1.6% 
and 5.2% (Merrick et al., 1987), the 
abundance in the eastern distinct 
population segment (EDPS) increased 
at 3.1% per year from 1977 through 
2002 (Pitcher et al., 2007; NMFS, 
2008). The hypothesized role of poor 
diet in the decline of the WDPS, con- 
trasted with the increasing EDPS, 
begs the question as to what type of 
prey the Steller sea lion EDPS con- 
sumes and how does it compare with 
that of the WDPS. The majority of 
information on Steller sea lion diet, 
however, has come from Alaska (e.g.. 
Pitcher, 1981; Merrick et al., 1997; 
Sinclair and Zeppelin, 2002; Womble 
and Sigler, 2006; Trites et al., 2007b; 
McKenzie and Wynne, 2008). In this 
