Scordino et al.: Dietary niche overlap and prey consumption for Eumetopias jubatus and Zalophus californianus 45 
iS 
5 
{o) 
ce) 
&, 2000 
© 
oO 
> 
<x 
1000 
th [bs IF 
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 
O California sea lion 
O Steller sea lion 
Figure 2 
Average monthly counts of age-1+ Steller sea lions (Eumetopias jubatus) and 
California sea lions (Zalophus californianus) in northwest Washington during 
vessel- and land-based surveys conducted from 2010 through 2013. Error bars 
indicate standard deviations. 
winter (0.3%) and spring (0.9%). Conversely, consump- 
tion of Gadidae was low in the summer (1.7%) and fall 
(3.6%) but common in the winter (5.7%) and spring 
(9.0%). Pleuronectiformes were common in all seasons 
(>5%) except winter. Clupeidae was the most common 
prey family in the diet of Steller sea lions in summer 
with an SSFO of 36.7% but accounted for only 13.8% of 
Average percentage of total count (%) 
Jan Feb Mar Apr May 
@ Adult male 
f& Adult female 
the diet in spring, likely driven by the 
presence of Pacific sardine (Sardinops 
sagax), which had an SSFO of 18.5% 
in summer but was rarely consumed in 
other seasons. 
Annual comparisons of diet included 
only data from 2011 and 2012 because 
samples were collected in all seasons 
(Table 1). Diet diversity was greater in 
2012 than in 2011 for both measures 
(Table 3). Consumption of common prey 
species also varied between years. The 
SSFO of Clupeidae declined from 27.9% 
in 2011 to 20.1% in 2012, likely due to 
a large reduction in the consumption 
of Pacific sardine. From 2011 to 2012, 
the consumption of northern anchovy 
(Engraulis mordax) and Pacific hake 
also declined. In 2012, Steller sea lions 
had higher consumption of common 
prey taxa, including Pleuronectiformes, 
Salmonidae, and Pacific spiny dogfish 
(Squalus suckleyi), and increases in 
consumption of a number of less com- 
mon prey taxa, in comparison to levels in 2011 (Suppl. 
Table 5) (online only). 
Diet of California sea lions 
The primary prey families (>5% SSFO) for all scat sam- 
ples from California sea lions were Clupeidae (38.2%), 
Aug Ses Oct Nov’ Dec 
&) Juvenile © Pup 
Figure 3 
Average monthly percentage of counts of Steller sea lions (Eumetopias jubatus) at haul-out sites 
during 2010-2013 in northwest Washington for 4 demographic groups. The demographic groups 
used in counts were adult males, adult females, juveniles, and pups. Error bars indicate standard 
deviations. 
