48 
Fishery Bulletin 120(1) 
Table 3 
Shannon—Wiener diversity index and Levin’s niche breadth for prey of 
Steller sea lions (Eumetopias jubatus) (SSL) and California sea lions 
(Zalophus californianus) (CSL), within season and year, from examina- 
tion of scat samples collected during 2010-2013 in northwest Washington. 
Corresponding 95% bias-corrected and accelerated bootstrap confidence 
intervals are provided in parentheses. Seasonal values are for samples 
collected during that season pooled across all years of study. Yearly val- 
ues reflect all samples collected for all seasons pooled within that year. 
Shannon—Wiener index 
Levin’s niche breadth 
Species Period 
SSL 
SSL 
SSL 
SSL 
SSL 
SSL 
CSL 
CSL 
CSL 
CSL 
CSL 
CSL 
Spring 
Summer 
Fall 
Winter 
2011 
2012 
Spring 
Summer 
Fall 
Fall 2010 
Fall 2011 
Fall 2012 
2.32 (2.27—2.40) 
2.04 (1.97—2.13) 
2.40 (2.34-2.49) 
2.11 (2.03—-2.24) 
2.23 (2.18-2.30) 
2.36 (2.32-2.44) 
2.20 (2.10—2.34) 
2.12 (2.04—2.25) 
1.77 (1.67-1.90) 
1.66 (1.51-1.95) 
1.64 (1.48-1.83) 
1.76 (1.60-1.97) 
8.32 (7.87-8.96) 
5.22 (4.68-5.87) 
8.60 (8.00—-9.44) 
6.26 (5.69-7.01) 
7.00 (6.47—7.60) 
8.29 (7.90-8.83) 
6.99 (6.01-8.40) 
6.90 (6.09-8.21) 
3.68 (3.17—4.24) 
3.88 (3.15—5.13) 
3.58 (2.84—4.49) 
3.34 (2.72—4.12) 
also increased each year during the fall from 11.6% of the 
diet in 2010 to 16.4% of the diet in 2012. 
Dietary niche overlap 
The Morisita—Horn index indicates significant dietary 
niche overlap between Steller and California sea lions. 
Using data from spring, summer, and fall pooled for all 
years of the study, we found significant dietary niche 
overlap between these species of sea lions (MH=0.88; 
95% confidence interval [CI]: 0.85—-0.91). Data pooled by 
season indicate significant overlap in spring (MH=0.82; 
95% CI: 0.75—-0.89), summer (MH=0.76; 95% CI: 0.66— 
0.86), and fall (MH=0.76; 95% CI: 0.69—0.83). There was 
also significant dietary niche overlap between the diets 
of California and Steller sea lions by year (pooling of all 
samples collected in spring, summer, and fall), with an 
MH of 0.77 in 2010 (95% CI: 0.65—0.91), an MH of 0.94 
in 2011 (95% CI: 0.92—0.97), and an MH of 0.82 in 2012 
(95% CI: 0.76—0.88). 
Prey consumption 
The average annual amount of prey consumed by Steller 
sea lions and California sea lions hauling out in north- 
west Washington from 2010 through 2013 was 11,327 t 
(SD 1600) and 9063 t (SD 4098), respectively. The amount 
of consumption was variable by season for both species; 
California sea lions had much higher consumption in fall 
than in other seasons (Fig. 4). In Tables 2 and 4, we report 
prey consumption by prey group and by lowest taxonomic 
group identified. 
Discussion 
Diet characterization 
This study is the first one to describe the diets of Califor- 
nia and Steller sea lions in northwest Washington. Like 
in past studies, we found that California and Steller sea 
lions eat diverse assemblages of prey that are seasonally 
and annually variable (Bailey and Ainley, 1981; Lowry 
et al., 1991; Merrick et al., 1997; Reimer and Brown’; Sin- 
clair and Zeppelin, 2002; Weise and Harvey, 2008; Womble 
et al., 2009; Scordino?; Orr et al., 2011; Riemer et al., 2011; 
Waite et al., 2012; Sinclair et al., 2013; Wiles, 2015; Trites 
and Rosen’). Steller and California sea lions in northwest 
Washington have diets among the more diverse that have 
been reported for these species on the basis of diet diver- 
sity indices (Aurioles-Gamboa and Camacho-Rios, 2007; 
Orr et al., 2011; Waite et al., 2012; Sinclair et al., 2013). 
We expected seasonal fluctuations in the diets of Steller 
and California sea lions for migratory prey taxa, such as 
the Pacific hake, the Pacific sardine, and Pleuronecti- 
formes (Eschmeyer et al., 1983; Demer et al., 2012; Malick 
et al., 2020), but other seasonal fluctuations in their diets 
were a surprise. Resident, nonmigratory prey taxa, such 
as Rajidae and Sebastidae, composed a larger portion of 
the sea lion diets in winter and spring than in summer 
and fall, likely a result of compensation for the reduc- 
tions of seasonally available prey. We expected to see a 
marked increase in consumption of salmon during the 
summer and early fall when adult Salmonidae migrate 
through the project area to their natal rivers (Weitkamp 
and Neely, 2002; Weitkamp, 2010), as observed in other 
