44 
Fishery Bulletin 120(1) 
Table 1 
Summary of the sampling of scat of Steller sea lions (Eumetopias jubatus) and California sea lions (Zalophus californianus) con- 
ducted from 2010 through 2013 in northwest Washington, by year, season, and month. 
Spring 
Species 
Steller sea lion 
California sea lion 
Monte Carlo simulations were used to incorporate uncer- 
tainty in estimating prey consumption by California and 
Steller sea lions. In each run of the model, parameter values 
were randomly selected from the parameter sampling dis- 
tributions listed in Supplementary Table 2 (online only) for 
California sea lions and in Supplementary Table 3 (online 
only) for Steller sea lions. The simulation was run in R 10,000 
times to estimate the mean and standard deviation (SD) of 
the total mass of prey consumed by California and Steller 
sea lions. We developed and used R code for estimating prey 
consumption by sea lions (the code is available from website). 
We multiplied our seasonal prey consumption estimates 
by the SSFO of each prey taxon to estimate the average 
metric tons eaten by sea lions in each season. Because we 
had no winter diet data for California sea lions, we used 
the pooled SSFO from all scat collections to estimate the 
amount eaten in winter. Seasonal estimates were added 
together to obtain annual consumption of each prey type 
by each sea lion species. 
Results 
During the study, 776 scat samples from Steller sea 
lions and 263 scat samples from California sea lions 
were collected (Table 1). Collections of scat of Steller sea 
lions were conducted primarily at the Tatoosh Complex 
in winter, Sea Lion Rock in summer, and a mix of all 
sites in spring and fall (Suppl. Table 4) (online only). The 
majority of scats from California sea lions were collected 
at East Bodelteh Island, with 2 collections totaling 13 
samples taken at Carroll Island. All raw count data from 
haul-out surveys and results from scat analysis are pub- 
licly available (Scordino et al.°). 
® Scordino, J., A. Akmajian, and S. Riemer. 2021. Steller and 
California sea lion count and diet data in northwest Washington, 
2010-2013. Mendeley Data, V1. [Available from website.] 
No. of samples 
Summer 
Counts of sea lions 
Counts indicate very different patterns in utilization of 
haul-out sites by California and Steller sea lions in north- 
west Washington (Fig. 2). Steller sea lions were present 
throughout the year with slightly greater counts in fall 
and slightly lower counts in winter. The greatest counts of 
California sea lions were recorded in fall. The proportion 
of the population of Steller sea lions composed of adult 
males, adult females, juveniles, and pups was variable by 
month, with adult females and juveniles accounting for the 
majority of counts (Fig. 3). The exception was during sum- 
mer when males composed a percentage of the hauled-out 
population similar to that for adult females. For individual 
haul-out sites within the study area, counts reveal variable 
patterns in utilization by sea lion species (Suppl. Figs. 1—7) 
(online only) and by demographic group of Steller sea lions 
(Suppl. Figs. 8-14) (online only). 
Diet of Steller sea lions 
The primary prey groups (those with SSFO >5%) for 
all scat samples from Steller sea lions were Clupeidae 
(22.7%), Sebastidae (14.0%), Rajidae (13.1%), Salmonidae 
(11.7%), Pleuronectiformes (9.7%), Squalidae (8.3%), and 
Merlucciidae (5.6%) (Table 2). 
For Steller sea lions, diet varied between seasons both 
in the SSFO of common prey species and diet diversity 
indices. Diet diversity was similar in spring and fall 
and in summer and winter, with the greatest diversity 
occurring in spring and fall for all measures (Table 3). 
The Levin’s niche breadth values are likely driven by 
the fact that the 3 most common prey families in the 
spring and fall made up 47.5% and 47.1% of the Steller 
sea lion diet, respectively; whereas, the 3 most common 
prey families in the summer and winter made up 62.6% 
and 61.4% of the diet, respectively (Table 2). Pacific 
hake (Merluccius productus) were common in the sum- 
mer (10.8%) and fall (8.5%) but rarely consumed in the 
