40 
Fishery Bulletin 120(1) 
Better knowledge of the spatial and temporal variability of 
the diets of California and Steller sea lions is needed. 
It is likely that California and Steller sea lions uti- 
lize the same marine environment for hunting prey 
because the sympatric species use the same haul-out 
sites (Mate, 1975) and because adult male California sea 
lions are similar in size to adult female Steller sea lions 
and, therefore, have similar physiological limits to dive 
performance (Weise et al., 2010). It is noteworthy that, 
although the abundance of the eastern distinct popula- 
tion segment of Steller sea lions increased at an annual 
rate of 4.25% for pups and of 3.22% for age-1+ sea lions 
from 1987 to 2017 (Muto et al., 2020), the growth rate 
of the population was not uniform across its range. The 
range of the Steller sea lion has contracted northward 
since the early 20th century with rookeries abandoned 
in Southern California and abundance at rookeries in 
central California declining significantly (Pitcher et al., 
2007; NMFS, 2013). One hypothesis is that niche compe- 
tition with California sea lions, which were increasing 
in abundance (Laake et al., 2018), caused the range con- 
traction (Mate, 1975; NMFS, 2013). Understanding to 
what degree the dietary niches of California and Steller 
sea lions overlap will improve our understanding of the 
ecology of the 2 species and the potential for competition 
between them. 
In this study, we had 3 objectives to advance our 
knowledge of the diets of California and Steller sea lions 
in northwest Washington. First, we characterized the 
diets of both species, including documenting prey diver- 
sity and seasonal and annual variability in diet. Second, 
we performed calculations to estimate the biomass (in 
metric tons) of fish and invertebrates consumed per year 
and season by California and Steller sea lions. Last, 
we used our diet data to evaluate the hypothesis that 
these sympatric sea lion species have significant dietary 
niche overlap. 
Materials and methods 
Study area 
Study activities were conducted in northwest Wash- 
ington (Fig. 1), which is the northernmost extent of the 
California Current. Relative to other portions of the 
northern California Current, the waters of northwest 
Washington have elevated productivity and enhanced 
biomass of high trophic levels due to geomorphic fea- 
tures and the confluence of the California Current and 
the Strait of Juan de Fuca (McFarlane et al., 1997; Mar- 
chetti et al., 2004; MacFadyen et al., 2008). Year-round 
sea lion haul-out sites in the study area include the 
Tatoosh Island Complex (haul-out sites: Tatoosh Island 
East, Tatoosh Island Cut, and Duncan Rock), Bodelteh 
Island Complex (haul-out sites: East Bodelteh Island, 
West Bodelteh Island, Umatilla Reef, and Guano Rock), 
Carroll Island, and Sea Lion Rock (Fig. 1). At Carroll 
Island and Sea Lion Rock, 10—20 Steller sea lion pups 
were born each year during the study period of 2010-— 
2013; both sites now meet the definition of a rookery 
(Pitcher et al., 2007) with greater than 50 newborn pups 
counted annually at Carroll Island since 2015 and Sea 
Lion Rock since 2019 (senior author, unpubl. data). We 
also surveyed Waadah Island, which is a seasonal haul- 
out site primarily utilized in the spring. 
Field methods 
Haul-out counts Vessel-based surveys were conducted by 
circling haul-out sites and counting sea lions with 7x50 or 
8x40 binoculars. When possible, we conducted land-based 
surveys of East Bodelteh Island. Sea lions at haul-out 
sites were counted in sections because of the size of the 
sites and to minimize disturbance by only circling each 
site once. For each section, we counted the total number 
of Steller and California sea lions present, and then we 
recounted the section for 4 demographic groups of Steller 
sea lions: pups, juveniles, adult females, and adult males. 
Pups were identified by their darker brown color, chubbier 
features, and smaller size and were counted from birth 
(May—July) through 11 months of age (May) (Pitcher et al., 
2001). Juveniles were identified as individuals older and 
larger than pups that had not developed the secondary 
sexual characteristics of adult males or the size of adult 
females and were assumed to be between 1 and roughly 5 
years of age. Adult females were identified by size, shape, 
and presence of a pup or dependent juvenile and by having 
longer whiskers than juveniles (King et al., 2007; Stricker 
et al., 2015). Adult males were identified by their overall 
larger size, coarse fur on chest and neck, and large head 
and foreflippers. 
We used the known age of branded individuals (see 
Wright et al., 2017) to calibrate our methods for classifying 
demographic groups. No demographic count was assigned 
if the sea lions entered the water or were arranged too 
close together to evaluate body shape. We assumed all 
California sea lions were adult males even though we did 
observe at least one female identified by the presence of 
a newborn pup and many juveniles including one that 
was identified as a 1 year old from San Miguel Island, 
California, on the basis of its brand number (see DeLong 
et al., 2017). 
We used the demographic counts to calculate the propor- 
tion of age-1+ Steller sea lions observed that were adult 
male, adult female, and juvenile; pups and sea lions not 
classified to a demographic group were excluded from this 
calculation. Olesiuk* found no significant difference in the 
proportion of time that age-1+ sea lions spent hauled out 
by sex or age, indicating that our demographic counts of 
age-1+ sea lions that were hauled out are representative 
of the whole population that utilized haul-out sites in 
northwest Washington during surveys. 
* Olesiuk, P. F. 2018. Recent trends in abundance of Steller sea 
lions (Eumetopias jubatus) in British Columbia. Can. Sci. Advis. 
Secr. Res. Doc. 2018/006, 67 p. [Available from website.] 
