Scordino et al.: Dietary niche overlap and prey consumption for Eumetopias jubatus and Zalophus californianus 51 
availability that were not representative of seasonal or 
annual trends. However, in a study of penguin diet, fre- 
quent samplings with small collections were compared 
to infrequent samplings with large collections, and no 
significant differences in diet estimates were found (Ber- 
row et al., 1999). Additionally, we chose to use SSFO to 
document diet; however, other approaches, such as bio- 
mass reconstruction, have been proven to have more 
accurately reconstructed the diet of sea lions (Tollit et al., 
2007). The use of SSFO in our study may have resulted 
in overreporting the importance of small-bodied prey 
and underreporting the importance of large-bodied prey 
(Laake et al., 2002; Tollit et al., 2007). 
Prey consumption 
We estimated that Steller sea lions ate an average of 
11,327 t (SD 1600) of prey per year and that California 
sea lions ate an average of 9063 t (SD 4098) of prey per 
year from 2010 through 2013 in northwest Washington. 
It is important when considering this result to remember 
that the scope of inference of this study was limited to sea 
lions hauled out in northwest Washington in 2010-2013. 
From 2010 through 2018, the average count of California 
and Steller sea lions hauled out in northwest Washington 
increased at a rate of 7.8% and 7.9%, respectively (Allyn 
and Scordino, 2020). At these observed rates of increase, 
our estimate of prey biomass consumed is likely less than 
half of what the 2 sea lion species are eating per year at 
the time of this publication. 
Results from our model of consumption of prey by sea 
lions are best characterized as estimates rather than as 
definitive values for a number of reasons. First, we used 
published estimates for many parameters of the model. 
The proportion of body weight eaten per day published by 
Winship et al. (2006) is based on an energetic model with 
fixed caloric density of prey, although caloric densities are 
known to vary between species (Logerwell and Schaufler, 
2005) and within species because of reproductive state and 
environmental factors (von Biela et al., 2019). Winship 
et al. (2006) did not account for fluctuations in caloric 
demands, but results from captive studies indicate that 
caloric demands of both Steller and California sea lions 
change seasonally (Kastelein et al., 1990, 2000). 
We used haul-out count correction factors in our 
model that were developed for aerial surveys of sea 
lions (Lowry and Forney, 2005; Olesiuk*), although in 
our study, counts were conducted by vessel or from land. 
Vessel- and land-based counts are known to be nega- 
tively biased, compared to counts from aerial surveys 
(Westlake et al., 1997); however, no correction factor was 
available to compare values produced with our count 
method to aerial counts. We also did not account for the 
possibility that sea lions hauled out at sites in Canada, 
such as Carmanah Point, could forage in the same area 
as the sea lions hauled out in northwest Washington. 
Together, not accounting for the negative bias of vessel- 
based and land-based counts and not accounting for sea 
lions from Canada likely negatively biased our estimates 
of prey consumption for both sea lion species in our study 
area by an unknown amount. Despite the limitations and 
assumptions needed for our model of consumption, we 
believe it provided important perspective for the role of 
Steller and California sea lions in the northern California 
Current Ecosystem. 
Dietary niche overlap 
We hypothesized that Steller and California sea lions 
would have dietary niche overlap because they are central 
place foragers (Womble et al., 2009) that utilize the same 
haul-out sites (Mate, 1975) and have similar physiological 
dive limits (Weise et al., 2010). Our findings indicate that 
the sympatric sea lion species use the same dietary niche 
and are therefore competing for prey resources, but there 
are a number of caveats to this conclusion. First, inter- 
preting niche overlap analyses is difficult because both a 
finding of no dietary niche overlap and a finding of signif- 
icant dietary niche overlap could be signs of competition 
(Litvaitis et al., 1996). Further, significant overlap can 
occur without competition if prey species are abundant 
(Szabo and Meszéna, 2006). The observed rapid increase 
in counts of sea lions at haul-out sites in northwest Wash- 
ington (Allyn and Scordino, 2020) indicates that prey 
resources were not limiting during our study. 
Another consideration is that we evaluated dietary 
niche overlap by using prey grouped to family or a higher 
taxonomic order, an approach that assumes that prey spe- 
cies within groupings by family or higher taxonomic order 
represented prey items that were identical (Greene and 
Jaksi¢, 1983; Krebs, 1999). Some families of fish have spe- 
cies with very different behaviors, distributions, and sizes 
that affect their vulnerability to predation by the 2 sea 
lion species. For instance, in Sebastidae, there are species 
that are pelagic and schooling, solitary and benthic, pri- 
marily distributed in nearshore waters, and primarily dis- 
tributed in offshore slope habitat (Eschmeyer et al., 1983). 
Even when we documented that the sea lions were eating 
the same species of prey, they could have exploited differ- 
ent size classes of that species. In a study in the Barents 
Sea, ringed seals (Pusa hispida) and harp seals (Pagoph- 
ilus groenlandicus) had almost complete niche overlap 
strictly on the basis of identification of prey species in scat 
samples (Wathne et al., 2000), yet a closer examination of 
prey remains revealed niche partitioning with harp seals 
diving deeper for larger fish of the same species. Although 
our results indicate significant dietary niche overlap, it is 
possible that the sea lions of the 2 species in this study 
were partitioning their dietary niches in a way that our 
study design could not detect. 
Conclusions 
We found that Steller and California sea lions in north- 
west Washington have similar diets with seasonal and 
annual variability, and we found significant dietary 
niche overlap between the 2 species. Currently, there 
